Apply a callback to an array
In this task, the goal is to take a combined set of elements and apply a function to each element.
You are encouraged to solve this task according to the task description, using any language you may know.
Ada
<ada>with Ada.Text_Io;
with Ada.Integer_text_IO;
procedure Call_Back_Example is
-- Purpose: Apply a callback to an array
-- Output: Prints the squares of an integer array to the console
-- Define the callback procedure
procedure Display(Location : Positive; Value : Integer) is
begin
Ada.Text_Io.Put("array(");
Ada.Integer_Text_Io.Put(Item => Location, Width => 1);
Ada.Text_Io.Put(") = ");
Ada.Integer_Text_Io.Put(Item => Value * Value, Width => 1);
Ada.Text_Io.New_Line;
end Display;
-- Define an access type matching the signature of the callback procedure
type Call_Back_Access is access procedure(L : Positive; V : Integer);
-- Define an unconstrained array type
type Value_Array is array(Positive range <>) of Integer;
-- Define the procedure performing the callback
procedure Map(Values : Value_Array; Worker : Call_Back_Access) is
begin
for I in Values'range loop
Worker(I, Values(I));
end loop;
end Map;
-- Define and initialize the actual array
Sample : Value_Array := (5,4,3,2,1);
begin
Map(Sample, Display'access);
end Call_Back_Example;</ada>
ALGOL 68
PROC call back proc = (INT location, INT value)VOID:
(
printf(($"array["g"] = "gl$, location, value))
);
PROC map = (REF[]INT array, PROC (INT,INT)VOID call back)VOID:
(
FOR i FROM LWB array TO UPB array DO
call back(i, array[i])
OD
);
main:
(
[4]INT array := ( 1, 4, 9, 16 );
map(array, call back proc)
)
Output:
array[ +1] = +1 array[ +2] = +4 array[ +3] = +9 array[ +4] = +16
C
callback.h
#ifndef __CALLBACK_H #define __CALLBACK_H /* * By declaring the function in a separate file, we allow * it to be used by other source files. * * It also stops ICC from complaining. * * If you don't want to use it outside of callback.c, this * file can be removed, provided the static keyword is prepended * to the definition. */ void map(int* array, int len, void(*callback)(int,int)); #endif
callback.c
#include <stdio.h>
#include "callback.h"
/*
* We don't need this function outside of this file, so
* we declare it static.
*/
static void callbackFunction(int location, int value)
{
printf("array[%d] = %d\n", location, value);
}
void map(int* array, int len, void(*callback)(int,int))
{
int i;
for(i = 0; i < len; i++)
{
callback(i, array[i]);
}
}
int main()
{
int array[] = { 1, 2, 3, 4 };
map(array, 4, callbackFunction);
return 0;
}
Output
array[0] = 1 array[1] = 2 array[2] = 3 array[3] = 4
C#
using System;
static class Program
{
// Purpose: Apply a callback (or anonymous method) to an Array
// Output: Prints the squares of an int array to the console.
// Compiler: Visual Studio 2005
// Framework: .net 2
[STAThread]
public static void Main()
{
int[] intArray = { 1, 2, 3, 4, 5 };
// Using a callback,
Console.WriteLine("Printing squares using a callback:");
Array.ForEach<int>(intArray, PrintSquare);
// or using an anonymous method:
Console.WriteLine("Printing squares using an anonymous method:");
Array.ForEach<int>
(
intArray,
delegate(int value)
{
Console.WriteLine(value * value);
});
}
public static void PrintSquare(int value)
{
Console.WriteLine(value * value);
}
}
This version uses the C# 3 lambda notation.
