嵌套for循环多维数组。 概念上的

Question

I've been working on some exercises on freecodecamp and I am stuck on this nesting for loop exercise. I was able to find the solution, but I don't quite understand.

Can someone explain to me how the second loop with the variable J works? I have read online saying the first for loop is for the outer array and the second one is the inner array, but why stop at two for loops, why not three?

function multiplyAll(arr) { 
    var product = 1;

    // Only change code below this line

    for (var i=0;i<arr.length;i++) {
        for (var j=0;j<arr[i].length;j++) {
            product *= arr[i][j];
        }
    }

    // Only change code above this line
    return product;
}

// Modify values below to test your code
multiplyAll([[1,2],[3,4],[5,6,7]]);

Answer 1

This is very basic logical questions. The ideal way to understand this is the way @Abhinav mentioned in the comment.

// Module that multiplies all the number in 2D array and returns the product
function multiplyAll(arr) { 
   var product = 1;

    // Iterates the outer array
    for (var i=0;i<arr.length;i++) {
        // 1st Iter: [1,2]
        // 2nd Itr: [3,4]
        // 3rd Itr: [5,6,7]
        console.log('i: ' + i, arr[i]);
        for (var j=0;j<arr[i].length;j++) {
            // Outer loop 1st inner 1st : 1
            // Outer loop 1st inner 2nd : 2
            // Outer loop 2nd inner 1st : 3
            // ...
            console.log('j: ' + j, arr[i][j]);

            // Save the multiplication result

            // Prev multiplication result * current no;
            // Outer loop 1st inner 1st : 1 * 1 = 1 
            // Outer loop 1st inner 2nd : 1 * 2 = 2
            // Outer loop 2nd inner 1st : 2 * 3 = 6
            // Outer loop 2nd inner 1st : 6 * 4 = 24
            // ...
            product *= arr[i][j];
       }
    }

    // Only change code above this line
    return product;
}


multiplyAll([[1,2],[3,4],[5,6,7]]);

Run this in browser console. This might give you some clarity.