The snippets are under the CC-BY-SA license.

# Programming-Idioms.org

• The snippets are under the CC-BY-SA license.
• Please consider keeping a bookmark
Lua
1
Print a literal string on standard output
``print("Hello World")``
Alternative implementation:
``io.write("Hello world")``
2
Loop to execute some code a constant number of times
``````for i=1, 10 do
print('Hello')
end``````
Alternative implementation:
``print(("Hello\n"):rep(10))``
Alternative implementation:
``print(string.rep("Hello\n", 10))``
Alternative implementation:
``````local i = 0
repeat
i = i + 1
print("Hello")
until i == 10``````
3
Like a function which doesn't return any value, thus has only side effects (e.g. Print to standard output)
``````function finish(name)
print('My job here is done. Goodbye, '..name..'.')
end``````
4
Create a function which returns the square of an integer
``````function square(x)
return x*x
end``````
5
Declare a container type for two floating-point numbers x and y
``point = { x = 123, y = 456 }``
6
Do something with each item x of the list (or array) items, regardless indexes.
``````for _, x in ipairs(items) do
print(x)
end``````
7
Print each index i with its value x from an array-like collection items
``````for i, x in ipairs(items) do
print('Index: '..i..', Value: '..x)
end``````
8
Create a new map object x, and provide some (key, value) pairs as initial content.
``x = {one = 1, two = 2}``
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.
``````local function BinTree(v, l, r)
return {
val = v,
left = l,
right = r
}
end``````
10
Generate a random permutation of the elements of list x
``````shuffled = {}
for i, v in ipairs(x) do
local pos = math.random(1, #shuffled+1)
table.insert(shuffled, pos, v)
end``````
Alternative implementation:
``````function(x)
for i = #x, 2, -1 do
local j = math.random(i)
x[i], x[j] = x[j], x[i]
end
end
``````
11
The list x must be non-empty.
``x[math.random(#x)]``
12
Check if the list contains the value x.
list is an iterable finite container.
``````function contains(list, x)
for _, v in ipairs(list) do
if v == x then return true end
end
return false
end``````
13
Access each key k with its value x from an associative array mymap, and print them.
``````for k, x in pairs(mymap) do
print('Key: '..k..', Value: '..x)
end``````
14
Pick a random number greater than or equals to a, strictly inferior to b. Precondition : a < b.
``````function pick(a,b)
return a + (math.random()*(b-a))
end
``````
15
Pick a random integer greater than or equals to a, inferior or equals to b. Precondition : a < b.
``math.random(a, b)``
16
Call a function f on every node of binary tree bt, in depth-first infix order
``````local function dfs(bt, f, ...)
local tt = type(bt)
if tt~='table' and tt~='userdata' then return end
dfs(t.left, f, ...)
f(t.data, ...)
dfs(t.right, f, ...)
end``````
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.
``````local function Tree(v, c)
return {
val = v,
children = c
}
end``````
19
Reverse the order of the elements of the list x.
This may reverse "in-place" and destroy the original ordering.
``````rev = {}
for i=#x, 1, -1 do
rev[#rev+1] = x[i]
end
x = rev``````
Alternative implementation:
``````rev = {}
for i=#x, 1, -1 do
rev[#rev+1] = x[i]
end

-- in-situ reversal
function reverse(t)
local n = #t
local i = 1
for i = 1, n do
t[i],t[n] = t[n],t[i]

