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1
Print a literal string on standard output
``print *, 'Hello, world!'``
2
Loop to execute some code a constant number of times
``````program main
implicit none
integer :: i
do i=1,10
write (*,'(A)') "Hello"
end do
end program main
``````
3
Like a function which doesn't return any value, thus has only side effects (e.g. Print to standard output)
``````module foo
implicit none
contains
subroutine bar
print *,"Hello"
end subroutine bar
end module foo``````
4
Create a function which returns the square of an integer
``````module foo
implicit none
contains
function square(i) result(res)
integer, intent(in) :: i
integer :: res
res = i * i
end function square
end module foo
``````
5
Declare a container type for two floating-point numbers x and y
``````type point
real :: x
real :: y
end type point``````
Alternative implementation:
``````module x
type point
real :: x, y
end type point
end module x
``````
6
Do something with each item x of the list (or array) items, regardless indexes.
``````elemental subroutine foo(x)
real, intent(in) :: x
end subroutine foo

call foo(x)``````
7
Print each index i with its value x from an array-like collection items
``````do i=1, size(items)
print *,i, items(i)
end do``````
9
The structure must be recursive because left child and right child are binary trees too. A node has access to children nodes, but not to its parent.
``````type binary_tree
integer :: value
type(binary_tree), allocatable :: left
type(binary_tree), allocatable :: right
end type binary_tree``````
10
Generate a random permutation of the elements of list x
``````module M_shuffle
contains
function scramble( number_of_values ) result(array)
integer,intent(in)    :: number_of_values
integer,allocatable   :: array(:)
array=[(i,i=1,number_of_values)]
n=1; m=number_of_values
do k=1,2
do i=1,m
call random_number(u)
j = n + FLOOR((m+1-n)*u)
itemp=array(j); array(j)=array(i); array(i)=itemp
enddo
enddo
end function scramble
end module M_shuffle
:
use M_shuffle, only : scramble
newlist=list(scramble(size(list))
``````
11
The list x must be non-empty.
``````
call random_number (a)
x(lbound(x) + int(a*ubound(x))
``````
12
Check if the list contains the value x.
list is an iterable finite container.
``if (findloc (list, x, 1) != 0) then``
Alternative implementation:
``if (any(x == list)) ...``
14
Pick a random number greater than or equals to a, strictly inferior to b. Precondition : a < b.
``````call random_number(c)
d = a + (b-a) * c``````
15
Pick a random integer greater than or equals to a, inferior or equals to b. Precondition : a < b.
``````real :: c
integer :: res

call random_number(c)
res = int((b-a+1)*c)
``````
16
Call a function f on every node of binary tree bt, in depth-first infix order
``````module x
type tree
type (tree), pointer :: left, right
contains
procedure, pass:: trav
end type tree
contains
recursive subroutine trav (t, f)
class (tree), intent(inout) :: t
interface
subroutine f(t)
import
class (tree), intent(inout) :: t
end subroutine f
end interface
if (associated (t%left)) call trav (t%left, f)
call f(t)
if (associated (t%right)) call trav (t%right, f)
end subroutine trav
end module x``````
17
The structure must be recursive. A node may have zero or more children. A node has access to its children nodes, but not to its parent.
``````type node_t
integer:: value
type(node_t), pointer :: next_sibling;
type(node_t), pointer :: first_child;
end type node_t``````
18
Call a function f on every node of a tree, in depth-first prefix order
``````  recursive subroutine depth_first (node, f)
type (tr), pointer :: node
interface
subroutine f(node)
import
type(tr), pointer :: node
end subroutine f
end interface
if (associated(node%left)) call depth_first (node, f)
if (associated(node%right)) call depth_first (node, f)
call f(node)
end subroutine depth_first``````
19
Reverse the order of the elements of the list x.
This may reverse "in-place" and destroy the original ordering.
``  a = a(ubound(a,dim=1)::-1)``
20
Implement a function search which looks for item x in a 2D matrix m.
Return indices i, j of the matching cell.
Think of the most idiomatic way in the language to return the two values at the same time.
``````function search(m,x)
integer, dimension(:,:), intent(in) :: m
integer, intent(in) :: x
integer, dimension(2) :: search
search = findloc(x,m)
end function search``````
21
Swap the values of the variables a and b
``````tmp = a
a = b
b = tmp``````
Alternative implementation:
``````subroutine swap(a, b)
integer, intent(inout) :: a, b
integer :: temp
temp = a
a = b
b = temp
end subroutine swap``````
22
Extract the integer value i from its string representation s (in radix 10)
``read (unit=s,fmt=*) i``
23
Given a real number x, create its string representation s with 2 decimal digits following the dot.
``  write (unit=s,fmt="(F20.2)") x``
24
Declare a new string s and initialize it with the literal value "ネコ" (which means "cat" in japanese)
``````  use, intrinsic :: iso_fortran_env
implicit none
integer, parameter :: u = selected_char_kind('ISO_10646')

character(kind=u,len=2) :: cat
cat = u_"ネコ"
``````
25
Share the string value "Alan" with an existing running process which will then display "Hello, Alan"
``````str = "Alan"
sync all
if (this_image() == 1) then
print *,"Hello, ", str
end if``````
26
Declare and initialize a matrix x having m rows and n columns, containing real numbers.
``  real, dimension(m,n) :: x``
27
Declare and initialize a 3D array x, having dimensions boundaries m, n, p, and containing real numbers.
``````  real, dimension(:,:,:), allocatable :: x

allocate (x(m,n,p))
``````
29
Remove i-th item from list items.
This will alter the original list or return a new list, depending on which is more idiomatic.
Note that in most languages, the smallest valid value for i is 0.
``````integer, allocatable, dimension(:) :: items
items = [items(:i-1), items(i+1:)]``````
30
Launch the concurrent execution of procedure f with parameter i from 1 to 1000.
Tasks are independent and f(i) doesn't return any value.
Tasks need not run all at the same time, so you may use a pool.
``````  integer :: tasks, n, t, i
n = num_images()
t = this_image()
do i = t, tasks, n
call f(i)
end do
sync all``````
31
Create the recursive function f which returns the factorial of the non-negative integer i, calculated from f(i-1)
``````module x
implicit none
contains
recursive function f (i) result (res)
integer, intent(in) :: i
integer :: res
if (i <= 0) then
res = 1
else
res = f(i-1) * i
end if
end function f
end module x``````
32
Create function exp which calculates (fast) the value x power n.
x and n are non-negative integers.
``````function exp(x,n) result(I)
I=x**n
end function exp``````
33
Assign to the variable x the new value f(x), making sure that no other thread may modify x between the read and the write.
``x = f(x)``
Alternative implementation:
``````integer, dimension[*] :: x

