Javascript Adding leading zeros works with while loop but not for loop

Question

I'm extracting number from a string and passing it to a function. I want to add 1 to it and then return the string while retaining the leading zeros. I was able to do it using a while loop but not a for loop. The for loop simply skips the zeros.

var addMoreZeros = (numStr)=> {
    let newNumStr = (parseInt(numStr)+1).toString();
    let zerosToAdd = numStr.length - newNumStr.length;
    let zeroPadStr = "";
    let i = 0;
    while (i < zerosToAdd) {
        zeroPadStr += "0";
        i++;
    }
   //This doesn't work
   //for(let i = 0; i++; i < zerosToAdd) {
   //   zeroPadStr+="0";
   //}
    return zeroPadStr + newNumStr;
}

`for (let i = 0; i < zerosToAdd; i++)`. Also, note that declaring `i` two times is not needed for the `for`. — Marco Bonelli, Nov 19 '18 at 17:53
@MarcoBonelli - I assumed the initial `let i = 0;` was part of the `while` solution. — T.J. Crowder, Nov 19 '18 at 17:59
Ha yes, thanks for pointing that. Yes, I was just testing it in the console. — l0h1t, Nov 19 '18 at 17:59
FYI: Zero-padding might be done in much simpler [way](https://stackoverflow.com/a/9744576/2118955), without loops. — hindmost, Nov 19 '18 at 18:00
Are you familiar with [`padStart()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/padStart)? — Patrick Roberts, Nov 19 '18 at 18:04

dotconnor · Accepted Answer · 2018-11-19T18:09:16.410

1

You have your for loop syntax wrong, it should be:

(initializer; condition; increments / decrements)

so:

for(let i = 0; i < zerosToAdd; i++) {}

var addMoreZeros = (numStr)=> {
    let newNumStr = (parseInt(numStr)+1).toString();
    let zerosToAdd = numStr.length - newNumStr.length;
    let zeroPadStr = "";
    for(let i = 0; i < zerosToAdd; i++) {
      zeroPadStr+="0";
    }
    return zeroPadStr + newNumStr;
}
console.log(addMoreZeros("00125"))

edited Nov 19 '18 at 18:09

answered Nov 19 '18 at 17:56

dotconnor

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Fun fact: In the third part of loop syntax you can insert any code (not onyl incrementation or decrementation). So this code could look like this: `for(let i = 0; i < zerosToAdd; i++, zeroPadStr+="0") {}` But please don;t do this, it's unreadable ;) – Konrad Nov 19 '18 at 23:44

score 0 · Answer 2 · edited Nov 19 '18 at 18:14

0

Alternative approach (instead of using for/while):

var addMoreZeros = (numStr, numLength = numStr.length) => `${++numStr}`.padStart(numLength,'0');

// example of usage
console.log(addMoreZeros("124", 5));

Explanation ++numStr coerces the numeric string to a number and increments it, then `${...}` casts the number back to a string. Finally padStart() adds leading zeros.

edited Nov 19 '18 at 18:14

Patrick Roberts

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answered Nov 19 '18 at 17:55

Kamil Kiełczewski

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1

`(numStr, numLength = numStr.length) => ...` is probably closer to what OP wants. Also this isn't code golf, no need to use cryptic expressions like `((+numStr+1)+'')`. I think `\`${parseInt(numStr) + 1}\`` is a bit more readable there. I didn't downvote, just giving feedback. – Patrick Roberts Nov 19 '18 at 18:08
Thanks. We can also use ++numstr here. – l0h1t Nov 19 '18 at 18:09

Javascript Adding leading zeros works with while loop but not for loop

2 Answers2