n = n - 1
end
end``````
Alternative implementation:
``````function array_reverse(x)
local n, m = #x, #x/2
for i=1, m do
x[i], x[n-i+1] = x[n-i+1], x[i]
end
return x
end
``````
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)
for i,v1 in ipairs(m) do
for j,v2 in ipairs(v1) do
if v2 == x then return i,j end
end
end
end``````
21
Swap the values of the variables a and b
``a, b = b, a``
22
Extract the integer value i from its string representation s (in radix 10)
``i = tonumber(s)``
23
Given a real number x, create its string representation s with 2 decimal digits following the dot.
``s = string.format("%.2f",x)``
24
Declare a new string s and initialize it with the literal value "ネコ" (which means "cat" in japanese)
``s = 'ネコ'``
26
Declare and initialize a matrix x having m rows and n columns, containing real numbers.
``````local x = setmetatable({},{
__index = function(t1,k1)
t1[k1] = setmetatable({},{
__index = function(t2,k2)
t2[k2] = 0
return t2[k2]
end
})
return t1[k1]
end
})``````
Alternative implementation:
``````x = {}
for i=1,m do
x[i] = {}
for j=1,n do
x[i][j] = 0
end
end``````
27
Declare and initialize a 3D array x, having dimensions boundaries m, n, p, and containing real numbers.
``````local x = {}
for i=1,m do
x[i] = {}
for j=1,n do
x[i][j] = {}
for k=1,p do
x[i][j][k] = 0
end
end
end``````
28
Sort the elements of the list (or array-like collection) items in ascending order of x.p, where p is a field of the type Item of the objects in items.
``````table.sort(items, function(a,b)
if a.p < b.p then return true end
end)``````
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.
``local removedItem = table.remove(items,i)``
31
Create the recursive function f which returns the factorial of the non-negative integer i, calculated from f(i-1)
``````function f(n)
local function f_(factorial,n)
if n < 2 then return 1
return f_(factorial*n, n-1)
end
return f_(1,n)
end``````
32
Create function exp which calculates (fast) the value x power n.
x and n are non-negative integers.
``````function exp(x, n)
return x^n
end``````
35
Implement a function compose (A -> C) with parameters f (A -> B) and g (B -> C), which returns the composition function g ∘ f
``````function compose(f,g)
return function(x)
return g(f(x))
end
end``````
36
Implement a function compose which returns composition function g ∘ f for any functions f and g having exactly 1 parameter.
``````function compose(f,g)
return function(x)
return g(f(x))
end
end``````
37
Transform a function that takes multiple arguments into a function for which some of the arguments are preset.
``````function curry2(f)
return function(a)
return function(b)
return f(a,b)
end
end
end

return a + b
end

Alternative implementation:
``````local unpack = unpack or table.unpack
local function aux_ncurry(n, m, fn, args)
if m>n then return fn(unpack(args,1,n))end
local new_args, new_m = {}, m
for i=1,m-1 do new_args[i]=args[i]end
return function(a)
new_args[new_m]=a
return aux_ncurry(n, new_m + 1, fn, new_args)
end
end
local function ncurry(n, fn) return aux_ncurry(n,1,fn,{}) end``````
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.
``local t = s:sub(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 = s:find(word, 1, true)``
Alternative implementation:
``````ok = false
if s:find(word, 1, true) then ok = true end``````
Alternative implementation:
``ok = s:find(word, 1, true) ~= nil``
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.
``````function utf8.reverse(s)
local r = ""
for p,c in utf8.codes(s) do
r = utf8.char(c)..r
end
return r
end