critical
x = f(x)
end critical``````
38
Find substring t consisting in characters i (included) to j (excluded) of string s.
Character indices start at 0 unless specified otherwise.
Make sure that multibyte characters are properly handled.
``````character(len=:), allocatable :: t

t = s(i:j-1)
``````
39
Set the boolean ok to true if the string word is contained in string s as a substring, or to false otherwise.
``ok = index(string, word) /= 0``
41
Create string t containing the same characters as string s, in reverse order.
Original string s must remain unaltered. Each character must be handled correctly regardless its number of bytes in memory.
``````  character(len=:), allocatable :: t
integer :: i, n

allocate (t, mold=s)
n = len(s)
do i = 0, n - 1
t(n-i : n-i) = s(i+1 : i+1)
end do``````
42
Print each item v of list a which is not contained in list b.
For this, write an outer loop to iterate on a and an inner loop to iterate on b.
``````  outer: do i=1,size(a)
do j=1,size(b)
if (a(i) == b(j)) cycle outer
end do
print *,a(i)
end do outer``````
43
Look for a negative value v in 2D integer matrix m. Print it and stop searching.
``````  outer: do j=1, size(m,2)
do i=1, size(m,1)
v = m(i,j)
if (v < 0) then
print *,v
exit outer
end if
end do
end do outer``````
44
Insert the element x at position i in the list s. Further elements must be shifted to the right.
``````integer, dimension(:), allocatable :: s

s = [s(1:i-1), x, s(i:)]``````
45
Sleep for 5 seconds in current thread, before proceeding with the next instructions.
``````module M_time
contains
subroutine system_sleep(wait)
use,intrinsic :: iso_c_binding, only: c_int
integer,intent(in) :: wait
integer(kind=c_int):: waited
interface
function c_usleep(msecs) bind (C,name="usleep")
import
integer(c_int) :: c_usleep
integer(c_int),intent(in),VALUE :: msecs
end function c_usleep
end interface
if(wait.gt.0)then
waited=c_usleep(int(wait,kind=c_int))
endif
end subroutine system_sleep
end module M_time
program x
use M_time
call system_sleep(5000000)
end  ``````
46
Create string t consisting of the 5 first characters of string s.
Make sure that multibyte characters are properly handled.
`````` character(len=5) :: t
t = s(1:5)``````
47
Create string t consisting in the 5 last characters of string s.
Make sure that multibyte characters are properly handled.
``````character(len=5) :: t
t = s(len(s)-4:)``````
48
Assign to variable s a string literal consisting in several lines of text, including newlines.
``````  s = "Hello &
&World"
``````
49
Build list chunks consisting in substrings of the string s, separated by one or more space characters.
``````program main
use stringifor_string_t ! https://github.com/szaghi/StringiFor
implicit none
type( string ) ::  string1
type( string ), allocatable :: substrings( : )
integer :: i

string1 =  " Build list _chunks consisting in substrings of input string _s  separated by one or more space characters"
call string1%split(sep=' ', tokens=substrings )
do i=1,size(substrings, dim=1)
write(*,*) substrings(i)
enddo
end program main``````
50
Write a loop that has no end clause.
``````do
end do``````
53
Concatenate elements of string list x joined by the separator ", " to create a single string y.
``````  write (unit=y,fmt='(*(A,:,", "))') x

``````
54
Calculate the sum s of the integer list or array x.
``   s = sum(x)``
55
Create the string representation s (in radix 10) of the integer value i.
``  write (unit=s,fmt=*) i``
56
Fork-join : launch the concurrent execution of procedure f with parameter i from 1 to 1000.
Tasks are independent and f(i) doesn't return any value.
Tasks need not run all at the same time, so you may use a pool.
Wait for the completion of the 1000 tasks and then print "Finished".
``````  integer :: tasks, n, t, i
n = num_images()
t = this_image()
do i = t, tasks, n
call f(i)
end do
sync all
print *,"Finished"
``````
57
Create the list y containing the items from the list x that satisfy the predicate p. Respect the original ordering. Don't modify x in-place.
``y = pack(x,mask=p(x))``
58
Create string lines from the content of the file with filename f.
``````program p
character(len=:),allocatable :: s
open(unit=10,file='myfile',access='stream')
inquire(10,size=i)
allocate(character(len=i) :: s)
write(*,*)s
end program p
``````
59
Print the message "x is negative" to standard error (stderr), with integer x value substitution (e.g. "-2 is negative").
``````program write_to_stderr
use iso_fortran_env, only : stderr=>ERROR_UNIT
implicit none
integer :: x=-2
write(stderr,'(i0," is negative")') x
end program write_to_stderr
``````
60
Assign to x the string value of the first command line parameter, after the program name.
``````  character(len=:), allocatable :: x
integer :: n
call get_command_argument (1, length=n)
allocate (character(n):: x)
call get_command_argument (1, x)
``````
61
Assign to the variable d the current date/time value, in the most standard type.
``````integer, dimension(8) :: d

