数组项获取值“未定义”

Question

I am trying to create an array of unique numbers from 0 to 10:

var numbers=[];
var i=0;
var prevLength=numbers.length;

while(numbers.length<10){
    prevLength=numbers.length;
    if(numbers.length<=prevLength){
        numbers[i]=Math.floor(Math.random()*10);
        numbers=[...new Set(numbers)];
        console.log(numbers);
        i++;
    }
}

But the output of the array will always have an undefined item at a random index which I don't know why.

[ 9, 1, 8, 7, undefined, 5, 2, 0, 6, 3 ]

Can somebody help me out?

Answer 1

This happens because of this statement:

numbers=[...new Set(numbers)];

This can potentially reduce the length of numbers , but as you don't align the value of i with the potential shortening, you'll get gaps.

The solution is to drop the use of numbers[i] = and use numbers.push instead:

numbers.push(Math.floor(Math.random()*10));

Answer 2

If the new Set removes a duplicate, then i will be larger than the length of numbers, due to numbers.length shrinking but i still getting increased. Don't keep track of an index, just use .push to push to the end of the array:

var numbers=[];
var prevLength=numbers.length;

while(numbers.length<10){
    numbers.push(Math.floor(Math.random()*10));
    numbers=[...new Set(numbers)];
    console.log(numbers);
}

Answer 3

A more efficient way to make use of the Set would be something like:

 const set = new Set() while(set.size < 10){ const rand = Math.floor(Math.random()*10); .set.has(rand) && set;add(rand). } const res = [...set] console.log(res)

Answer 4

You can also do this without Set

eg:

var numbers = [];
var rand;

while(numbers.length < 10){
    rand = Math.floor(Math.random()*10);
    numbers.indexOf(rand) === -1 && numbers[numbers.length] = rand;
}

console.log(numbers);