如何迭代对象中所有唯一的条目对？

Question

I currently have an array data structure that I iterate over like this, calling foo on each unique pair of elements.

for(var i = 0; i < arr.length; i++) {
    for(var j = i + 1; j < arr.length; j++) {
        foo(arr[i], arr[j]);
    }
}

However, I've realized that I'd rather use an object instead of an array, since I can then add and remove elements by name very easily.

However, I can't see an obvious way to iterate over such an object. The closest I can get is:

for(i in obj) {
    for(j in obj) {
        foo(obj[i], obj[j]);
    }
}

Obviously, this will do each pair twice, and even produce a pair of identical elements. Is there an easy way to iterate over an object in the same way as I do in the array in my first code sample?

Update:

Performance testing the solutions on jsperf .

Answer 1

My solution that was at first written as a comment:

Add an if (i < j) condition in the inner loop. It might not be the best solution, but it would work as long as the foo function does the same thing for foo(2, 10) and foo(10, 2) :

for(i in obj) {
    for(j in obj) {
        if (i < j) {
            foo(obj[i], obj[j]);
        }
    }
}

Answer 2

Assuming I understand your question... maybe check to see if the value has already been visited by the outer loop?

var visited = {}
for(i in obj) {
    visited[i] = true;
    for(j in obj) {
        if(j in visited){ continue; }
        foo(obj[i], obj[j]);
    }
}

Answer 3

Use Object.keys() to get the list of keys out as an array:

keys = Object.keys();
for(i=0;i<keys.length;i++) {
    for(j=i+1;j<keys.length;j++) {
        foo(obj[keys[i]], obj[keys[j]]);
    }
}

Answer 4

Maybe You can try unset used objects:

for(i in obj) {
    var a = obj[i];
    delete obj[i];
    for(j in obj) {
        foo(a, obj[j]);
    }
}

http://jsfiddle.net/bXcvb/

If you need to original obj in tact see: How do I correctly clone a JavaScript object?

Answer 5

You can push the object keys into an array:

var obj_keys = [];
for (i in obj) {
  obj_keys.push(i);
}

for(i = 0; i < obj_keys.length; ++i) {
    for(j = i + 1; j < obj_keys.length; ++j) {
        foo(obj[obj_keys[i]], obj[obj_keys[j]]);
    }
}