call date_and_time (values=d)

``````
62
Set i to the first position of string y inside string x, if exists.

Specify if i should be regarded as a character index or as a byte index.

Explain the behavior when y is not contained in x.
``I=index(x,y)``
63
Assign to x2 the value of string x with all occurrences of y replaced by z.
Assume occurrences of y are not overlapping.
``````  character(len=:), allocatable :: x
character(len=:), allocatable :: x2
character(len=:), allocatable :: y,z
integer :: j, k
k=1
do
j = index(x(k:),y)
if (j==0) then
x2 = x2 // x(k:)
exit
end if
if (j>1) then
x2 = x2 // x(k:j+k-2)
end if
x2 = x2 // z
k = k + j + len(y) - 1
end do``````
65
From the real value x in [0,1], create its percentage string representation s with one digit after decimal point. E.g. 0.15625 -> "15.6%"
``write (*,'(F5.1,"%")') x*100.``
67
Calculate binom(n, k) = n! / (k! * (n-k)!). Use an integer type able to handle huge numbers.
``````integer, parameter :: i8 = selected_int_kind(18)
integer, parameter :: dp = selected_real_kind(15)
n = 100
k = 5
print *,nint(exp(log_gamma(n+1.0_dp)-log_gamma(n-k+1.0_dp)-log_gamma(k+1.0_dp)),kind=i8)``````
69
Use seed s to initialize a random generator.

If s is constant, the generator output will be the same each time the program runs. If s is based on the current value of the system clock, the generator output will be different each time.
``````  call random_seed(size = n)
allocate(seed(n))
! ...
call random_seed(put=seed)
``````
70
Get the current datetime and provide it as a seed to a random generator. The generator sequence will be different at each run.
``````  call random_seed (size=k)
allocate (seed(k))
call date_and_time (values=val)
seed = [(173*i**2+4567,i=1,k)]
m = min(8,k)
seed(1:m) = seed(1:m) + val(m:1:-1)
call random_seed (put=seed)``````
71
Basic implementation of the Echo program: Print all arguments except the program name, separated by space, followed by newline.
The idiom demonstrates how to skip the first argument if necessary, concatenate arguments as strings, append newline and print it to stdout.
``````program x
implicit none
character (len=:), allocatable :: a
integer :: n, i, l
n = command_argument_count()
a = ''
do i=1,n
call get_command_argument(i, a, l)
if (l > len(a)) then
deallocate (a)
allocate (character(len=l) :: a)
call get_command_argument(i, a)
end if
if (i < n) then
else
write (unit=*,fmt='()')
end if
end do
end``````
74
Compute the greatest common divisor x of big integers a and b. Use an integer type able to handle huge numbers.
``````function gcd(m,n) result(answer)
implicit none
integer(kind=int64),intent(in)  :: m, n
ifirst=iabs(m)
else
do
if(irest == 0)  exit
enddo
endif
end function gcd
``````
76
Create the string s of integer x written in base 2.

E.g. 13 -> "1101"
``write (unit=s,fmt='(B0)') x``
Alternative implementation:
``write (unit=s,fmt='(B32)') x``
77
Declare a complex x and initialize it with value (3i - 2). Then multiply it by i.
``````  complex :: x
x = (-2,3)
x = x * (0,1)
``````
78
Execute a block once, then execute it again as long as boolean condition c is true.
``````do
call do_something
if (.not. c) exit
end do``````
79
Declare the floating point number y and initialize it with the value of the integer x .
``````real :: y
y = x``````
80
Declare integer y and initialize it with the value of floating point number x . Ignore non-integer digits of x .
Make sure to truncate towards zero: a negative x must yield the closest greater integer (not lesser).
``````integer :: y
real  :: x
y=x``````
81
Declare the integer y and initialize it with the rounded value of the floating point number x .
Ties (when the fractional part of x is exactly .5) must be rounded up (to positive infinity).
``````integer :: y

y = nint(x)``````
82
Find how many times string s contains substring t.
Specify if overlapping occurrences are counted.
``````  lt = len(t) - 1
k = 1
num = 0
do
print *,s(k:)
i = index(s(k:),t)
if (i==0) exit
num = num + 1
k = k + i + lt
end do
print *,num
``````
84
Count number c of 1s in the integer i in base 2.

E.g. i=6 → c=2
``c = popcnt(i)``
85
Write boolean function addingWillOverflow which takes two integers x, y and return true if (x+y) overflows.

An overflow may be above the max positive value, or below the min negative value.
``````  logical function adding_will_overflow (x,y) result(res)
integer, intent(in) :: x, y
res = (x > 0 .and. y > huge(x) - x) .or. (x < 0 .and. y < huge(x) - x)
86
Write boolean function multiplyWillOverflow which takes two integers x, y and return true if (x*y) overflows.

An overflow may be above the max positive value, or below the min negative value.
``````  logical function multiply_will_overflow (x, y) result(res)
integer, intent(in) :: x, y
integer, parameter :: ik = selected_int_kind (int(digits(y)*log10(2.)*2))

res = int(x,kind=ik) * int(y,kind=ik) > huge(x)
end function multiply_will_overflow``````
87
Exit immediately.
If some extra cleanup work is executed by the program runtime (not by the OS itself), describe it.
``stop``
88
Create a new bytes buffer buf of size 1,000,000.
``````  integer(kind=c_int8_t), dimension(:), allocatable :: a

allocate (a(10**6))``````
89
You've detected that the integer value of argument x passed to the current function is invalid. Write the idiomatic way to abort the function execution and signal the problem.
``  if (x > largest_value) error stop "Illegal value in function."``
90
Expose a read-only integer x to the outside world while being writable inside a structure or a class Foo.
``````module x
implicit none
type foo
integer, private :: x
contains
end type foo
contains
class(foo) :: f
end module x``````
93
Implement the procedure control which receives one parameter f, and runs f.
``````module x
implicit none
contains
subroutine control(f)
interface
subroutine f()
end subroutine f
end interface
call f
end subroutine control
end module x``````
95
Assign to variable x the length (number of bytes) of the local file at path.
``````program xx
implicit none
character(len=256) :: message
character(len=4096) :: filename
integer :: ios,x
logical :: foundit
filename='myfile'
inquire(file=filename,exist=foundit,size=x,iostat=ios,iomsg=message)
if(.not.foundit)then
elseif(ios.ne.0)then
write(*,*)trim(message)
else
write(*,*)'size =',x,'bytes'
endif
end program xx
``````
96
Set boolean b to true if string s starts with prefix prefix, false otherwise.
``````  logical :: b
b = index (string, prefix) == 1``````
97
Set boolean b to true if string s ends with string suffix, false otherwise.
``  b = (s(len(s)-len(suffix)+1:) == suffix)``
99
Assign to the string x the value of the fields (year, month, day) of the date d, in format YYYY-MM-DD.
``````program p
integer, dimension(8) :: d
character(len=10) :: x
call date_and_time (values=d)
write(x,'(i4.4,"-",i2.2,"-",i2.2)')d(1),d(2),d(3)
print *,'DATE=',x
end program p
``````
105
Assign to the string s the name of the currently executing program (but not its full path).
``````program p
implicit none
character(len=:),allocatable :: s
integer                      :: stat,l,i,j,k
call get_command_argument (0,length=l)
allocate(character(len=l) :: s)
call get_command_argument (0,s,status=stat)
if (stat == 0) then
i=index(s,'/',back=.true.)
j=index(s,'\',back=.true.)
k=max(i,j)
if(k.ne.0)s=s(k+1:)
print *, "The program's name is " // trim (s)
endif
end program p
``````
106
Assign to string dir the path of the working directory.
(This is not necessarily the folder containing the executable itself)
``````  interface
function c_getcwd (buf, size) bind(C,name="getcwd") result(r)
import
type(c_ptr) :: r
character(kind=c_char), dimension(*), intent(out) :: buf
integer(kind=c_size_t), value :: size
end function c_getcwd
end interface

