I'm doing a learning exercise and am trying to understand the following code. I thought I had a handle on arrays and loops, but this one has got me very confused.
The below code:
function zeroArray(m, n)
{
let newArray = [];
let row = [];
for (let i = 0; i < m; i++)
{
for (let j = 0; j < n; j++)
{
row.push(0);
}
newArray.push(row);
}
return newArray;
}
let matrix = zeroArray(3, 2);
console.log(matrix);
Returns
[ [ 0, 0, 0, 0, 0, 0 ],
[ 0, 0, 0, 0, 0, 0 ],
[ 0, 0, 0, 0, 0, 0 ] ]
However I would have expected it to return
[ [ 0, 0, ],
[ 0, 0, 0, 0, ],
[ 0, 0, 0, 0, 0, 0 ] ]
Given that in each i loop, we are pushing (0) to row[] twice, before pushing row[] into newArray.
This isn't happening though, and in my VSCode debugger it looks as though in each i loop, every existing index of newArray is being updated with the latest version of the row[] array.
Why is this?
You need to make a copy of the array when pushing to newArray:
function zeroArray(m, n) {
let newArray = [];
let row = [];
for (let i = 0; i < m; i++) {
for (let j = 0; j < n; j++) {
row.push(0);
}
newArray.push(row.slice());
}
return newArray;
}
let matrix = zeroArray(3, 2);
console.log(JSON.stringify(matrix));1) Start outer loop with i = 1 upto i <= m, so the loop count will be m
for (let i = 1; i <= m; i++) {
2) You should create a new row every time the inner loop start and push row into newArray after the inner loop ends
3) Set inner loop condition as j < n * i
for (let j = 0; j < n * i; j++) {
function zeroArray(m, n) {
let newArray = [];
// const row = [] // (-)
for (let i = 1; i <= m; i++) {
const row = []; // (+)
for (let j = 0; j < n * i; j++) { // (+)
row.push(0);
}
newArray.push(row);
}
return newArray;
}
let matrix = zeroArray(3, 2);
console.log(matrix);/* This is not a part of answer. It is just to give the output fill height. So IGNORE IT */
.as-console-wrapper { max-height: 100% !important; top: 0; }Matrix m x n should be m rows and n cols.
So for 3, 2 you expect
[
[0, 0],
[0, 0],
[0, 0],
]
Just declare row inside the first loop:
function zeroArray(m, n) {
const newArray = [];
for (let i = 0; i < m; i++) {
const row = [];
for (let j = 0; j < n; j++) {
row.push(0);
}
newArray.push(row);
}
return newArray;
}
let matrix = zeroArray(3, 2);
console.log(matrix);