Looping through array using splice method infinite loop javascript

Question

When i loop through the array using the splice method, the page just freezes. It looks like i caused an infinite loop. lib.randomInt() works, so that is not the problem.

function() {
return function(string) {
    var arr = string.split("")
    arr.sort();
    for(var i = 0; arr.length;i++){
        arr.splice((i+1),0,lib.randomInt(9));
    }
    var pseudocryptarr = arr.join("");
}
})()("example");

This is from a different file that is placed above the main file in html

var lib = {
factorial: function(num){
  function _factorial(num){
    if(num === 1){
        return 1;
        } else {
            return num*_factorial(num-1);
        }
    }
    console.log(num+"! = " + _factorial(num));
    },
    randomInt: function(int,offset){
        if(offset == undefined || null || NaN){
            offset = 0;
        }
        return Math.floor(Math.random()*int)+offset;
    },
    display: function(m, fn){
        fn(m);
    }
};

You are making the array 1 element longer in each iteration, so your loop's end-condition is never getting closer to being fulfilled. What did you want to achieve? — trincot
– trincot, Commented Jan 21, 2018 at 0:15
My theory is that splice keeps adding elements to the array so that the loop will never end — Tom
– Tom, Commented Jan 21, 2018 at 0:20

user4723924 · Accepted Answer · 2018-01-21 00:24:24Z

1

You've got to loop in reverse when modifying the array itself to avoid corrupting the loop like this...

for (var i=arr.length-1; i>=0; i--){}

answered Jan 21, 2018 at 0:24

user4723924

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1 Comment

Tom Over a year ago

this does seem to be a better way than my answer

trincot · Accepted Answer · 2018-01-21 00:52:20Z

I guess that you wanted to insert a random value after every array element, so that the string "example" would become something like "e5x9a2m4p7l1e3"

There are two issues:

Your for loop has no end condition that will become false. You need to state i < arr.length instead of just arr.length which is always truthy for non-empty arrays.
You add array elements in every iteration, but then also visit them in the next iteration, and from there on you will only be visiting the new inserted values and never get to the next original element that keeps being 1 index away from i. You need to increment i once more. For that you can use ++i instead if i+1 as the splice argument.

So your loop should be:

    for(var i = 0; i < arr.length; i++) {
        arr.splice(++i,0,lib.randomInt(9));
    }

const lib = { randomInt: n => Math.floor(Math.random()*n) };
(function() {
    return function(string) {
        var arr = string.split("")
        arr.sort();
        for(var i = 0; i < arr.length; i++) {
            arr.splice(++i,0,lib.randomInt(9));
        }
        var pseudocryptarr = arr.join("");
        console.log(pseudocryptarr);
    }
})()("example");

Or to save an addition:

    for(var i = 1; i <= arr.length; i+=2) {
        arr.splice(i,0,lib.randomInt(9));
    }

const lib = { randomInt: n => Math.floor(Math.random()*n) };
(function() {
    return function(string) {
        var arr = string.split("")
        arr.sort();
        for(var i = 1; i <= arr.length; i+=2) {
            arr.splice(i,0,lib.randomInt(9));
        }
        var pseudocryptarr = arr.join("");
        console.log(pseudocryptarr);
    }
})()("example");

@Tom, I added the corresponding snippet as proof it does not loop forever.

Tom · Accepted Answer · 2018-01-21 00:43:51Z

I fixed it. I wanted after each character for there to be a number. Using the pre-looped array length and doubling it while iterating twice, means that the splice adds the number after the new number element and then the character.

Edit: My typo was the problem. I didnt even have to use len, just iterate by 2.

for(var i = 0;i < arr.length;i+=2){
  arr.splice((i+1),0,lib.randomInt(9));
}

(function() {
    return function(string) {
        var arr = string.split("")
        arr.sort();
        var len = arr.length
        for(var i = 0;i < len*2;i+=2){
            arr.splice((i+1),0,lib.randomInt(9));
        }
        var pseudocryptarr = arr.join("");
        console.log(pseudocryptarr);
    }
})()("example");

Edit: user4723924 method is better:

(function() {
    return function(string) {
        var arr = string.split("")
        arr.sort();
        for(var i = arr.length;i >= 0;i--){
            arr.splice((i+1),0,lib.randomInt(9));
        }
        var pseudocryptarr = arr.join("");
        console.log(pseudocryptarr);
    }
})()("example");

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