Compare array js

Question

I have the following

var a = [4,6,12];
var b = [4,6];

for (var i = 0; i < a.length; i++) {
    for (var j = 0; j < b.length; j++) {
       if (a[i] !== b[j]) {
         a.pop();
       }
    }
}

I want to compare the two and remove 12 from a if it is not found in b. I do not want to create a new array with the result just remove from a.

However, if I console log a I get a as empty.

Answer 1

The reason why a is coming out as empty is because of your double loop. You're comparing each element of a to every other element of b. When you hit a[0] == b[1], obviously, 4 != 6 so you pop it.

Answer 2

Try:

var a = [4,6,12];
var b = [4,6];

a.forEach(function(v,i){
    if (b.indexOf(v) === -1) {
        a.splice(i,v);
    }
});

console.log(a); // result [4,6]

JSFiddle

Iteration checks every element of a array and compares to b elements, if certain element doesn't exist in b then that element is removed from a array

Answer 3

var a = [4,6,12];
var b = [4,6];

for (var i = 0; i < a.length; i++) {
       if (!b.indexOf(a[i])) {
         a.pop();
       }
}

Answer 4

var a = [4,6,12];
var b = [4,6];

var common = $.grep(a, function(element) {
    return $.inArray(element, b ) !== -1;
});

console.log(common); // returns [4,6];

Answer 5

Try this:

var a = [4,6,12];
var b = [4,6];
var i = 0;
for (; i < a.length && i < b.length; i++)
    if (a[i] !== b[i]) {
        a.pop();
        break;
    }
while(i < a.length)
    a.pop();

Answer 6

a = a.filter(function(item){ return b.contains(item); });

More on .filter()

Answer 7

Your second for-loop causes this script to remove everything from a:

1. i = 0, j = 0 => a[0] = 4, b[0] = 4. (4 !== 4) => false, 
2. i = 0, j = 1 => a[0] = 4, b[1] = 6. (4 !== 6) => true, pop first element from a
3. i = 1, j = 0 => a[1] = 6, b[0] = 4. (6 !== 4) => true, pop second elmement from a

So you are popping one element after another from a until a is empty.

Try instead:

var a = [4,6,12];
var b = [4,6];

    for (var i = 0; i < a.length; i++) {
           if (b.indexOf(a[i]) == -1) {
             a.pop();

        }
    }