For loop inside another for loop

Question

Can't wrap my head around why this piece of JavaScript code outputs 100.

var num = 0;

for (var i=0; i < 10; i++) {
    for (var j=0; j < 10; j++) {
        num++;
    }
}

alert(num); // 100

Answer 1

The inner loop runs 10 times for every iteration of the outer loop. Since the outer loop runs 10 times as well, the inner loop runs 10 x 10 times, which is 100 .

Consider one iteration of the inner loop. It runs 10 times, right? Since num is 0 initially, after the inner loop runs the first time, num will be 10 . I mentioned earlier that the inner loop runs 10 times for each iteration of the outer loop, so after each iteration of the outer loop, num is basically incremented by 10 . This means that you eventually end up with num equal to 100 .

Answer 2

Try running this, you can watch the behavior of i , j , and num :

var num = 0;
for (var i = 0; i < 10; i++) {
    for (var j = 0; j < 10; j++) {
        alert("i = "+i+", j = "+j+", num = "+num);
        num++;
    }
}
alert(num);

i iterates 10 times, j iterates to 10 i times, and so num is incremented 100 times (10 * 10)

Answer 3

Because you have nested for loops. So the the inner 10 will execute 10 times. 10^2 = 100.

Answer 4

It does the inner loop 10 times.

    for (var j=0; j < 10; j++) {
        num++; // 10
    }

Then it does it 10 more times because it's in a for loop.

for (var i=0; i < 10; i++) {
   // each time this runs you will add 10.
}

In the end the inner for loop will run 100 times. 10 times every time it is called. It's called 10 times. 10 * 10 = 100.

Answer 5

For loops loop x times.

var count = 0;
for(var i = 0; i < 10; i++)
    count++;
//count = 10

So Sequential loops ADD

var count = 0;
for(var i = 0; i < 7; i++)
    count++;
for(var i = 0; i < 10; i++)
    count++;
//count = 17 // 7 (first) + 10 (second)

But loops Within loops MULTIPLY (because each INNER loop is executed for each OUTER loop)

var count = 0;
for(var i = 0; i < 4; i++)
    for(var i = 0; i < 10; i++)
        count++;
//count = 40 // 4 (outer) * 10 (inner)

So to calculate loop iterations multiply inner by outer, or for sequential add the first then the second

Answer 6

The inner loop is executed 10 times. The inner loop executes 10 times. The inner loop execution add 1 to num; 10*10 of adding 1 yields 100