I was working on Leetcode Problem 1770 and ran into some weird behavior that I'm hoping someone can help me understand.
It's a dynamic programming problem that requires you to instantiate a 2-dimensional array of length m. Pretty straightforward.
What's weird is that my syntax for generating a 2D array causes the problem to fail, but an alternative syntax passes all tests. As far as I can tell the two lines output an identical result.
My syntax:
// const m = 3
let memo = new Array(m).fill(new Array(m).fill(-Infinity))
console.log({memo})
// stdout: {
// memo: [
// [ -Infinity, -Infinity, -Infinity ],
// [ -Infinity, -Infinity, -Infinity ],
// [ -Infinity, -Infinity, -Infinity ]
// ]
// }
Alternative syntax:
// const m = 3
let memo = Array.from(Array(m), () => Array(m).fill(-Infinity))
console.log({memo})
// stdout: {
// memo: [
// [ -Infinity, -Infinity, -Infinity ],
// [ -Infinity, -Infinity, -Infinity ],
// [ -Infinity, -Infinity, -Infinity ]
// ]
// }
To my eye the output of the two is exactly identical. Yet one passes all tests and what doesn't. What gives?
In the first case you are filling the array with the same Array instance. In the second case every row is a new Array because you provided a factory function.
const m = 3;
let memo = new Array(m).fill(new Array(m).fill(-Infinity));
console.log(memo[0] === memo[1]); // trueconst m = 3
let memo = Array.from(Array(m), () => Array(m).fill(-Infinity));
console.log(memo[0] === memo[1]); // falseAs a side note, you don't need to pass an Array as the first parameter of Array.from, an ArrayLike is enough:
const m = 3
let memo = Array.from({ length: m }, () => Array(m).fill(-Infinity));
console.log(memo);