if (c_associated(c_getcwd (buf, size(buf,kind=c_size_t)))) then
n = findloc(buf,achar(0),1)
allocate (character(len=n-1) :: dir)
dir(1:n-1) = transfer(buf(1:n-1),dir(1:n-1))
end if``````
108
Print the value of variable x, but only if x has been declared in this program.
This makes sense in some languages, not all of them. (Null values are not the point, rather the very existence of the variable.)
``````implicit none

print *,x``````
109
Set n to the number of bytes of a variable t (of type T).
``n = c_sizeof(i)``
110
Set the boolean blank to true if the string s is empty, or null, or contains only whitespace ; false otherwise.
``blank = s == ''``
111
From current process, run program x with command-line parameters "a", "b".
``````program p
integer :: i

call execute_command_line("x a b", exitstat=i)
print *, "Exit status of x was ", i

call execute_command_line("x a b", wait=.false.)
print *, "running x in the background"
end program p
``````
116
Remove all occurrences of string w from string s1, and store the result in s2.
``````  character(len=:), allocatable :: s1
character (len=:), allocatable :: s2

character(len=:), allocatable :: w
integer :: i, j, k, tc

allocate (s2, mold=s1)
i = 1
j = 1
do
k = index (s1(i:), w)
if (k == 0) then
s2(j:j+len(s1)-i) = s1(i:)
s2 = s2(:j+len(s1)-i)
exit
else
tc = k - 1
if (tc > 0) s2(j:j+tc-1) = s1(i:i+tc-1)
i = i + tc + len(w)
j = j + tc
end if
end do``````
117
Set n to the number of elements of the list x.
``n = size(x)``
120
Read an integer value from the standard input into the variable n
``````   integer :: n
122
Create an enumerated type Suit with 4 possible values SPADES, HEARTS, DIAMONDS, CLUBS.
``````enum, bind(c)
enumerator :: spades, hearts, diamonds, clubs
end enum``````
123
Verify that predicate isConsistent returns true, otherwise report assertion violation.
Explain if the assertion is executed even in production environment or not.
``if (.not. isconsistent) stop "Inconsistent state"``
125
measure the duration t, in nanoseconds, of a call to the function foo. Print this duration.
``````integer, parameter :: i8 = selected_int_kind(15)
integer (kind=i8) :: count, count_rate, count_2
x = 1.2
call system_clock (count, count_rate)
call foo
call system_clock (count_2)
if (count_rate == 10**9) then
print *,"The call to foo used ", (count_2-count), "Nanoseconds"
else
print *,"The call to foo used ", (count_2-count)/real(count_rate)*1e9, "Nanoseconds"
end if
``````
126
Write a function foo that returns a string and a boolean value.
``````subroutine foo(c, b)
character(len=:), allocatable, intent(out) :: c
logical, intent(out) :: b
c = 'string'
b = .false.
end subroutine foo``````
127
Import the source code for the function foo body from a file "foobody.txt".
``````function foo()
include "foobody.txt"
end function foo``````
131
Execute f1 if condition c1 is true, or else f2 if condition c2 is true, or else f3 if condition c3 is true.
Don't evaluate a condition when a previous condition was true.
``````if (c1) then
call f1
else if (c2) then
call f2
else if (c3) then
call f3
end if``````
132
Run the procedure f, and return the duration of the execution of f.
``````integer, parameter :: i8 = selected_int_kind(15)
integer(kind=i8) :: start, finish, rate

call system_clock (count_rate=rate)
call system_clock (count=start)
call f()
call system_clock (count=finish)
print *,(finish-start)/real(rate)``````
133
Set boolean ok to true if string word is contained in string s as a substring, even if the case doesn't match, or to false otherwise.
`````` function u_i(string, substr)
character (len=*), intent(in) :: string, substr
integer :: i,j, c1, c2, u_i
u_i = 0
out: do i=1,len(string)-len(substr)+1
c1 = iachar(string(i:i))
if (c1 >= iachar('a') .and. c1 <= iachar('z')) c1 = c1 - 32
do j=0,len(substr)-2
c2 = iachar(substr(j+1:j+1))
if (c2 >= iachar('a') .and. c1 <= iachar('z')) c2 = c2 - 32
if (c1 /= c2) cycle out
end do
u_i = i
return
end do out
end function u_i

ok = u_i(string, word) /= 0``````
134
Declare and initialize a new list items, containing 3 elements a, b, c.
``````integer, dimension(3) :: items
items = [a,b,c]``````
135
Remove at most 1 item from list items, having the value x.
This will alter the original list or return a new list, depending on which is more idiomatic.
If there are several occurrences of x in items, remove only one of them. If x is absent, keep items unchanged.
``````integer, dimension(:), allocatable :: items

i = findloc(items, x)
if (i /= 0) items = [items(1:i-1), items(i+1:)]``````
136
Remove all occurrences of the value x from list items.
This will alter the original list or return a new list, depending on which is more idiomatic.
``items = pack (items,items != x)``
137
Set the boolean b to true if the string s contains only characters in the range '0'..'9', false otherwise.
``````b = .true.
do i=1, len(s)
if (s(i:i) < '0' .or. s(i:i) > '9') then
b = .false.
exit
end if
end do``````
138
Create a new temporary file on the filesystem.
``open (newunit=u,status="scratch")``
141
Iterate in sequence over the elements of the list items1 then items2. For each iteration print the element.
``print *,items1,items2``
142
Assign to string s the hexadecimal representation (base 16) of integer x.

E.g. 999 -> "3e7"
``````write (*,'(Z8.8)') x
``````
143
Iterate alternatively over the elements of the lists items1 and items2. For each iteration, print the element.