int[] intArray = { 1, 2, 3, 4, 5 };
Array.ForEach(intArray, i => Console.WriteLine(i * i));
C++
C-Style Array
#include <iostream> //cout for printing
#include <algorithm> //for_each defined here
//create the function (print the square)
void print_square(int i) {
std::cout << i*i << " ";
}
int main() {
//create the array
int ary[]={1,2,3,4,5};
//stl for_each
std::for_each(ary,ary+5,print_square);
return 0;
}
//prints 1 4 9 16 25
std::vector
#include <iostream> //cout for printing
#include <algorithm> //for_each defined here
#include <vector> //stl vector class
//create the function (print the square)
void print_square(int i) {
std::cout << i*i << " ";
}
int main() {
//create the array
std::vector<int> ary;
ary.push_back(1);
ary.push_back(2);
ary.push_back(3);
ary.push_back(4);
ary.push_back(5);
//stl for_each
std::for_each(ary.begin(),ary.end(),print_square);
return 0;
}
//prints 1 4 9 16 25
More tricky with binary function
#include <iostream> //cout for printing
#include <algorithm> //for_each defined here
#include <vector> //stl vector class
#include <functional> //bind and ptr_fun
//create a binary function (print any two arguments together)
template<class type1,class type2>
void print_juxtaposed(type1 x, type2 y) {
std::cout << x << y;
}
int main() {
//create the array
std::vector<int> ary;
ary.push_back(1);
ary.push_back(2);
ary.push_back(3);
ary.push_back(4);
ary.push_back(5);
//stl for_each, using binder and adaptable unary function
std::for_each(ary.begin(),ary.end(),std::bind2nd(std::ptr_fun(print_juxtaposed<int,std::string>),"x "));
return 0;
}
//prints 1x 2x 3x 4x 5x
Boost.Lambda
using namespace std; using namespace boost::lambda; vector<int> ary(10); int i = 0; for_each(ary.begin(), ary.end(), _1 = ++var(i)); // init array transform(ary.begin(), ary.end(), ostream_iterator<int>(cout, " "), _1 * _1); // square and output
Clean
Define a function and an initial (unboxed) array.
square x = x * x
values :: {#Int}
values = {x \\ x <- [1 .. 10]}
One can easily define a map for arrays, which is overloaded and works for all kinds of arrays (lazy, strict, unboxed).
mapArray f array = {f x \\ x <-: array}
Apply the function to the initial array (using a comprehension) and print result.
Start :: {#Int}
Start = mapArray square values
Common Lisp
Imperative: print 1, 2, 3, 4 and 5:
(map nil #'print #(1 2 3 4 5))
Functional: collect squares into new vector that is returned:
(defun square (x) (* x x)) (map 'vector #'square #(1 2 3 4 5))
Destructive, like the Javascript example; add 1 to every slot of vector *a*:
(defvar *a* (vector 1 2 3)) (map-into *a* #'1+ *a*)
D
<d>U[] map(T, U)(T[] array, U delegate(T) dg) {
auto result = new U[array.length];
foreach (index, element; array)
result[index] = dg(element);
return result;
}
void main() {
writefln(
[1, 2, 3, 4, 5].map( (int i) { return i+5; } )
);
}</d>
E
def array := [1,2,3,4,5]
def square(value) {
return value * value
}
Example of builtin iteration:
def callback(index, value) {
println(`Item $index is $value.`)
}
array.iterate(callback)
There is no built-in map function yet. The following is one of the ways one could be implemented, returning a plain list (which is usually an array in implementation).
def map(func, collection) {
def output := [].diverge()
for item in collection {
output.push(func(item))
}
return output.snapshot()
}
println(map(square, array))
Forth
This is a word that will call a given function on each cell in an array.
