with two arrays of potentially different sizes, what is best way to see if they are the same as far as it goes

for example

var a1 = [ 1, 2, 3 ];
var a2 = [ 1, 2 ];
var a3 = [ 1, 3 ];

a1 == a2 => true;
a1 == a3 => false;

am positive this has been done thousands of times and the syntax is well memorized

with two arrays of potentially different sizes, what is best way to see if they are the same as far as it goes

for example

var a1 = [ 1, 2, 3 ];
var a2 = [ 1, 2 ];
var a3 = [ 1, 3 ];

a1 == a2 => true;
a1 == a3 => false;

am positive this has been done thousands of times and the syntax is well memorized

• javascript
• arrays

• Why is the first true and the second false? – Gumbo Commented Feb 12, 2012 at 12:10
• why al & a2 are same?? because a2 content is there in a1?? – Fahim Parkar Commented Feb 12, 2012 at 12:10

7 Answers 7

What about this (I'll just demonstrate on a1 and a2 -> I presume you can make function out of this):

var min_val = min(a1.length, a2.length);
var equals = true;

for(i = 0; i < min_val; i++)
{
    if(a1[i] != a2[i])
    {
        equals = false;
        break;
    }
}

The result will be in equals variable of course. If you want to make function out of this, just pass a1 and a2 as arguments and return equals.

function pareArraySeq(a, b) {
    return a.slice(0, b.length).join(' ') == b.slice(0, a.length).join(' ');
}

function pareArraySeq(a1, a2) {
  var i, l = Math.min(a1.length, a2.length); 

  for (i=0; i<l; i++) {
    if (a1[i] !== a2[i]) return false;
  }
  return true;
}

[edit] based on Tomalaks ments I'd say JSON can e to the rescue.

So, again: here's an Array extension that does what [I suppose] you want to do:

function parePartial(arr1,arr2){
  var arr2 = this, l1 = arr1.length, l2 = arr2.length;

  return ( l1<1 || l2<1
            ? false :
              JSON.stringify(arr1.slice(0, l2)) ===
              JSON.stringify(arr2.slice(0, l1))
         );
}
Array.prototype.parePartial = 
    Array.prototype.parePartial || parePartial;

//usage
    var a1 = [ 1, 2, 3 ]
   ,a2 = [ 1, 2 ]
   ,a3 = [ 1, 3 ]
   ,a4 = ['','']
   ,a5 = ['','','']
   ,a6 = []
   ,a7 = ['bla','doh',1]
   ,a8 = ['bla','doh',1,'yeah','really']
   ,a9 = [1,3,5,'doh']
   ,a10= ['1','3','5','doh']
   ,a11= [{a:1,b:2},{c:3,d:4}]
   ,a12= [{a:1,b:2},{c:3,d:4},{e:5,f:6}]

console.log(
  [ a1.parePartial(a2)
   ,a2.parePartial(a1)
   ,a1.parePartial(a3)
   ,a4.parePartial(a5)
   ,a5.parePartial(a6)
   ,a1.parePartial(a6)
   ,a8.parePartial(a7)
   ,a10.parePartial(a9)  //=> 'type safe' parison
   ,a11.parePartial(a12) //=> can pare arrays of Objects
  ].join(' - ')
); //=> true - true - false - true - false - false - true - false - true

function prefixEqual(a, b) {
    var prefixLength = a.length < b.length ? a.length : b.length;
    for(var i = 0; i < prefixLength; i+=1)
        if( a[i] != b[i] )
            return false;
    return true;
}

Make a loop checking one spot at a time. I have made this:

var pare = function (a1, a2) {
    var l = Math.min(a1.length, a2.length);
    for (var i = 0; i < l; i++) {
        if (a1[i] !== a2[i]) {
            return false;
        }
    }
    return true;
}

Now you can pare arrays like this:

var a = [0, 1, 2, 3];
var b = [0, 1, 2];
var c = [0, 1, 3];

pare(a, b); //true
pare(a, c); //false

Hope this works for you :)

Fiddle link: http://jsfiddle/8zbJj/1/

If your arrays are strings or numbers or booleans you can pare their String values.

function pareSimpleValues(a,b){
  if(a.length>=b.length)return String(a).indexOf(String(b))===0;
  return String(b).indexOf(String(a))===0;
}