js - loop through multidimensional array

Question

I'm trying to implement jquery hotkeys in a for loop.

acKey = [
    ["keydown", "alt+d", open],
    ["keydown", "alt+a", close],
    ["keydown", "alt+s", max],
];
for(i=0;i<=acKey.length;i++)
{
    $(document).bind(acKey[i][0], acKey[i][1], acKey[i][2]);
}

However, it turned out error Uncaught TypeError: Cannot read property '0' of undefined. What's wrong with my code?

Your loop is for(i=0;i<=acKey.length;i++) (note the i<=acKey.length). What happens when i==acKey.length. Does acKey[3] exist? — Matt
– Matt, Commented Mar 11, 2014 at 9:29
Sometimes you do need "<=" in a for loop, but it's very much the exception. Any time you do use "<=", think about adding a comment to say why. — slim
– slim, Commented Mar 11, 2014 at 9:31

slim · Accepted Answer · 2014-03-11 09:41:36Z

4

With due credit to @Matt who's comment points it out.

The commonest format for looping through an array is:

 for(var i=0; i<array.length; i++) {
     doSomethingWith(array[i];
 }

Note that's a "less than" operator, not a "less than or equal to" operator.

This loop counts from 0 to array.length - 1, because the second part of the for statement: i < array.length, means "keep repeating for as long as i is less than array.length.

... and that's what you want, because arrays are numbered from 0 to length-1. That is, an array of length 4 is numbered 0,1,2,3.

If you loop while i <= 4, then the loop will execute for 0,1,2,3,4 -- and in Javascript, it will get undefined when it references array[4].

Sometimes you do need "<=" in a for loop, but it's very much the exception. Any time you do use "<=", think about adding a comment to say why.

edited Mar 11, 2014 at 9:41

answered Mar 11, 2014 at 9:38

slim

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Comments

cardeol · Accepted Answer · 2014-03-11 09:45:30Z

1

Your problem is your index (out of bounds) when i=acKey.length.

You can use i<acKey.lenght or implement a "for each" iteration to avoid confusion:

acKey = [
    ["keydown", "alt+d", open],
    ["keydown", "alt+a", close],
    ["keydown", "alt+s", max],
];

var sub;
for(i in acKey) {
    sub = acKey[i];
    $(document).bind(sub[0], sub[1], sub[2]);
}

edited Mar 11, 2014 at 9:45

answered Mar 11, 2014 at 9:37

cardeol

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7 Comments

Lewis Over a year ago

That's quite new to me. Thank you, cardeol.

Emil L Over a year ago

in shouldn't be used to loop over arrays generally. See explanation here: stackoverflow.com/a/3010848/355499

cardeol Over a year ago

In this case is completely safe.

Lewis Over a year ago

Could you guys all point out this in several lines? Which cases are not safe?

cardeol Over a year ago

I.E. If you use an object inherited properties are also enumerated. but in case of arrays shouldn't be any problem. I use a lot this structure. Even with objects. stackoverflow.com/questions/11846484/each-for-object

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