t = utf8.reverse(s)``````
43
Look for a negative value v in 2D integer matrix m. Print it and stop searching.
``````breakout = false
for i,v1 in ipairs(m) do
for j,v2 in ipairs(v1) do
if v2 < 0 then
print(v2)
state = "found"
breakout = true
break
end
end
if breakout then break end
end``````
Alternative implementation:
``````for _,v1 in ipairs(m) do
for _,v2 in ipairs(v1) do
if v2<0 then
print(v2)
goto outer
end
end
end
::outer::``````
44
Insert the element x at position i in the list s. Further elements must be shifted to the right.
``table.insert(s,i,x)``
45
Sleep for 5 seconds in current thread, before proceeding with the next instructions.
``````local start = os.clock()
while os.clock() - start < 5 do end``````
Alternative implementation:
``os.execute(package.config:sub(1,1) == "/" and "sleep 5" or "timeout 5")``
46
Create the string t consisting of the 5 first characters of the string s.
Make sure that multibyte characters are properly handled.
``t = s:sub(1,5)``
48
Assign to variable s a string literal consisting in several lines of text, including newlines.
``````s = [[
Huey
Dewey
Louie
]]
``````
Alternative implementation:
``````s = 'Huey \n'..
'Dewey \n'..
'Louie'``````
49
Build list chunks consisting in substrings of the string s, separated by one or more space characters.
``````chunks = {}
for substring in s:gmatch("%S+") do
table.insert(chunks, substring)
end``````
50
Write a loop that has no end clause.
``````while true do
-- Do something
end``````
Alternative implementation:
``````::loop::
goto loop``````
Alternative implementation:
``````repeat
until false``````
51
Determine whether the map m contains an entry for the key k
``m[k] ~= nil``
53
Concatenate elements of string list x joined by the separator ", " to create a single string y.
``y = table.concat(x, ", ")``
54
Calculate the sum s of the integer list or array x.
``````s = 0
for i,v in ipairs(x) do
s = s + v
end``````
55
Create the string representation s (in radix 10) of the integer value i.
``s = tostring(i)``
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 = {}
for _, v in ipairs(x) do
if p(v) then y[#y+1] = v end
end``````
Alternative implementation:
``````local y={}
for _,v in ipairs(x) do
if p(v)==true then
table.insert(y,v)
end
end``````
58
Create the string lines from the content of the file with filename f.
``lines = io.input(f):read('a')``
59
Print the message "x is negative" to standard error (stderr), with integer x value substitution (e.g. "-2 is negative").
``io.stderr:write(string.format("%d is negative\n",x))``
60
Assign to x the string value of the first command line parameter, after the program name.
``x = arg[1]``
61
Assign to the variable d the current date/time value, in the most standard type.
``d = os.date()``
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 = x:find(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.
``x2 = x:gsub(y,z)``
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%"
``s = string.format("%.1f%%", x*100)``
Alternative implementation:
``local s=(x-x%10^-3)*100 .."%"``
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.
``math.randomseed(s)``
70
Get the current datetime and provide it as a seed to a random generator. The generator sequence will be different at each run.
``math.randomseed( os.time() )``
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.
``print( table.concat( arg, " " ) )``
74
Compute the greatest common divisor x of big integers a and b. Use an integer type able to handle huge numbers.
``````function gcd(x, y)
if (y == 0) then
return x
else
return gcd(y, x%y)
end
end``````
Alternative implementation:
``````function gcd(a, b)
return b==0 and a or gcd(b,a%b)
end``````
76
Create the string s of integer x written in base 2.

E.g. 13 -> "1101"
``````local s = {}

while x > 0 do
local tmp = math.fmod(x,2)
s[#s+1] = tmp
x=(x-tmp)/2
end

s=table.concat(s)``````
78
Execute a block once, then execute it again as long as boolean condition c is true.
``````repeat
someThing()
someOtherThing()
until not c``````
79
Declare the floating point number y and initialize it with the value of the integer x .
``y = x + .0``
Alternative implementation:
``y = (16 ^ 13) + x - (16 ^ 13)``
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).
``y = math.modf(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).
``````function round(float)
local int, part = math.modf(float)
if float == math.abs(float) and part >= .5 then return int+1    -- positive float
elseif part <= -.5 then return int-1                            -- negative float
end
return int
end``````
Alternative implementation:
``````function round(float)
return math.floor(float + .5)
end``````
83
Declare regular expression r matching strings "http", "htttp", "httttp", etc.
``r = "htt+p"``
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.
``````local function intadding_will_overflow(x,y)
return x>0
and y>0 and x+y<x
or y<0 and x+y>x
end``````
87
Exit immediately.
If some extra cleanup work is executed by the program runtime (not by the OS itself), describe it.
``os.exit()``
Alternative implementation:
``return``
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.
``return nil, "Invalid argument x"``
Alternative implementation:
``error("Invalid argument x")``
90
Expose a read-only integer x to the outside world while being writable inside a structure or a class Foo.
``````local Foo = {}
do
local x = 0
Foo.getX = function()
return x
end
end
print(Foo.getX()) -- 0
print(x) -- nil``````
Alternative implementation:
``````local function Foo()
local private = {x = 9}
local mt = {
__index = private,
__newindex = function (t, k, v)
error("attempt to update a read-only table", 2)
end
}
return setmetatable({}, mt)
end

local foo = Foo()
print(foo.x) -- 9
foo.x = 3    -- error: attempt to update a read-only table``````
91
Read from the file data.json and write its content into the object x.
Assume the JSON data is suitable for the type of x.
``````file = io.open("data.json")
file:close()
``````
92
Write the contents of the object x into the file data.json.
``````file = io.open("data.json", "w")
file:write(cjson.encode(x))
file:close()``````
93
Implement the procedure control which receives one parameter f, and runs f.
``````function control(f)
f()
end``````
94
Print the name of the type of x. Explain if it is a static type or dynamic type.

This may not make sense in all languages.
``print(type(x))``
96
Set the boolean b to true if string s starts with prefix prefix, false otherwise.
``b = s:find(prefix, 1, true) == 1``
Alternative implementation:
``````function startswith(text, prefix)
return text:find(prefix, 1, true) == 1
end

