Take the following function to generate a square multiplication table:
function getMultTable(n) {
/* generate multiplication table from 1 to n */
let table = new Array(n);
for (let i=1; i<=n; i++) {
table[i-1] = new Array(n);
for (let j=1; j<=n; j++) {
table[i-1][j-1] = i*j;
}
}
return table;
}
console.log(getMultTable(3));
// [
// [ 1, 2, 3 ],
// [ 2, 4, 6 ],
// [ 3, 6, 9 ]
// ]Why can't the following be done instead?
function getMultTable(n) {
let table = new Array(n);
for (let i=1; i<=n; i++) {
for (let j=1; j<=n; j++) {
table[i-1][j-1] = i*j;
}
}
return table;
}
In other words, why can't a multi-dimensional array be created on-the-fly in javascript, or is it possible to do that some other way?
Let's start from the basics, why you can't do the following
let arr;
arr[0] = 1
The answer is because arr is undefined. Well, when an array is initialized it is initialized with all its entries undefined. The operation table[i-1][j-1] = i*j is equivalent to
const row = table[i-1];
row[j-1] = i*j
So, when the table is created all its items are undefined, and the statement row[j-1] = i*j, is trying to set a property of row, in other words setting a property of undefined.
The reason is similar to why you can't run this
function getMultTable(n) {
let table;
for (let i=1; i<=n; i++) {
for (let j=1; j<=n; j++) {
table[i-1][j-1] = i*j;
}
}
return table;
}
Only that in this case your problem is trying to read a property from undefined
You can do it something like this:
let arr = new Array(n).fill().map(el => new Array(n).fill());