When you use this code
const x = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
var y = []
x.forEach(elt => {
for(let i = 0; i < elt; i++){
y.unshift(i)
}
})
console.log(y)it keeps counting down the array 'x' the length of x times. Please explain this to me.
The code is doing exactly what you're asking it to do.
You are running a for-loop for every element in x.
So you are running
for (let i = 0; i < x[0]; i++) ... //i goes from 0 to 0
for (let i = 0; i < x[1]; i++) ... //i goes from 0 to 1
for (let i = 0; i < x[2]; i++) ... //i goes from 0 to 2
for (let i = 0; i < x[3]; i++) ... //i goes from 0 to 3
...
That's why you're getting those 9...0, 8...0, 7...0, and so on.
const x = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
let y = []
x.forEach(elt => {
console.log(`For the element ${elt}`);
//For each element in x, you are executing the following loop:
for(let i = 0; i < elt; i++){
console.log(`unshifting ${i}...`);
y.unshift(i)
}
console.log(`End of each element (${elt}): ${y}`);
})
//if you want each element in x to unshift into y, you need to simply:
const z = [];
x.forEach(elem => z.unshift(elem))
console.log(z);Here is a cleaner way to achieve the same result:
const x = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
const y = []
x.forEach(n => x.slice(0, n).forEach((_, i) => y.unshift(i)))
Code with explanation:
const x = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
const y = []
// For each number in x array:
x.forEach(n =>
// Create a new array from 0 to the current n number:
// [0, ..., n]
x.slice(0, n).forEach((_, i) =>
// And iterate adding the current i index to the beginning of y array
y.unshift(i)
)
)
console.log(y)