Explain what happens if items1 and items2 have different size.
``````do i=1, min(size(items1), size(items2))
print *,items1(i), items2(i)
end do``````
144
Set boolean b to true if file at path fp exists on filesystem; false otherwise.

Beware that you should never do this and then in the next instruction assume the result is still valid, this is a race condition on any multitasking OS.
``inquire (file=fp,exist=b)``
145
Print message msg, prepended by current date and time.

Explain what behavior is idiomatic: to stdout or stderr, and what the date format is.
``````integer :: value(8)
msg = "asdf"
call date_and_time (values=value)
write (unit=*,fmt='(I4,"-",I2.2,"-",I2.2," ",I2.2,":",I2.2,".",I4.4,": ",A)') value(1:3), value(5:7), msg``````
146
Extract floating point value f from its string representation s
``read(s,'(g0)')f``
Alternative implementation:
``read (unit=s,fmt=*) f``
147
Create string t from string s, keeping only ASCII characters
``````n = 0
do i=1,len(s)
if (iachar(s(i:i)) <= 127) n = n + 1
end do
allocate (character(len=n) :: t)
j = 0
do i=1,len(s)
if (iachar(s(i:i)) <= 127) then
j = j + 1
t(j:j) = s(i:i)
end if
end do``````
148
Read a list of integer numbers from the standard input, until EOF.
``````integer :: I,j,k ,l(3)
``````
150
Remove the last character from the string p, if this character is a forward slash /
``````program x
character(len=:),allocatable :: string
integer :: ii
string='my string/'
ii=len(string)
string=trim(merge(string(:ii-1)//' ',string,string(ii:ii).eq.'/'))
write(*,*)string
end program x
``````
152
Create string s containing only the character c.
``````character(len=:),allocatable :: s
character(len=1) :: c
s=c``````
153
Create the string t as the concatenation of the string s and the integer i.
``write (unit=t,fmt='(A,I0)') s, i``
154
Find color c, the average between colors c1, c2.

c, c1, c2 are strings of hex color codes: 7 chars, beginning with a number sign # .
Assume linear computations, ignore gamma corrections.
``````character (len=7) :: c, c1, c2
integer, dimension(3) :: v, v1, v2

v = (v1 + v2)/2
write (unit=c,fmt='("#",3Z2)') v``````
155
Delete from filesystem the file having path filepath.
``````open (10,file=filepath,status="old", iostat=ierr)
if (ierr == 0) close (10,status="delete")
``````
156
Assign to the string s the value of the integer i in 3 decimal digits. Pad with zeros if i < 100. Keep all digits if i1000.
``write (unit=s,fmt='(I0.3)') i``
157
Initialize a constant planet with string value "Earth".
``  character(len=*), parameter :: planet = "Earth"``
Alternative implementation:
``character(*), parameter :: planet = "Earth"``
158
Create a new list y from randomly picking exactly k elements from list x.

It is assumed that x has at least k elements.
Each element must have same probability to be picked.
Each element from x must be picked at most once.
Explain if the original ordering is preserved or not.
``````allocate (sample(k))
do i=1,k
sample(i) = x(i)
end do
do i=k+1,n
call random_number(a)
j = 1 + int(i*a)
if (j .le. k) sample(j) = x(i)
end do``````
159
Define a Trie data structure, where entries have an associated value.
(Not all nodes are entries)
``````  type trie_p
type(trie), pointer :: p => NULL()
end type trie_p
type trie
class(*), allocatable :: value
type(trie_p), dimension(:), allocatable :: nodes
end type trie``````
160
Execute f32() if platform is 32-bit, or f64() if platform is 64-bit.
This can be either a compile-time condition (depending on target) or a runtime detection.
``````program main
use iso_c_binding
implicit none
type(c_ptr) :: x
logical, parameter :: is_64 = c_sizeof(x) == 8
if (is_64) then
call f64
else
call f32
end if
end program main``````
161
Multiply all the elements of the list elements by a constant c
``elements = elements * c``
162
execute bat if b is a program option and fox if f is a program option.
``````  do i=1, command_argument_count ()
call get_command_argument (i, length=length)
if (length > len(opt)) then
deallocate (opt)
allocate (character(length) :: opt)
end if
call get_command_argument (i, opt)
if (opt(1:1) /= '-') exit
do j=2, length
select case (opt(j:j))
case ('b')
print *,"bat"
case ('f')
print *,"fox"
end select
end do
end do``````
163
Print all the list elements, two by two, assuming list length is even.
``write (*,'(2I8,:," ")') list``
165
Assign to the variable x the last element of the list items.
``x = items(ubound(items,1))``
166
Create the list ab containing all the elements of the list a, followed by all the elements of the list b.
``````real, dimension(:), allocatable :: ab

ab = [a, b]``````
167
Create string t consisting of string s with its prefix p removed (if s starts with p).
``````  if (index(s,p) == 1) then
t = s(len(p)+1:)
else
t = s
end if``````
168
Create string t consisting of string s with its suffix w removed (if s ends with w).
``````  i = index(s,w,back=.true.) - 1
if (i == len(s) - len(w) ) then
allocate (t, source=s(:i))
else
allocate (t, source=s)
end if``````
169
Assign to the integer n the number of characters of the string s.
Make sure that multibyte characters are properly handled.
n can be different from the number of bytes of s.
``n = len(s)``
171
Append the element x to the list s.
``````real, allocatable, dimension(:) :: s

s = [s, x]``````
175
From array a of n bytes, build the equivalent hex string s of 2n digits.
Each byte (256 possible values) is encoded as two hexadecimal characters (16 possible values per digit).
``````  character(len=:), allocatable :: s
allocate (character(len=2*size(a)) :: s)
write(unit=s,fmt='(*(Z2.2))') a``````
176
From hex string s of 2n digits, build the equivalent array a of n bytes.
Each pair of hexadecimal characters (16 possible values per digit) is decoded into one byte (256 possible values).
``````  integer(kind=c_int8_t), dimension(:), allocatable :: a

allocate (a(len(s)/2))
``````
178
Set boolean b to true if if the point with coordinates (x,y) is inside the rectangle with coordinates (x1,y1,x2,y2) , or to false otherwise.
Describe if the edges are considered to be inside the rectangle.
``````logical :: b
b = (x1 < x) .and. (x < x2) .and. (y1 < y) .and. (y < y2)``````
179
Return the center c of the rectangle with coördinates(x1,y1,x2,y2)
``````real :: center(2)
center = [(x1 + x2)/2, (y1 + y2)/2]``````
182
Output the source of the program.
``! See original attribution URL``
186
Exit a program cleanly indicating no error to OS
``  STOP``
188
Perform matrix multiplication of a real matrix a with nx rows and ny columns, a real matrix b with ny rows and nz columns and assign the value to a real matrix c with nx rows and nz columns.
``c = matmul (a,b)``
189
Produce a new list y containing the result of the function T applied to all elements e of the list x that match the predicate P.
``y = pack (t(x), mask=p(x))``
190
Declare an external C function with the prototype

void foo(double *a, int n);

and call it, passing an array (or a list) of size 10 to a and 10 to n.