: map ( addr n fn -- ) -rot cells bounds do i @ over execute i ! cell +loop ;
Example usage:
create data 1 , 2 , 3 , 4 , 5 , data 5 ' 1+ map \ adds one to each element of data
Fortran
In ISO Fortran 95 or later, use elemental functions:
ELEMENTAL FUNCTION CUBE( X )
IMPLICIT NONE
REAL :: CUBE
REAL, INTENT(IN) :: X
CUBE = X * X * X
END FUNCTION CUBE
PROGRAM EXAMPLE
IMPLICIT NONE
INTEGER :: I, J
REAL, DIMENSION(3,4) :: B, &
A = RESHAPE( (/ ((0.1 * I + J, I = 1, 3), J = 1, 4) /), (/ 3,4 /) )
DO I = 1, 3
WRITE(*,*) A(I,:)
END DO
B = CUBE( A ) ! Applies CUBE to every member of A,
! and stores each result in the equivalent element of B
DO I = 1, 3
WRITE(*,*) B(I,:)
END DO
END PROGRAM EXAMPLE
ANSI FORTRAN 77 (with MIL-STD-1753 structured DO) Example:
program test
C
C-- Declare array:
integer a(5)
C
C-- Fill it with Data
data a /45,22,67,87,98/
C
C-- Do something with all elements (in this case: print their squares)
do i=1,5
print *,a(i)*a(i)
end do
C
end
FP
{square * . [id, id]}
& square: <1,2,3,4,5>
Groovy
Print each value in a list
[1,2,3,4].each { println it }
Create a new list containing the squares of another list
[1,2,3,4].collect { it * it }
Haskell
let square x = x*x let values = [1..10] map square values
Using list comprehension to generate a list of the squared values
[square x | x <- values]
Using function composition to create a function that will print the squares of a list
let printSquares = putStr.unlines.map (show.square) printSquares values
IDL
Hard to come up with an example that isn't completely contrived. IDL doesn't really distinguish between a scalar and an array; thus
b = a^3
will yield a scalar if a is scalar or a vector if a is a vector or an n-dimensional array if a is an n-dimensional array
JavaScript
Portable technique:
function map(a, func) {
for (var i in a)
a[i] = func(a[i]);
}
var a = [1, 2, 3, 4, 5];
map(a, function(v) { return v * v; });
With the BeyondJS library:
var a = (1).to(10).collect(Math.pow.curry(undefined,2));
With Firefox 2.0:
function cube(num) {
return Math.pow(num, 3);
}
var numbers = [1, 2, 3, 4, 5];
//get results of calling cube on every element
var cubes1 = numbers.map(cube);
//display each result in a separate dialog
cubes1.forEach(alert);
//array comprehension
var cubes2 = [cube(n) for each (n in numbers)];
var cubes3 = [n * n * n for each (n in numbers)];
Functional.map('x*x*x', [1,2,3,4,5])
Logo
to square :x output :x * :x end show map "square [1 2 3 4 5] ; [1 4 9 16 25] show map [? * ?] [1 2 3 4 5] ; [1 4 9 16 25] foreach [1 2 3 4 5] [print square ?] ; 1 4 9 16 25, one per line
Lua
Say we have an array:
myArray = {1, 2, 3, 4, 5}
A map function for this would be
map = function(f, data)
local result = {}
for k,v in ipairs(data) do
result[k] = f(v)
end
return result
end
Together with our array and and a square function this yields:
myFunc = function(x) return x*x end print(unpack( map(myFunc, myArray) )) --> 1 4 9 16 25
If you used pairs() instead of ipairs(), this would even work on a hash table in general.
OCaml
This function is part of the standard library:
Array.map
Usage example:
let square x = x * x;; let values = Array.init 10 ((+) 1);; Array.map square values;;
Oz
functor
import
Application System
define
Print = System.showInfo
fun{Square A}
A*A
end
fun{FuncEach Func A}
{Map A Func}
end
proc{ProcEach Proc A}
{ForAll A Proc}
end
Arr = [1 2 3 4 5]
{ProcEach Print {FuncEach Square Arr}}
{ForAll {Map Arr Square} Print} %% same
{Application.exit 0}
end
Perl
# create array my @a = (1, 2, 3, 4, 5);
# create callback function
sub mycallback {
return 2 * shift;
}
# use array indexing
my $i;
for ($i = 0; $i < scalar @a; $i++) {
print "mycallback($a[$i]) = ", mycallback($a[$i]), "\n";
}
# using foreach
foreach my $x (@a) {
print "mycallback($x) = ", mycallback($x), "\n";
}
# using map (useful for transforming an array) my @b = map mycallback($_), @a; # @b is now (2, 4, 6, 8, 10)
# and the same using an anonymous function
my @c = map { $_ * 2 } @a; # @c is now (2, 4, 6, 8, 10)
# use a callback stored in a variable my $func = \&mycallback; my @d = map $func->($_), @a; # @d is now (2, 4, 6, 8, 10)
PHP
function cube($n)
{
return($n * $n * $n);
}
$a = array(1, 2, 3, 4, 5);
$b = array_map("cube", $a);
print_r($b);
PL/SQL
set serveroutput on
declare
type myarray is table of number index by binary_integer;
x myarray;
i pls_integer;
begin
-- populate array
for i in 1..5 loop
x(i) := i;
end loop;
i :=0;
-- square array
loop
i := i + 1;
begin
x(i) := x(i)*x(i);
dbms_output.put_line(x(i));
exception
when no_data_found then exit;
end;
end loop;
end;
/
Pop11
;;; Define a procedure
define proc(x);
printf(x*x, '%p,');
enddefine;
;;; Create array
lvars ar = { 1 2 3 4 5};
;;; Apply procedure to array appdata(ar, proc);
If one wants to create a new array consisting of transformed values then procedure mapdata may be more convenient.
