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I want to ask a question that I have about a 2D array in JavaScript, similar to this:

var array = [[2,45],[3,56],[5,67],[8,98],[6,89],[9,89],[5,67]]

That is, on every index, I have an array as well.

Suppose that I have a value in my second index like 56, and I want the corresponding 1st index (i.e. 3 in above example).

I can do it using a loop (if no other option), but is there any better way?

Also, I know indexOf method in JavaScript, but it's not returning the index when I call it like this:

array.indexOf(56);

Any insight will be helpful.

I want to ask a question that I have about a 2D array in JavaScript, similar to this:

var array = [[2,45],[3,56],[5,67],[8,98],[6,89],[9,89],[5,67]]

That is, on every index, I have an array as well.

Suppose that I have a value in my second index like 56, and I want the corresponding 1st index (i.e. 3 in above example).

I can do it using a loop (if no other option), but is there any better way?

Also, I know indexOf method in JavaScript, but it's not returning the index when I call it like this:

array.indexOf(56);

Any insight will be helpful.

• javascript
• indexing

Share Improve this question Follow edited Jul 2, 2016 at 23:15 Warren Sergent 2,59744 gold badges3737 silver badges4444 bronze badges asked Jun 18, 2012 at 10:09 A_userA_user 2,15766 gold badges2626 silver badges3333 bronze badges

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5 Answers 5

Sorted by: Reset to default 14

Use some iterator function.

filter allows you to iterate over an array and returns only the values when the function returns true.

Modern browsers way:

var arr = array.filter( function( el ) {
    return !!~el.indexOf( 56 );
} );
console.log( arr ); // [3, 56]
console.log( arr[ 0 ] ); // "3"

Legacy browsers way, using jQuery:

var arr = $.filter( array, function() {
    return !!~$.inArray( 56, $( this ) );
} );
console.log( arr ); // [3, 56]
console.log( arr[ 0 ] ); // "3"

You could do like this:

var ret;
var index = array.map(function(el){return el[1];}).indexOf(56);
if (index !== -1) {
  ret = array[index][0];
}

array.indexOf( x ) returns the position of x in the array, or -1 if it is not found. In your 2D array this is useless, since 56 isn't in array; it is in an array which is contained in array.

You will have to loop over the first array, then use indexOf to check each sub-array.

I found that in IE6 at least (I'm not sure about the more recent versions), the built-in Array type didn't have an indexOf method. If you're testing in IE, perhaps that's why you couldn't get started. I wrote this little addition to include in all my code:

if(!Array.prototype.indexOf){
    Array.prototype.indexOf = function(obj){
        for(var i=0; i<this.length; i++){
            if(this[i]===obj){
                return i;
            }
        }
     return -1;
   };
}

I can't really take credit for this. It's probably a fairly generic piece of code I found in some tutorial or one of Doug Crockford's articles. I'm quite sure someone will come along with a better way of writing it.

I find working in Firefox is a good place to start because you can install the Firebug console and get a better idea where things are going wrong. I admit that doesn't solve cross-browser problems, but at least it allows you to get a start on your project.

This little piece of code will ensure you can use the indexOf method to find where a given value is in a one-dimensional array. With your 2-dimensional array, each element in the outer array is an array not a single value. I'm going to do a little test to see exactly how that works and get back to you, unless someone else comes in with a better answer.

I hope this at least gets you started.

UPDATE Assuming you have a browser that implements Array.indexOf, I've tried every which way to pass one of the array elements in your outer array.

alert(array.indexOf([3,56]))
alert(array.indexOf("3,56"))

just return -1.

Oddly,

alert(array.indexOf(array[1]))

correctly returns 1. But you need to know the index of the inner array to get the index! I can't see a way of passing an array to the indexOf method.

Please try the below code, it is working

 var array = [[2, 45], [3, 56], [5, 67], [8, 98], [6, 89], [9, 89], [5, 67]];
array.map((x, index) => {
    if (x.includes(56)) {
        console.log(index);
    }
})

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