Decreasing values of an array by one where value > x?

**kaisellgren** · 04-04-2009 09:21 AM

Hi,

I have an array similar to this:

Code:

0 -> 0
1 -> 5
2 -> 9
3 -> 2
4 -> 1
5 -> 6
6 -> 8
7 -> 7
...

There is no clear logic and that does not matter. I need to decrease the values of those by one where the value is higher than x. How would I do this?

For instance, if x = 5, then that array becomes

Code:

0 -> 0
1 -> 5
2 -> 8
3 -> 2
4 -> 1
5 -> 5
6 -> 7
7 -> 6
...

All values higher than 5 were decreased by one.

Can someone help me?

**astupidname** · 04-04-2009 10:16 AM

I have an array prototype method for doing things like this, similar to php's array_walk, just pass it the name of a function you have defined to do the job on each member of the array, each member of the array will have the function performed on it. Note the function defined, in this case alterNum() must accept an argument which will be the member of the array, and return a value.

Code:

<script type="text/javascript">
Array.prototype.changeEach = function (func) {
    var z = 0;
    while (z < this.length) {
        if (this[z] instanceof Array) { //go multiple levels deep if member is another array
            this[z].changeEach(func);
        } else {
            this[z] = func(this[z]); //run the function on the individual member
        }
        z++;
    }
};

var arr = [0, 5, 9, 2, 1, 6, 8, 7];

function alterNum(num) {
    return (typeof num == 'number' && num > 5) ? num - 1 : num;
}

arr.changeEach(alterNum);
alert(arr); //0,5,8,2,1,5,7,6
</script>

**astupidname** · 04-04-2009 10:55 AM

Actually, I just got around to re-working that changeEach method (I've been meaning to, but forgot til now..) as it only works on numerical arrays. The newly modified version works on associative arrays also, might as well share it:

HTML Code:

<html>
<body>

<script type="text/javascript">

Array.prototype.changeEach = function (func) {
    if (typeof func == 'function') {
        var prop;
        for (prop in this) {
            if (this[prop] instanceof Array) { //if member is another array, go multiple levels deep
                this[prop].changeEach(func);
            } else { //run func on individual member
                this[prop] = func(this[prop]);
            }
        }
    }
};

var arr = [0, 5, 9, 2, 1, 6, 8, 7];

var arr2 = [];
arr2["bob"] = 3;
arr2["fred"] = 7;
arr2["john"] = 5;
arr2["sue"] = 22;
arr2["multi"] = [4,8,12];


function alterNum(num) {
    return (typeof num == 'number' && num > 5) ? num - 1 : num;
}

arr.changeEach(alterNum);
alert(arr);

arr2.changeEach(alterNum);
alert(arr2.fred);
alert(arr2.multi[2]);
</script>

</body>
</html>

**JMRKER** · 04-04-2009 12:12 PM

While I think 'astupidname's script is a little more elegant,
if you just need a quick and dirty way to reduce the array size
take a look at this ...

Code:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html lang="en">
<head>
<title>Depopulate List</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">

<script type="text/javascript">
// From: http://www.webdeveloper.com/forum/showthread.php?t=206258

var InitArray = [0, 5, 9, 2, 1, 6, 8, 7];
var arr = InitArray;

function ShowArr() {
  alert(arr.join('\n'));
}
  
function ReduceArray(a) {
  var tmp = [];
  for (i=0; i<a.length; i++) { 
    if (a[i] <= 5) { tmp.push(a[i]); }
  }
  arr = tmp;  
  ShowArr();
}

function Initialize(a) {
  arr = InitArray;
  ShowArr();
}
</script>
</head>
<body>
<button onClick="ShowArr()">Current Array Contents</button>
<button onClick="ReduceArray(arr)">Reduce Array</button>
<button onClick="Initialize(arr)">Initialize</button>
</body>
</html>

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