b = startswith(s, prefix)``````
Alternative implementation:
``b = string.sub(s, 1, #prefix) == prefix``
97
Set boolean b to true if string s ends with string suffix, false otherwise.
``b = s:sub(-string.len(suffix)) == suffix``
99
Assign to the string x the value of the fields (year, month, day) of the date d, in format YYYY-MM-DD.
``x = os.date("%F",d)``
100
Sort elements of array-like collection items, using a comparator c.
``table.sort(items,c)``
105
Assign to the string s the name of the currently executing program (but not its full path).
``s = arg[0]``
106
Assign to string dir the path of the working directory.
(This is not necessarily the folder containing the executable itself)
``dir = os.getenv("PWD") or io.popen("cd"):read()``
107
Assign to string dir the path of the folder containing the currently running executable.
(This is not necessarily the working directory, though.)
``dir = arg[1]``
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.)
``if x then print(x) end``
110
Set the boolean blank to true if the string s is empty, or null, or contains only whitespace ; false otherwise.
`` blank = (s == nil or #string.gsub(s, "^%s*(.-)%s*\$", "%1") == 0)``
111
From current process, run program x with command-line parameters "a", "b".
``os.execute(x .. 'a b')``
112
Print each key k with its value x from an associative array mymap, in ascending order of k.
``````for k, x in pairs(mymap) do
print(k, x)
end``````
116
Remove all occurrences of string w from string s1, and store the result in s2.
``s2 = s1:gsub(w,"")``
117
Set n to the number of elements of the list x.
``local n = #x``
118
Create the set y from the list x.
x may contain duplicates. y is unordered and has no repeated values.
``````local hash = {}
local y = {}
for _,v in ipairs(x) do
if (not hash[v]) then
y[#y+1] = v
hash[v] = true
end
end``````
119
Remove duplicates from the list x.
Explain if the original order is preserved.
``````local seen = {}
for index,item in ipairs(x) do
if seen[item] then
table.remove(x, index)
else
seen[item] = true
end
end``````
120
Read an integer value from the standard input into the variable n
``n = io.read("n")``
122
Create an enumerated type Suit with 4 possible values SPADES, HEARTS, DIAMONDS, CLUBS.
``````Suit = {
HEARTS=2,
DIAMONDS=3,
CLUBS=4
}``````
123
Verify that predicate isConsistent returns true, otherwise report assertion violation.
Explain if the assertion is executed even in production environment or not.
``assert(isConsistent() , "Assertion violation") ``
126
Write a function foo that returns a string and a boolean value.
``````function foo()
return "bar", false
end``````
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 f1()
elseif (c2) then f2()
elseif (c3) then f3()
end``````
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.
``ok = string.find(string.lower(s), string.lower(word)) and true or false``
Alternative implementation:
``local ok=string.find(string.lower(s),string.lower(word),nil,true)~=nil``
134
Declare and initialize a new list items, containing 3 elements a, b, c.
``items = {a, b, c}``
137
Set the boolean b to true if the string s contains only characters in the range '0'..'9', false otherwise.
``b = tonumber(s) ~= nil``
138
Create a new temporary file on the filesystem.
``file = io.tmpfile()``
140
Delete from map m the entry having key k.

Explain what happens if k is not an existing key in m.
``m[k]=nil``
142
Assign to string s the hexadecimal representation (base 16) of integer x.

E.g. 999 -> "3e7"
``s = string.format("%x",x)``
150
Remove the last character from the string p, if this character is a forward slash /
``p = string.gsub(p,"/\$","")``
151
Remove last character from string p, if this character is the file path separator of current platform.

Note that this also transforms unix root path "/" into the empty string!
``````if string.sub(p, -1, -1) == "/" then
p=string.sub(p, 1, -2)
end``````
152
Create string s containing only the character c.
``s = c``
153
Create the string t as the concatenation of the string s and the integer i.
``t = 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.
``````local c = "#"
for i=2,#c1,2 do
local avg = (tonumber(c1:sub(i,i+1), 16) + tonumber(c2:sub(i,i+1), 16)) / 2
c = c .. string.format("%02x", math.floor(avg))
end
``````
155
Delete from filesystem the file having path filepath.
``os.remove(filepath)``
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.
``string.format("%03d", i)``
157
Initialize a constant planet with string value "Earth".
``PLANET = "Earth"``
159
Define a Trie data structure, where entries have an associated value.
(Not all nodes are entries)
``````local function New(base,new)
return setmetatable(new,{__index=base})
end
local Trie={}
function Trie.New()
return New(Trie,{
children={},
value=nil
})
end``````
161
Multiply all the elements of the list elements by a constant c
``````local function map(t,fn)
for i,v in ipairs(t) do
t[i]=fn(v)
end
return t
end

