Apply a callback to an array: Difference between revisions

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Revision as of 21:45, 24 August 2007

Ada

Tested With:

Gnat GPL 2005
- Amd-64bit-3500+-WinXP

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;

C

Tested With:

GCC 3.3.6
- i686-pc-linux-gnu
GCC 3.4.6
- i686-pc-linux-gnu
GCC 4.0.3
- i686-pc-linux-gnu
GCC 4.1.1
- i686-pc-linux-gnu
- powerpc-unknown-linux-gnu
TCC 0.9.23
- i686-pc-linux-gnu
ICC 9.1
- i686-pc-linux-gnu

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#

Platform: .NET

Language Version: 2.0+

Compiler: Visual C# 2005

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);
  }
}

C++

Compiler: GNU Compiler Collection 4.1.1

Using 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

Using 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

Using 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*)

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 builtin 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

      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

Interpreter : "fp"

 {square * . [id, id]}
 & square: <1,2,3,4,5>

Haskell

Interpreter : GHCi

Compiler : GHC

 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 is 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)];

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;;

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

Interpreter : Oracle compiler

 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

squares2 = map(square, numbers)         # discouraged nowadays

squares3 = [n * n for n in numbers]     # no need for a function,
                                        # anonymous or otherwise

isquares = (n * n for n in numbers)     # iterator, lazy

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 => Console.println(i)}

Same for an array

 val a = Array(1,2,3,4)
 a.foreach {i => Console.println(i)}

 // Or for an externally defined function
 def doSomething(in: int) = {Console.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 define on a

for(val i <- a) Console.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} //returns  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

 | anArray |
 anArray = #( 1 2 3 4 5 )
 anArray do: [ :x | Transcript nextPut: x * x ]

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 -- )
 {
   variable| array fn |
   [ i 1- array @ ] is I
   [ fn ! swap array ! [ I get-element fn @ invoke I put-element ] +iterate ]
 } is map-array
 
 ( Build an array )
 5 cells is-array a
 10 0 a put-element
 11 1 a put-element
 12 2 a put-element
 13 3 a put-element
 14 4 a put-element
 
 ( Add 1 to each item in the array )
 a 5 ` 1+ map-array