Use only standard features of your language.
``````module x
use iso_c_binding
interface
subroutine foo (a, n) bind(c)
import
real(kind=c_double), dimension(*) :: a
integer (kind=c_int), value :: n
end subroutine foo
end interface
contains
subroutine call_foo
real(kind=c_double) :: a(10)
call foo(a,10)
end subroutine call_foo
end module x``````
191
Given a one-dimensional array a, check if any value is larger than x, and execute the procedure f if that is the case
``if (any(a > x)) call f``
192
Declare a real variable a with at least 20 digits; if the type does not exist, issue an error at compile time.
``````  integer, parameter :: qp = selected_real_kind(20)
real(kind=qp) :: a
``````
193
Declare two two-dimensional arrays a and b of dimension n*m and m*n, respectively. Assign to b the transpose of a (i.e. the value with index interchange).
``````real :: a(n,m), b(m,n)

b = transpose(a)``````
194
Given an array a, set b to an array which has the values of a along its second dimension shifted by n. Elements shifted out should come back at the other end.
``b = cshift(a,n,dim=2)``
195
Pass an array a of real numbers to the procedure (resp. function) foo. Output the size of the array, and the sum of all its elements when each element is multiplied with the array indices i and j (assuming they start from one).
``````subroutine foo(a)
implicit none
real, dimension(:,:) :: a
real :: s
integer :: i,j
print *,size(a,1), size(a,2)
s = 0
do j=1,size(a,2)
do i=1,size(a,1)
s = s + a(i,j) * i * j
end do
end do
print *,s
end subroutine foo
!
call foo(a)``````
196
Given an integer array a of size n, pass the first, third, fifth and seventh, ... up to the m th element to a routine foo which sets all these elements to 42.
``````! Caller:
integer, dimension(n) :: a
call foo(a(1:m:2))

! Callee:

subroutine foo(a)
integer, dimension(:), intent(inout) :: a
a = 42
end subroutine foo``````
198
Abort program execution with error condition x (where x is an integer value)
``STOP 1``
200
Returns the hypotenuse h of the triangle where the sides adjacent to the square angle have lengths x and y.
``h = hypot(x,y)``
201
Calculate n, the Euclidean norm of data (an array or list of floating point values).
``n = norm2( data )``
202
Calculate the sum of squares s of data, an array of floating point values.
``s = sum( data**2 )``
203
Calculate the mean m and the standard deviation s of the list of floating point values data.
``````real, allocatable :: data(:)
real :: m, s
...
m = sum( data ) / size( data )
s = sqrt( sum( data**2 ) / size( data ) - m**2 )``````
204
Given a real number a, print the fractional part and the exponent of the internal representation of that number. For 3.14, this should print (approximately)

0.785 2
``````a = 3.14
print *,fraction(a), exponent(a)
``````
205
Read an environment variable with the name "FOO" and assign it to the string variable foo. If it does not exist or if the system does not support environment variables, assign a value of "none".
``````  call get_environment_variable ("FOO", length=n, status=st)
if (st /= 0) then
foo = "none"
else
allocate (character(len=n) :: foo)
call get_environment_variable ("FOO", value=foo)
end if ``````
206
Execute different procedures foo, bar, baz and barfl if the string str contains the name of the respective procedure. Do it in a way natural to the language.
``````  select case (str)
case ("foo")
call foo
case ("bar")
call bar
case ("baz")
call baz
case ("barfl")
call barfl
end select``````
207
Allocate a list a containing n elements (n assumed to be too large for a stack) that is automatically deallocated when the program exits the scope it is declared in.
``````  integer, dimension(:), allocatable :: a
allocate (a(n))``````
208
Given the arrays a,b,c,d of equal length and the scalar e, calculate a = e*(a+b*c+cos(d)).
Store the results in a.
``a = e*(a+b*c+cos(d))``
209
Declare a type t which contains a string s and an integer array n with variable size, and allocate a variable v of type t. Allocate v.s and v.n and set them to the values "Hello, world!" for s and [1,4,9,16,25], respectively. Deallocate v, automatically deallocating v.s and v.n (no memory leaks).
``````  type t
character(len=:), allocatable :: c
integer, dimension(:), allocatable :: n
end type t
type(t), allocatable :: v

allocate (v)
v%s = 'Hello, world!'
v%n = [1,4,9,16,25]

deallocate (v)``````
210
Assign, at runtime, the compiler version and the options the program was compilerd with to variables version and options, respectively, and print them. For interpreted languages, substitute the version of the interpreter.

Example output:

GCC version 10.0.0 20190914 (experimental)
-mtune=generic -march=x86-64
``````  character(len=:), allocatable :: version, options
version = compiler_version()
options = compiler_options()
print *,version
print *,options
``````
214
Append extra character c at the end of string s to make sure its length is at least m.
The length is the number of characters, not the number of bytes.
``````CHARACTER(N) ::  FOO
``````
215
Prepend extra character c at the beginning of string s to make sure its length is at least m.
The length is the number of characters, not the number of bytes.
``repeat(x,max(n-len(s),0)) // s``
216
Add the extra character c at the beginning and ending of string s to make sure its length is at least m.
After the padding the original content of s should be at the center of the result.
The length is the number of characters, not the number of bytes.

E.g. with s="abcd", m=10 and c="X" the result should be "XXXabcdXXX".
``````  i = (m-len(s))/2
j = (m - len(s)) - (m-len(s))/2
s = repeat(c,i) // s // repeat(c,j)``````
222
Set i to the first index in list items at which the element x can be found, or -1 if items does not contain x.
``````i = findloc(items, x)
if (i == 0) i = -1``````
223
Loop through list items checking a condition. Do something else if no matches are found.

A typical use case is looping through a series of containers looking for one that matches a condition. If found, an item is inserted; otherwise, a new container is created.

These are mostly used as an inner nested loop, and in a location where refactoring inner logic into a separate function reduces clarity.
``````  found = .false.
do i=1,size(items)
if (items(i) == "asdf") then
found = .true.
exit
end if
end do
if (.not. found) call do_something
``````
224
Insert the element x at the beginning of the list items.
``items = [x, items]``
225
Declare an optional integer argument x to procedure f, printing out "Present" and its value if it is present, "Not present" otherwise
``````subroutine f(x)
integer, optional :: x
if (present(x)) then
print *,"Present", x
else
print *,"Not present"
end if
end subroutine f
``````
226
Remove the last element from list items.
``items = items(1:size(items)-1)``
227
Create the new list y containing the same elements as the list x.

Subsequent modifications of y must not affect x (except for the contents referenced by the elements themselves if they contain pointers).
``````type (foo), allocatable, dimension(:) :: y
y = x``````
228
Copy the file at path src to dst.
``````character, dimension(n_buff) :: buffer
access="stream")
open(newunit=u_w,file="dst", action="write", form="unformatted",&
access="stream")
inquire(unit=u_r,size=sz)
do while (sz > 0)
n_chunk = min(sz, n_buff)
write (unit=u_w) buffer(1:n_chunk)
sz = sz - n_chunk
end do``````
237
Assign to c the result of (a xor b)
``c = ieor(a,b)``
238
Write in a new byte array c the xor result of byte arrays a and b.

a and b have the same size.
``````integer(kind=int8), dimension(:) :: a, b, c
! Assign values to a and b
c = ieor(a,b)``````
243
Print the contents of the list or array a on the standard output.
``write (*,*) a``
247
Remove all the elements from list x that don't satisfy the predicate p, without allocating a new list.
Keep all the elements that do satisfy p.

