Javascript, how to match and get similar objects containing in two arrays?

Question

I have two array of objects. I want to check the list of objects from array one existing in array two by name.

arr1 = [{name: "krishna", age: 27}, {name: "chandan", age 30}]
arr2 = [{name: "krishna", age: 27}, {name: "chandan", age 30}, {name: "someone", age: 30}]

arr1 intersec arr2 = [{name: "krishna", age: 27}, {name: "chandan", age 30}]

First I tried to iterate over the arrays and find the similar objects by name like below

var intersec = [];
for(var i = 0; i < arr1.length; i++) {
  for(var j = 0; j < arr2.length; j++) {
    if(arr1[i].name === arr2[j].name) {
      intersec.push(arr1[i]);
    }
  }
}

Later I thought this code will become complicated if the data size grows.

So I came up with another logic where I stringify arr1 and iterate over arr2 and get the name and use indexOf function on the stringify-ed arr1 to check if it exists.

var exp = "name: \""+arr2[i].name+"\"";
stringArr1.indexOf(exp);

I want to know which one is efficient and are there any other efficient ways to do the same?

KJ Price · Accepted Answer · 2015-03-17 14:40:00Z

2

You are correct when talking about efficiency, what you have above is a O(n^2) level of complexity as you have two "nested" loops. You want to run through each loop just one time.

First, I'd loop through the second array and create an object of this so that you can easily search by name:

var obj2 = {};
for (var i = 0; i < arr2.length; i++) {
    obj2[arr2[i].name] = arr2[i];
}

Then, loop through the first array and see if the name property exists in the new obj2 object we created:

var intersec = [];
for (var i = 0; i < arr1.length; i++) {
    if (obj2[arr1[i].name]) {
        intersec.push(arr1[i]);
    }
}

This solution will give you a O(2n) level of complexity...Much more efficient!

answered Mar 17, 2015 at 14:40

KJ Price

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4 Comments

Krishna Chaitanya Over a year ago

As far as I know, when we ask obj2[arr1[i].name] the javascript engine also does an iteration over the keys in obj2 and returns the value. So I think I am iterating (arr2.length + (arr1.length * obj2_key_size)) times. which is again complex. Am I right?

Alex Pakka Over a year ago

This is the right answer :) Small possible improvement: check which of the arrays is smaller in size and put it into a hash map (your obj2) then iterate over the longer array, because building hash map is typically slower. Also one could stop once intersec reaches the size of smaller array in some cases.

Alex Pakka Over a year ago

@KrishnaChaitanya: Incorrect. obj2["key"] is a hash map access operation. It takes building a hash (integer) out of "key" and accessing array slot at position hash("key") % mapSize and then maybe iterating over couple of elements if there are collisions. So it is very fast with near constant access times.

KJ Price Over a year ago

Nice, a few things I did not know there. Thanks @AlexPakka.

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