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Programming-Idioms

  • Java
  • Perl

Idiom #196 Pass a sub-array

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.

$n=25;
$m=15;

$A[$n-1] = undef;         # autovivify @A to length $n

sub foo  {
    my ($aref, @i) = @_;
    @$aref[@i] = (42) x @i;
    return;
}

foo(\@A, grep { 0 == $_ % 2 } 0 .. $m);

use Data::Dumper;
print Dumper(@A);

Perl list A is accessed in list context as @A and in scalar context as $A. Passing A as \@A turns A into a list reference, which is received in foo as $aref and accessed as a list slice using the @$aref[@i] syntax; @i on the rhs is the length of the sublist and x is the repeat operator. The empty return ensures we don't return the sublist and emphasizes that the caller should get results from arg1.
use strict;
my $n = 25;
my $m = 16;

my $a = [ 0..$n-1 ];            # create list reference

sub foo  {
    my ($aref, @idx) = @_;      # unpack sub arguments
    foreach my $i ( @idx ) {
        $aref->[$i] = 42;       # dereference element using ->
    }
}

my @subarray;                   # create list
for ( my $i=0; $i < $m; $i += 2 ) {
    push @subarray, $a->[$i];   # dereference element using ->
}

foo($a, @subarray);             # pass list reference and list

print join ', ', @{ $a };       # deref

Less idiomatic implementation which modifies array a in place by defining a as a list reference and accessing its elements.
interface F { void set(int a[], int m); }
F f = (a, m) -> {
    for (int i = 1; i <= m; a[i] = 42, i = i + 2);
};
void foo(int a[], int m) {
    for (int i = 1; i <= m; a[i] = 42, i = i + 2);
}

Passes the entire `a` reference.
void Foo(out int element)
{
    element = 42;
}

for (int i = 0; i < m; i += 2)
{
    Foo(out a[i]);
}

a.Length is the size of a at run-time.

Requires m <= a.Length; the code does not verify this beforehand.

References to individual elements are passed to Foo().
New implementation...
< > ⌨
tkoenig 
$n=25;
$m=15;

$A[$n-1] = undef;         # autovivify @A to length $n

sub foo  {
    my ($aref, @i) = @_;
    @$aref[@i] = (42) x @i;
    return;
}

foo(\@A, grep { 0 == $_ % 2 } 0 .. $m);

use Data::Dumper;
print Dumper(@A);
my $n = 25;
my $m = 16;

my $a = [ 0..$n-1 ];            # create list reference

sub foo  {
    my ($aref, @idx) = @_;      # unpack sub arguments
    foreach my $i ( @idx ) {
        $aref->[$i] = 42;       # dereference element using ->
    }
}

my @subarray;                   # create list
for ( my $i=0; $i < $m; $i += 2 ) {
    push @subarray, $a->[$i];   # dereference element using ->
}

foo($a, @subarray);             # pass list reference and list

print join ', ', @{ $a };       # deref
interface F { void set(int a[], int m); }
F f = (a, m) -> {
    for (int i = 1; i <= m; a[i] = 42, i = i + 2);
};
void foo(int a[], int m) {
    for (int i = 1; i <= m; a[i] = 42, i = i + 2);
}
void Foo(out int element)
{
    element = 42;
}

for (int i = 0; i < m; i += 2)
{
    Foo(out a[i]);
}
void foo(Range)(Range r) {
	r.fill(42);
}

foo(a.indexed(iota(0,m,2)));
var a = List.filled(n,0);
foo(a.take(m).toList());
foo(List<int> a) {
  a.fillRange(0, a.length, 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
func foo(a []int, m int) {
	n := len(a)
	for i := 0; i < m && i < n; i += 2 {
		a[i] = 42
	}
}

fun foo(a : IntArray, idx: IntProgression) = 
  idx.forEach{ a[it] = 42 }
foo(a, 0 .. (m-1) step 2)

procedure foo(var L: Integer);
begin
  L := 42;
end;

begin
  for i := 0 to Min(m, n-1) do
    if not odd(i) then foo(a[i]);
end.
def foo(data, r):
    for i in r: 
        data[i] = 42

foo(a, range(0, m+1, 2))
def foo(s):
    global a
    m = (s.stop - 1) - s.start
    a[s] = [42] * ((m // s.step) + 1)
foo(slice(0, m + 1, 2))
# @param a [Array<Integer>]
# 
# @return [Array<Integer>]
# 
def foo(a)
  a.fill(42)
end

foo(arry.select(&:odd?))

# For older versions of ruby:
# foo(arry.select { |x| x.odd? })
fn foo(el: &mut i32) {
    *el = 42;
}
a.iter_mut().take(m).step_by(2).for_each(foo);
Sub Foo(ByRef element As Integer)
    element = 42
End Sub

' Statements in caller:
For i = 0 To m - 1 Step 2
    Foo(a(i))
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