# https://www.educative.io/courses/learn-html-css-javascript-from-scratch/gxZAOwDB7pZ#Exercise- more quetions

## Steel, have difficulty in understanding row 6, 7, 8, 9: what is the different between numbers.length and numbers[i].length? what is: numbers[i][j]} on row 7? sums[i] on row 8? and sum = 0 on row 9.

Type your question above this line.

Hey Masha!

• `numbers.length` is the length of the array.

• `numbers[i].length` is the length of the ith sub-array.

• On row 6

for (j = 0; j< numbers[i].length;j++){

We iterate the whole sub-array using sub-array’s length property `numbers[i].length`.

• On row 7

sum = sum + numbers[i][j]

We add each element of sub-array in each iteration. If we talk about the `i=0` then in the first iteration, number will be added. In second, number will be added.In third, number will be added. In fourth, number will be added.
After four iterations, we will get out of the loop as for i=0 the numbers[i].length is four, and the `sum` will have the sum of all elements of ith sub-array.

• On row 8

sums[i] = sum;

We store the calculated sum of all elements of ith sub-array on the ith position of a new array `sums` that we have declared earlier.

• On row 9

sum = 0;

We reset the `sum` variable to use it for the next ith iteration (the next sub-array).

Thank you for asking the question.