# 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[0][0] will be added. In second, number[0][1] will be added.In third, number[0][2] will be added. In fourth, number[0][3] 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.