Repeat

Repeat is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.

The task is to write a procedure which accepts as arguments another procedure and a positive integer. The latter procedure is executed a number of times equal to the accepted integer.

C

<lang C>

1. include <stdio.h>

void repeat(void (*f)(void), int n) {

for(int i=n; 0<i; i--)
(*f)();

}

void example() {

printf("Example\n");

}

void main(char *argv[], int argc) {

repeat(example, 4);

} </lang>

Haskell

<lang Haskell> rep :: Monad m => m () -> Integer -> m () rep _ 0 = return () rep f n = f >> rep f (n-1)

sampleFunction :: IO () sampleFunction = putStrLn "a"

main = rep sampleFunction 5 </lang>

Perl 6

Perl 6 has a built in infix repeat operator (xx) that does this. It returns a list of whatever is on the left side (string, function, subroutine, whatever) times whatever integer is on the right. Nominally it is for building lists but can be used for side effects too. It could be argued that this is an operator, not a procedure, but in Perl 6, operators are just subroutines with funny calling conventions.

Print a list consisting of repeated strings. <lang Perl6>say <Again> xx 4;</lang>

Output:

   Again Again Again Again

Use it for side effects. Execute the code inside the parethesis repeatedly. <lang Perl6>(say <Again>) xx 3;</lang>

Output:

   Again
   Again
   Again

Print a list built by executing the built in rand function repeatedly. <lang Perl6>say rand xx 5;</lang>

Output:

   0.313024826146821 0.231583221230768 0.439756228609952 0.870180182595826 0.650911888509852

Create a small subroutine, call it repeatedly then print the result. <lang Perl6>sub boguscode () { sleep 1; time; } my @times = boguscode() xx 4; say @times;</lang>

Output:

   1400450920 1400450921 1400450922 1400450923

Python

<lang Python>

1. !/usr/bin/python

def repeat(f,n):

 for i in range(n):
   f();

def procedure():

 print("Example");

repeat(procedure,3); #prints "Example" (without quotes) three times, separated by newlines. </lang>

Racket

The racket guide has a section called "Iterators and Comprehensions", which shows that for isn't just for repeating n times!

<lang Racket>#lang racket/base (define (repeat f n) ; the for loop is idiomatic of (although not exclusive to) racket

 (for ((_ n)) (f)))

(define (repeat2 f n) ; This is a bit more "functional programmingy"

 (when (positive? n) (f) (repeat2 f (sub1 n))))

(display "...") (repeat (λ () (display " and over")) 5) (display "...") (repeat2 (λ () (display " & over")) 5) (newline)</lang>

Output:

... and over and over and over and over and over... & over & over & over & over & over