local ret=map(elements,function(x) return x*c end)``````
165
Assign to the variable x the last element of the list items.
``local x = items[#items]``
166
Create the list ab containing all the elements of the list a, followed by all the elements of the list b.
``````ab = {}

table.foreach(a, function(k, v) table.insert(ab, v) end)
table.foreach(b, function(k, v) table.insert(ab, v) end)``````
Alternative implementation:
``````ab = {}

table.move(a, 1, #a, 1, ab)
table.move(b, 1, #b, #ab + 1, ab)``````
167
Create the string t consisting of the string s with its prefix p removed (if s starts with p).
``local t = (s:sub(0, #p) == p) and s:sub(#p+1) or s``
168
Create string t consisting of string s with its suffix w removed (if s ends with w).
``local t = s:gsub(w.."\$", "")``
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.
``local n = utf8.len(s)``
Alternative implementation:
``````local utf8={}
function utf8.bytes(str,index)
local byte=string.byte(str,index)
local ret
if     byte==nil then ret=0
elseif byte<128  then ret=1
elseif byte<192  then ret=2
elseif byte<224  then ret=3
else                  ret=4
end
return ret
end
function utf8.len(str)
local count=0
local fini=#str+1
local index=1
while index~=fini do
count=count+1
index=index+utf8.bytes(str,index)
end
return count
end
local n=utf8.len(s)``````
170
Set n to the number of elements stored in mymap.

This is not always equal to the map capacity.
``````local n=0
for _ in pairs(mymap) do n=n+1 end``````
171
Append the element x to the list s.
``s[#s + 1] = x``
Alternative implementation:
``table.insert(s, x)``
175
From the 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).
``````for i,v in ipairs(a) do
a[i]=string.format("%02x",v)
end
local s=table.concat(a)``````
182
Output the source of the program.
``````x = [["x = [" .. "[" .. x .. "]" .. "]\nprint(" .. x)]]
print("x = [" .. "[" .. x .. "]" .. "]\nprint(" .. x)``````
186
Exit a program cleanly indicating no error to OS
``os.exit(0)``
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).
``````local a = {}

for x = 1, n do
local t = {}
for y = 1, m do
t[y] = {x, y}
end
a[x] = t
end

local b = {}

for y = 1, m do
local t = {}
for x = 1, n do
t[x] = a[x][y]
end
b[y] = t
end
``````
198
Abort program execution with error condition x (where x is an integer value)
``os.exit(x)``
200
Compute the hypotenuse h of the triangle where the sides adjacent to the square angle have lengths x and y.
``local h = math.sqrt(x^2 + y^2)``
Alternative implementation:
``local h=(x^2+y^2)^0.5``
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
``````local function frexp(a)
return math.frexp(a)
end``````
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".
``foo = os.getenv("FOO") or "none"``
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.
``````local procedures={
foo=function() end,
bar=function() end,
baz=function() end,
barfl=function() end,
}
local function process(str)
local procedure=procedures[str]
if procedure~=nil then
procedure()
end
end
process(str)``````
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
``print(_VERSION)``
219
Create the string t from the value of string s with each sequence of spaces replaced by a single space.

