javascript while循环正确迭代但是对于具有相同逻辑的循环不是，在具有整数值的数组和那里的一些空值

Question

Iterating through a javascript array which has some data in, and some null or not defined values also, is giving funny behaviors with a for loop, but not with a while loop. It is not returning when it should and is stuck in an infinite loop

I have investigated the outputs extensively, the condition whether the number exists in the array is never evaluated to be true , only ever false , but it sometimes enters the if statement region as if it is true. It is seemingly arbitrary.

//function called within this code

function randomArrayOfIndexes() {
    var randNumbArray = new Array(4);
    var indexToAssign = Math.floor(Math.random() * Math.floor(4));
    randNumbArray[0] = indexToAssign;


for (i = 1; i < randNumbArray.length; i++) {
    indexToAssign = Math.floor(Math.random() * Math.floor(4));
    while (arrayContains(randNumbArray, indexToAssign)) {
        indexToAssign = Math.floor(Math.random() * Math.floor(4));
    }
    randNumbArray[i] = indexToAssign;
}

return randNumbArray;
}

//this works

function arrayContains(arrayin, numberIn) {
    var i = arrayin.length;
    while (i--) { //takes one from i so highest index is accurate on first iteration
        if (arrayin[i] === numberIn) {
            return true;
        }
    }
    return false;
}

//this doesn't... not even backwards like the above iteration

function arrayIncludes(arrayin, numberIn) {
    for (i = 0; i < arrayin.length; i++) {
        if (arrayin[i] === numberIn) {
            return true;
        }
    }
    return false;

}

At first each function above is passed in an array with [int value, null, null, null], and a random number; when the function returns, the next null value is filled with the random number that doesn't exist in it already, so [int value, int value, null, null]... until all values are filled... the final array is filled with unique random numbers from 0 to 3, to provide an index for a piece of data in another array... to make sure that it is only used once in the program I am writing.

I would expect it to return true if the number passed in is already in there, another random number then generated outside of the broken function, and the process repeated until a unique random number is found. When it is found, the array being passed back in will be populated at the next available index, and the process repeated. This is not happening. It is getting stuck in an infinite loop, and never returning

Answer 1

you are just missing a var before i :

function arrayIncludes(arrayin, numberIn) {
    for (var i = 0; i < arrayin.length; i++) {
//     in ^ here
        if (arrayin[i] === numberIn) {
            return true;
        }
    }
    return false;

}

You may also declare it before loop, like

var i;
for (i = 0; i < arrayin.length; i++) {
...

By the way, this way of generating random numbers without duplicates is very inefficient, I suggest something like having an array of 0-3 (in your current example) or 0-n and then just randomly taking items out of it. then you don't have to loop through the whole array each time you find a new number. every time you just find a random index between 0 and the length of remaining items.

Imagine that the array length is 1000, and the last item remaining is a number like 100, how many times you have to find a random number and loop through whole array till your random number is 100?

 var n = 5; var a = new Array(n); for(var i=0;i<n;i++) a[i] = i; var result = new Array(n); var i = n; while(i) { var index = Math.floor(Math.random() * i); result[--i] = a[index]; a.splice(index,1); } document.getElementById('a').innerHTML = result; 

 <div id="a"></div> 

Answer 2

You need to declare variables in you loops with for i=0 . if you don't do this the variable is global and when you use the same loop variable in nested loops one can change the other.

You are using i in both loops so when you call the for loop with:

function arrayIncludes(arrayin, numberIn) {
    for (i = 0; i < arrayin.length; i++) {
 // etc 
 }

You set i back to 0 ad iterate it — this is the same i you are using in randomArrayOfIndexes so it interferes with that loop. This is a common cause of hard-to-find bugs and is hy you should always declare loop variables.

Here's the bug in it's simplest form. Notice that the out loop only runs once because i is incremented in the inner loop causing the outloop to exit early:

 for (i = 0; i < 4; i++){ console.log("out loop number: ", i) for (i = 0; i < 4; i++){ console.log("inner_loop: ", i) } } 

If you declare the variables for for let i = , each loop gets its own version of i both loops run independently:

 for (let i = 0; i < 4; i++){ console.log("out loop number: ", i) for (let i = 0; i < 4; i++){ console.log("inner_loop: ", i) } }