For languages that don't have mutable lists, refer to idiom #57 instead.
``x = pack(x,.not.p())``
248
Construct the "double precision" (64-bit) floating point number d from the mantissa m, the exponent e and the sign flag s (true means the sign is negative).
``````  if (s) then
else
end if

``````
249
Define variables a, b and c in a concise way.
Explain if they need to have the same type.
``integer :: a, b, c``
251
Extract integer value i from its binary string representation s (in radix 2)
E.g. "1101" -> 13
``````character (len=:), allocatable :: s
integer :: i
s = '1101'

``````
252
Assign to the variable x the value "a" if calling the function condition returns true, or the value "b" otherwise.
``````if (condition())
x = a
else
x = b
end if``````
254
Replace all exact occurrences of "foo" with "bar" in the string list x
``````  where (x == "foo")
x = "bar"
endwhere``````
256
Print the numbers 5, 4, ..., 0 (included), one line per number.
``````do i=5,0,-1
print *,i
end do``````
257
Print each index i and value x from the list items, from the last down to the first.
``````do i=size(items),1,-1
print *,i,items(i)
end do``````
258
Convert the string values from list a into a list of integers b.
``````  allocate (b(size(a)))
``````
260
Declare a new list items of string elements, containing zero elements
``````character(len=:), allocatable, dimension(:) :: items
allocate (items(0),source="")
``````
261
Assign to the string x the value of fields (hours, minutes, seconds) of the date d, in format HH:MM:SS.
``````integer, dimension(8) :: d
character (len=8) :: x
call date_and_time (values=d)
write (unit=x,fmt='(I2.2,":",I2.2,":",I2.2)') d(5), d(6), d(7)``````
262
Assign to t the number of trailing 0 bits in the binary representation of the integer n.

E.g. for n=112, n is 1110000 in base 2 ⇒ t=4
``t = trailz(n)``
263
Write two functions log2d and log2u, which calculate the binary logarithm of their argument n rounded down and up, respectively. n is assumed to be positive. Print the result of these functions for numbers from 1 to 12.
``````program main
implicit none
integer :: i
do i=1,12
print *,i,log2d(i),log2u(i)
end do
contains
integer function log2d (n)
integer, intent(in) :: n
log2d = bit_size(n) - 1 - leadz(n)
end function log2d

integer function log2u (n)
integer, intent(in) :: n
end function log2u
end program main``````
264
Pass a two-dimensional integer array a to a procedure foo and print the size of the array in each dimension. Do not pass the bounds manually. Call the procedure with a two-dimensional array.
``````module x
contains
subroutine foo(a)
integer, dimension(:,:) :: a
print *,size(a,1), size(a,2)
end subroutine foo
end module x

program main
use x
integer, dimension(5,10) :: a
call foo(a)
end program main``````
265
Calculate the parity p of the integer variable i : 0 if it contains an even number of bits set, 1 if it contains an odd number of bits set.
``print *,poppar(i)``
266
Assign to the string s the value of the string v repeated n times, and write it out.

E.g. v="abc", n=5 ⇒ s="abcabcabcabcabc"
``````character(len=:), allocatable :: s,v
integer :: n
s = repeat(v,20)
write (*,'(A)') s
``````
267
Declare an argument x to a procedure foo that can be of any type. If the type of the argument is a string, print it, otherwise print "Nothing."

Test by passing "Hello, world!" and 42 to the procedure.
``````program main
call foo("Hello, world!")
call foo(42)
contains
subroutine foo(x)
class(*), intent(in) :: x
select type(x)
type is (character(len=*))
write (*,'(A)') x
class default
write (*,'(A)') "Nothing."
end select
end subroutine foo
end program main``````
268
Define a type vector containing three floating point numbers x, y, and z. Write a user-defined operator x that calculates the cross product of two vectors a and b.
``````module vect
private
type, public:: vector
real :: x,y,z
end type vector
public:: operator(.x.)
interface operator(.x.)
procedure vector_cross
end interface operator(.x.)
contains
function vector_cross(a,b) result(c)
type(vector), intent(in) :: a,b
type(vector) :: c
c%x = a%y*b%z - a%z*b%y
c%y = a%z*b%x - a%x*b%z
c%z = a%x*b%y - a%y*b%x
end function vector_cross
end module vect
``````
270
Given a floating point number r1 classify it as follows:
If it is a signaling NaN, print "This is a signaling NaN."
If it is a quiet NaN, print "This s a quiet NaN."
If it is not a NaN, print "This is a number."
``````  character (len=:), allocatable :: msg
if (ieee_support_nan(r1)) then
if (ieee_class(r1) == ieee_quiet_nan) then
msg = "This s a quiet NaN."
else if (ieee_class(r1) == ieee_signaling_nan) then
msg = "This is a signaling NaN."
else
msg = "This is a number."
end if
else
msg = "NaNs are not supported."
end if
write (*,'(A)') msg``````
271
If a variable x passed to procedure tst is of type foo, print "Same type." If it is of a type that extends foo, print "Extends type." If it is neither, print "Not related."
``````  subroutine tst (x)
class (*), intent(in) :: x
type (foo) :: y
if (same_type_as (x,y)) then
write (*,'(A)') "Same type."
else if (extends_type_of (x,y)) then
write (*,'(A)') "Extends type."
else
write (*,'(A)') "Not related."
end if
end subroutine tst``````
272
Fizz buzz is a children's counting game, and a trivial programming task used to affirm that a programmer knows the basics of a language: loops, conditions and I/O.

