Comparing values in multiple arrays

Question

I have two arrays where I need to compare values and get the duplicates. I wrote most of the code but seem to be stumped on the comparison.

Here is my code:

function compare(arr1, arr2) {
 for (var i = 0; i< arr1.length; i++) {
 for (var j = 0; j < arr2.length; j++) {
 if (arr1[i] == arr2[j]) {
 console.log[i];
 }
 }
 }
}
compare([5, 3, 2, 5, 1, 6], [6, 4, 2, 7, 10]);

I get the for loops to print all of the numbers, but for some reason the if statement comparison doesn't work. Is there something I am not getting about comparing values in arrays?

I am not looking for a straight up answer but guidance if possible.

• pptaszek1990: @zzzzBov Sorry that was just a copying error.
• Alex K.: Typo: console.log[i]; -> console.log(i);
• Andy: You don't need a nested loop. Loop over the longest array and for each index check to see (arr2.indexOf(arr1[i]) > -1) if it's in the second array. Output the number to a new array if it is.
• zzzzBov: @Andy, although there are efficient libraries that have solved this problem, I believe OP is trying to figure out how to handle this problem for himself.
• Andy: He said "I am not looking for a straight up answer but guidance if possible." which is what I gave. In a comment.

Answer 1

Your code is quadratic in time since it iterates the second array for each item in the first array. A linear time solution is to convert the first array into a hash table, and then, for each item in the second one, instantly check if it is in the hash.

function intersect(a, b) {
 var hash = {};
 a.forEach(function(x) { hash[x] = 1 });
 
 return b.filter(function(x) { return hash[x] === 1 });
}
c = intersect([5, 3, 2, 5, 1, 6], [6, 4, 2, 7, 10]);
document.write(c)

Do note, however, that this only works if items to compare are primitives, you cannot put objects in a hash, so the code has to be quadratic:

function intersect(a, b) {
 return a.filter(function(x) {
 return b.indexOf(x) >= 0
 });
}
a = {x:'a'};
b = {x:'b'};
c = {x:'c'};
d = {x:'d'};
i = intersect([a,b,c], [a,b,d]);
document.write(JSON.stringify(i));
 

Regarding your bit about improving your current code, I suggest that you make your javascript more idiomatic, in particular,

• get used to iteration methods instead of for loops
• check the repertoire of built-in functions and use them wherever possible
• and, for sanity's sake, never ever use ==

• pptaszek1990: I got your example to work but I am a bit confused as to how it works. Can you direct me to a resource so that I can read up on it? Especially the iterator methods.
• georg: @pptaszek1990: for any Javascript research, MDN is always the starting point. BTW, my naming was incorrect, it's "iteration", not "iterator" methods.