JavaScript Incrementing a number in an array - Stack Overflow

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I want to increment a value in a array when a link is pressed in JavaScript

i Used the following code

<script type="text/javascript">

 var i=0;
 var numbers = new Array();


 function go(val){

 numbers[i]=val;
 i++;

 alert(numbers[i]);

 }

</script>

Called the Function like this

<a href='javascript:go(1)' </a> 

but always the alert prompts me 'undefined'

I want to increment a value in a array when a link is pressed in JavaScript

i Used the following code

<script type="text/javascript">

 var i=0;
 var numbers = new Array();


 function go(val){

 numbers[i]=val;
 i++;

 alert(numbers[i]);

 }

</script>

Called the Function like this

<a href='javascript:go(1)' </a> 

but always the alert prompts me 'undefined'

• javascript

Share Improve this question Follow asked Nov 19, 2010 at 17:22 SudanthaSudantha 16.2k4545 gold badges108108 silver badges162162 bronze badges 1

• 1 Your <a> markup is invalid. – jwueller Commented Nov 19, 2010 at 17:25

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5 Answers 5

Sorted by: Reset to default 7

The alert is correct -- before you do your alert, you incremented i. You're looking at the next element after the one you just entered.

After calling the method once, your array looks like this:

numbers[0] = 1;
numbers[1] = undefined;

and i == 1.

After calling it again, the array looks like:

numbers[0] = 1;
numbers[1] = 1;
numbers[2] = undefined;

and i == 2.

Hopefully you can see that this method will always alert undefined

That's because you increment "i"

i++;

right before you put up the alert! Thus "i" will alwuays refer to the next array slot to use, not the one you just populated.

You could change the alert to use "i-1"

alert(numbers[i - 1]);

You are setting numbers[0] = 1 and then incrementing i which becomes 1 so alert(numbers[1]) is undefined, because it is undefined.

Do the alert before you increment. Also, use onclick or even better unobtrusively attach the event handlers in JS, not in the HTML.

Yes, it does that because you:

1. Create a completely empty array, and a pointer at 0.
2. When the function is called, you set the current pointer value to whatever was passed in...
3. ...and then increment the pointer, so it's now pointing past the end of all the elements.
4. Now you look at the element in the array that's being pointed at, which has to be undefined because of the way you're managing the i pointer.

What were you hoping for this to do, by the way?

The question doesn't even match the code... or the code doesn't match the question?

"I want to increment a value in a array"

Your code is not incrementing the value, it's incrementing the index!

function go(val){
  numbers[i]=val;
  i++;
}

(where i is the index of the next undefined array element) is just the same as

numbers.push(val);

and if you need i to equal what will be the index of the next undefined array element then

i = numbers.length;

To increment the value you would have to first have numeric values for some array elements; then your function would need the index of which value to increment

var numbers = [0,0,0,0];

function go(i){
  numbers[i]++;
}

// testing

go(1);
go(3);
go(1);

alert(numbers);

will show 0,2,0,1

But if your entire goal is to put a value into a new element on the end of an array then just use .push()

and .length will tell you how many elements there are; there is no need to increment an i

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