JavaScript: Nesting For Loops (multiplyAll)

[mtahir2020]

Hi guys,

I’m having a real problem understanding this exercise. I KIND of get the nested loops concept, but I don’t get what we are producing by doing the following:

 product = product * arr[i][j]

What’s happening with the arr[i][j] here!

I also have no idea what the actual answer is as we are given three different outcomes, and this one in particular is baffling:

multiplyAll([[5,1],[0.2, 4, 0.5],[3, 9]]) should return 54

Any help appreciated, thank you

Your code so far

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 = product * arr[i][j];
  }
}
// Only change code above this line
return product;
}

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

Challenge: Nesting For Loops

Link to the challenge:

freeCodeCamp.org

[jwilkins.oboe]

Hi @mtahir2020 !

Welcome to the forum!

So let’s focus on this nested array.

@mtahir2020

[[1,2],[3,4],[5,6,7]]

Our task is to get the product of all of the numbers in the subarrays.

These are the sub arrays

[1,2]
[3,4]
[5,6,7]

When i=0 this piece of code arr[i] targets the first sub array [1,2].

We check the condition

Is i < arr.length; Yes

Now we enter the inner loop(or the j loop) and have this code arr[i][j]

That translates to arr[0][0] or the number 1.
That second [0] targets the first element of the subarray because remember arrays are zero based index.

So now we can execute this line of code

product = product * arr[i][j];

translates to

1 = 1*1

Now we increment j by 1 ( j++)
And product is still 1.

We check the condition

Is j < arr[0].length; Yes

Now arr[i][j] turns into arr[0][1]

arr[0][1] is the second element in our first subarray.
The second element is the number 2.

Now we execute this line of code

product = product * arr[i][j];

translates to

2 = 1 * 2;

We now have to increment j by 1 ( j++)
Now j =2
Now we check the condition of the j loop

Is j < arr[0].length?
No, because 2 is not less than 2. This subarray [1,2] only has two elements.

Now we leave the j loop and increment i by 1 (i++)
Now i=1

We check the condition
Is i < arr.length; Yes

Now arr[i][j] is arr[1][0]

Then we go through the same steps as before and always assigning the new result to the product.

Hope that makes sense on how nested arrays work.

[mtahir2020]

Thanks jwilkins for such a thorough and patient explanation, I really appreciate it.

Really helped boost my understanding of this exercise! I just wrote it all out on paper and it worked!