The typical fizz buzz game is to count from 1 to 100, saying each number in turn. When the number is divisible by 3, instead say "Fizz". When the number is divisible by 5, instead say "Buzz". When the number is divisible by both 3 and 5, say "FizzBuzz"
``````  integer :: i
logical :: fizz, buzz
do i=1,100
fizz = mod(i,3) == 0
buzz = mod(i,5) == 0
if (.not. fizz .and. .not. buzz) write (*,'(I0)',advance="no") i
end do
write (*,'()')
end program``````
275
From the string s consisting of 8n binary digit characters ('0' or '1'), build the equivalent array a of n bytes.
Each chunk of 8 binary digits (2 possible values per digit) is decoded into one byte (256 possible values).
``````  subroutine to_s (s, a)
use iso_fortran_env, only: int8
character (len=*), intent(in) :: s
integer (kind=int8), allocatable, intent(out), dimension(:) :: a
allocate (a(len(s)/8))
end subroutine to_s``````
277
Remove the element e from the set x.

Explains what happens if e was already absent from x.
``````integer,allocatable::x(:)
integer::e=6
x=[2,4,6,8,10]
!remove 6
x=pack(x,x/=e)``````
280
Remove all the elements from the map m that don't satisfy the predicate p.
Keep all the elements that do satisfy p.

Explain if the filtering happens in-place, i.e. if m is reused or if a new map is created.
``````  real, dimension(:), allocatable :: a
a = pack(a,p(a))
!
elemental logical function p(x)
real, intent(in) :: x
p = x > 0.7
end function p

``````
284
Create a new list a (or array, or slice) of size n, where all elements are integers initialized with the value 0.
``````integer, dimension(:), allocatable :: a

allocate(a(n),source = 0)``````
285
Given two floating point variables a and b, set a to a to a quiet NaN and b to a signalling NaN. Use standard features of the language only, without invoking undefined behavior.
`````` a = ieee_value (a, IEEE_QUIET_NAN)
b = ieee_value (b, IEEE_SIGNALING_NAN)``````
286
Print a line "Char i is c" for each character c of the string s, where i is the character index of c in s (not the byte index).

Make sure that multi-byte characters are properly handled, and count for a single character.
``````  integer, parameter :: ucs4  = selected_char_kind ('ISO_10646')
character(kind=ucs4,  len=30) :: s
open (output_unit, encoding='UTF-8')

do i=1,len(s)
print *,"Char ", i, " is ", s(i:i)
end do
``````
288
Set the boolean b to true if the set x contains the element e, false otherwise.
``b = x(e)``
289
Create the string s by concatenating the strings a and b.
``s = a // b``
291
Delete all the elements from index i (included) to index j (excluded) from the list items.
``items = [items(:,i-1), items(j:)]``
292
Write "Hello World and 你好" to standard output in UTF-8.
``````  integer, parameter :: ucs4  = selected_char_kind ('ISO_10646')
character(kind=ucs4,  len=30) :: hello_world
hello_world = ucs4_'Hello World and ' &
// char (int (z'4F60'), ucs4)     &
// char (int (z'597D'), ucs4)

open (output_unit, encoding='UTF-8')
write (*,*) trim (hello_world)``````
294
Given an array a containing the three values 1, 12, 42, print out
"1, 12, 42" with a comma and a space after each integer except the last one.
``````  integer, dimension(:), allocatable :: a
a = [1,12,42]
write (*,'(*(I0:", "))') a``````
296
Assign to x2 the value of string x with the last occurrence of y replaced by z.
If y is not contained in x, then x2 has the same value as x.
``````  character(len=:), allocatable :: x, x2, y, z

k = index(x,y,back=.true.)
if (k > 0) then
x2 = x(1:k-1) // z // x(k+len(y):)
else
x2 = x
end if
``````
299

This line will not be compiled or executed.
``! This is a comment.``
301
Compute the Fibonacci sequence of n numbers using recursion.

Note that naive recursion is extremely inefficient for this task.
``````  recursive function f (n) result(res)
integer, value, intent(in) :: n
integer :: res
if (n == 0) then
res = 0
else if (n <= 2) then
res = 1
else
if (mod(n,2) == 1) then
res = f((n + 1)/2)**2 + f((n - 1)/2)**2
else
res = f(n/2) * (f(n/2-1) + f(n/2 + 1))
end if
end if
end function f``````
302
Given the integer x = 8, assign to the string s the value "Our sun has 8 planets", where the number 8 was evaluated from x.
``  write (unit=s,fmt='(*(G0))') "Our sun has ",x," planets."``
303
Declare an array a of integers with six elements, where the first index is 42 and consecutive elements have the indices 43, 44, 45, 46, 47.
``  integer, dimension(42:47) :: a``
305
Compute and print a^b, and a^n, where a and b are floating point numbers and n is an integer.
``````  print *,a**b, a**n
``````
309
Create the new 2-dimensional array y containing a copy of the elements of the 2-dimensional array x.

x and y must not share memory. Subsequent modifications of y must not affect x.
``````integer, allocatable, dimension(:,:) :: y

y = x``````