Basic JavaScript - Nesting For Loops

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Describe your issue in detail here.
Can someone please describe how the math is done in this exercise? How is product * arr[i][j] calculated. I’m confused on everything in the arrays are mulitplied together.
Your code so far

function multiplyAll(arr) {
  let 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]]);

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Challenge: Basic JavaScript - Nesting For Loops

Link to the challenge:

I’m going to take this question as you are asking what the value of arr[i][j] is each time through the inner loop.

Let’s assume that arr holds the following array:

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

Forget about the [j] for a moment. If i is 0, what is the value of arr[i]?

Its 2 correct right?

Is it? Explain your thoughts

[1, 2] is 0, [3, 4] is 1, and [5, 6, 7] is 2.

If what you mean is that arr[0] = [1,2], arr[1] = [3,4], and arr[2] = [5,6,7], then yes, you are correct.

So when i is 0 then arr[i] is [1,2]. In other words, the 0’th index in the array [[1, 2], [3, 4], [5, 6, 7]] is [1,2].

So what is the 0th index in the array [1,2]? This would be equivalent to arr[i][j] where both i and j are 0. Do you see how that happens?

So is the function multiplying the indexes of i and j or the values inside of the array? So like 0*0 because of the index or is it 1 * 2 * 1 because the 0 index of i is [1, 2] and the 0 index of j is 1.

Let’s look at the code again:

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

You’ve initialized product to 1 before we begin the loops, so we know that the first time we get to that line, it is really going to be:

product = 1 * arr[i][j];

And since it is the first time through both the outer and inner for loop, arr[i][j] is really arr[0][0] which is 1. So the very first time we hit that line it is really:

product = 1 * 1;

So product still equals 1 after the first time through the inner loop.

But there are two elements in the first sub array, so while i is 0, j will first be 0 (as described above) and then the second time through the inner loop it will be 1:

product = product * arr[0][1];

We know that product is 1 at this point, so we can rewrite it as:

product = 1 * arr[0][1];

So what is the value of arr[0][1]? Based on that, what will the value of product be after the inner for loop is completed while i is 0?

1 Like

Would it be

product = 1 * 1 * 2 ? index 1 of [j] is 2. So the value is 2

Correct, the value of arr[0][1] is 2 and so that line would be:

product = 1 * 2;

Which means product will be 2 after going through the first sub array.

So based on what we just walked through, are you able to answer your original question now?

Since arr[i] is the outer array do we use the index value to find the product? So like say [i] is 1, so the [3, 4] index and say that [j] is on the fourth index which is 5. Would it be

product = 1 * 1 * 5
product = 1 * 1 * 4

The value of product isn’t multiplied by the values of i or j.

If i is 1 then arr[i] is [3, 4]. Since there are only two numbers in that array then j can only be either 0 or 1.

product = 1 * 1 * 5

This is never a possibility. The code is:

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

So there can only ever be two numbers on the right of the equals sign. The value of product will be whatever it currently is, and the value of arr[i][j] is the value retrieved from the array at those indices.

I think I understand now. So the first loop of [i] finds the values of [1, 2], [j] multiplies 1 and 2 and gives i [0] a value of 2. Next loop is the 3,4 which is 12, next is 5,6,7 which is 210. Then those values are multiplied together to get 5040.

I’m not quite sure you got it. The value of the items in the array never change. The only value that changes is product as you multiply it with new numbers.

Perhaps I did not read your explanation correctly? It would really help if you used correct syntax in your explanation.

And as @JeremyLT shows below, sometimes you just have to console log it out to see what’s going on :slightly_smiling_face:

function multiplyAll(arr) {
  let product = 1;
  // Only change code below this line
  for (let i = 0; i < arr.length; i++) {
    console.log("i is", i);
    console.log("arr[i] is", arr[i]);
    for (let j = 0; j < arr[i].length; j++) {
      console.log("i is", i);
      console.log("j is", j);
      console.log("arr[i][j] is", arr[i][j]);
      console.log("product is", product);
      product = product * arr[i][j];
      console.log("product * arr[i][j] is", product);
  console.log("final value of product is", product);
  // Only change code above this line
  return product;

Thank you I can see it perfectly now!

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