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Thread: [RESOLVED] Function with multiple parameters as parameter

  1. #1
    Join Date
    Jul 2013
    Posts
    16

    resolved [RESOLVED] Function with multiple parameters as parameter

    I want to do functional programming in javascript.
    Code:
    function forEach(array, function)
    {
      var newArray = new Array();
      for(var i=0; i < array.length; i++)
      {
        newArray[i] = function(this[i]);
      }
      return newArray;
    }
    If I have a function like isNumber(x) that returns true if x is a number, then I can use the function written above to apply the isNumber function to an array.
    Code:
    var arr = [1, 2, "three", 4, "five"];
    var boolArr = forEach(arr, isNumber);
    //boolArr = [true, true, false, true, false]
    But what if I want to apply a function that has multiple parameters? For example sum(a,b) that adds a and b and I would want that function to add all numbers in an array. Or maybe a function like isBetween(x,a,b) that would return true if x is between a and b and I want to be able to do something like forEach(array, isBetween(a,b)) which would check if each element is between a and b. Is there a way to do this?

  2. #2
    Join Date
    Jul 2013
    Posts
    46
    sorry i am sort of new to all this but "&&" is and eg
    if (A == 1 && B == 1) "this will only run if A == 1 AND B == 1"

    also "||" is or eg
    if (C == 1 || D == 1) "this will run if C == 1 and/or D == 1"

  3. #3
    Join Date
    Mar 2011
    Posts
    128
    A 'forEach' function normally works on the array passed to it. The function you have which returns a new array, is usually called a 'map' function.
    Chapter 6 of http://eloquentjavascript.net/ explains some nice tricks on functional programming.

    As a way to solve your questions.
    Code:
     function forEach(array, action){
      for(var i=0; i<array.length; i++){
       action.call(this, array[i], i);
      }
     };
    
     function reduce(combine, base, array){
      forEach(array,function(element){
       base = combine(base ,element);
      });
      return base;
     };
    
     function map(func, array){
      var result = [];
      forEach(array, function(element, i){
       result.push(func(element, i));
      });
      return result;
     };
    
     function asArray(quasiArray, start){
      return Array.prototype.splice.call(quasiArray, start)
     };
    
     function partial(func) {
      var fixedArgs = asArray(arguments, 1);
      return function(){
       return func.apply(null, fixedArgs.concat(asArray(arguments, 0)));
      };
     };
    
     function add(a, b){
      return a+b;
     };
    
     function isBetween(a, b, x){
      return x>a && x<b;
     };
    
     var sum = reduce(add, 0, [1,2,3]);
     var between = map(partial(isBetween, 3, 7), [1,2,3,4,5,6,7,8,9]);
     document.write(sum + '<hr>' +between);

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