Python
def square(n):
return n * n
numbers = [1, 3, 5, 7]
squares1 = [square(n) for n in numbers] # list comprehension
squares2a = map(square, numbers) # functional form
squares2b = map(lambda x: x*x, numbers) # functional form with `lambda`
squares3 = [n * n for n in numbers] # no need for a function,
# anonymous or otherwise
isquares1 = (n * n for n in numbers) # iterator, lazy
import itertools
isquares2 = itertools.imap(square, numbers) # iterator, lazy
To print squares of integers in the range from 0 to 9, type:
print " ".join(str(n * n) for n in range(10))
Or:
print " ".join(map(str, map(square, range(10))))
Result:
0 1 4 9 16 25 36 49 64 81
Raven
# To print the squared elements [1 2 3 4 5] each dup * print
# To obtain a new array group [1 2 3 4 5] each dup * list
Ruby
# You could use a traditional "for i in arr" approach like below:
for i in [1,2,3,4,5] do
puts i**2
end
# Or you could the more preferred ruby way of an iterator (which is borrowed from SmallTalk)
[1,2,3,4,5].each{ |i| puts i**2 }
# To create a new array of each value squared
[1,2,3,4,5].map{ |i| i**2 }
Scala
val l = List(1,2,3,4)
l.foreach {i => println(i)}
When the argument appears only once -as here, i appears only one in println(i) - it may be shortened to
l.foreach(println(_))
Same for an array
val a = Array(1,2,3,4)
a.foreach {i => println(i)}
a.foreach(println(_)) // same as previous line
Or for an externally defined function:
def doSomething(in: int) = {println("Doing something with "+in)}
l.foreach(doSomething)
There is also a for syntax, which is internally rewritten to call foreach. A foreach method must be defined on a
for(val i <- a) println(i)
It is also possible to apply a function on each item of an list to get a new list (same on array and most collections)
val squares = l.map{i => i * i} //squares is List(1,4,9,16)
Or the equivalent for syntax, with the additional keyword yield, map is called instead of foreach
val squares = for (val i <- l) yield i * i
Scheme
(define (square n) (* n n)) (define x #(1 2 3 4 5)) (map square (vector->list x))
A single-line variation
(map (lambda (n) (* n n)) '(1 2 3 4 5))
For completeness, the map function (which is R5RS standard) can be coded as follows:
(define (map f L)
(if (null? L)
L
(cons (f (car L)) (map f (cdr L)))))
Smalltalk
#( 1 2 3 4 5 ) collect: [:n | n * n].
Tcl
If I wanted to call "myfunc" on each element of dat and dat were a list:
foreach var $dat { myfunc $var }
if dat were an array, however:
foreach var [array names dat] { myfunc $dat($var) }
Toka
( array count function -- )
{
value| array fn |
[ i array ] is I
[ to fn swap to array 0 swap [ I array.get :stack fn invoke I array.put ] countedLoop ]
} is map-array
( Build an array )
5 cells is-array a
10 0 a array.put
11 1 a array.put
12 2 a array.put
13 3 a array.put
14 4 a array.put
( Add 1 to each item in the array )
a 5 [ 1 + ] map-array
V
apply squaring (dup *) to each member of collection
[1 2 3 4] [dup *] map
Visual Basic .NET
Platform: .NET
Dim intArray As Integer() = {1, 2, 3, 4, 5}
Array.ForEach(intArray, Function(i) Console.WriteLine(i * i))