Explain if only the space characters will be replaced, or the other whitespaces as well: tabs, newlines.
``local t = s:gsub("%s+", " ")``
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.
``````local function list_cond(t,cond)
for _,v in ipairs(t) do
if cond(v)==true then return true end
end
return false
end
if list_cond(items,function(x) return x==0 end) then
print("did find 0")
else
print("didn't find 0")
end``````
224
Insert the element x at the beginning of the list items.
``table.insert(items, 1, x)``
225
Declare an optional integer argument x to procedure f, printing out "Present" and its value if it is present, "Not present" otherwise
``````function f( x )
if x then
print("Present", x)
else
print("Not present")
end
end``````
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).
``````local function deepcopy(input)
local t=type(input)
if t~="table" then
return input
end
local copy={}
for k,v in pairs(input) do
k=deepcopy(k)
v=deepcopy(v)
copy[k]=v
end
return copy
end
local y=deepcopy(x)``````
238
Write in a new byte array c the xor result of byte arrays a and b.

a and b have the same size.
``````local c = {}
for i=1,#a do
c[i] = string.char(string.byte(a, i) ~ string.byte(b, i))
end
c = table.concat(c)
``````
243
Print the contents of the list or array a on the standard output.
``print(table.concat(a,", "))``
246
Set c to the number of distinct elements in the list items.
``local c=#items``
256
Print the numbers 5, 4, ..., 0 (included), one line per number.
``````for i=5, 0, -1 do
print(i)
end``````
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.
``````local Vector={x=0,y=0,z=0}
---@type metatable
local mt={
__index=Vector,
}
return Vector.new(
a.x+b.x,
a.y+b.y,
a.z+b.z
)
end
function Vector.new(x,y,z)
return setmetatable({x=x,y=y,z=z},mt)
end``````
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"
``````n=1
while(n<=100) do
out=""
if(n%3==0) then
out=out .. "Fizz"
end
if(n%5==0) then
out=out .. "Buzz"
end
if(out=="") then
out=out .. tostring(n)
end
print(out)
n=n+1
end``````
Alternative implementation:
``````for n=1,100 do
print(
n%3==0
and (n%5==0 and "FizzBuzz" or "Fizz")
or (n%5==0 and "Buzz")
or tostring(n)
)
end
``````
276
Insert an element e into the set x.
``x[e] = true``
282
Declare a type Foo, and create a new map with Foo as key type.

Mention the conditions on Foo required to make it a possible map key type.
``local map={[Foo]=true}``
283
Build the list parts consisting of substrings of input string s, separated by the string sep.
``````local function split(str,sep)
local ret={}
for sub in string.gmatch(str,"[^"..sep.."]+") do
table.insert(ret,sub)
end
return ret
end
local parts=split(s,",")``````
284
Create a new list a (or array, or slice) of size n, where all elements are integers initialized with the value 0.
``````local function new_zeroed_list(n)
local ret={}
for i=1,n do
ret[i]=0
end
return ret
end
local zeroed_list=new_zeroed_list(n)``````
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.
``local a = 0/0``
289
Create the string s by concatenating the strings a and b.
``local s = a..b``
291
Delete all the elements from index i (included) to index j (excluded) from the list items.
``````local function erase(t,i,j)
for i=1,j-1 do
t[i]=nil
end
end
erase(items,i,j)``````
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.
``local str=string.format("Our sun has %d planets",x)``
327
Assign to t the value of the string s, with all letters mapped to their lower case.
``local t = string.lower(s)``
328
Assign to t the value of the string s, with all letters mapped to their upper case.
``local t = string.upper(s)``
329
Assign to v the value stored in the map m for the key k.

Explain what happens if there is no entry for k in m.
``local v=m[k]``
335
Create the map m containing all the elements e of the list a, using as key the field e.id.
``````local m={}
for _,e in ipairs(a) do
m[e.id]=e
end``````