How to reverse a negative integer in javascript?

Question

So I came across a problem "How to reverse an integer in javascript?" I successfully managed to reverse the positive numbers for eg if I enter 123 then I get the output 321 but on the hand, if I am trying some negative number like -123 then I get 0 as the output. How can I solve this issue and get output as -321?

var reverse = function(x){
    let a = 0;
    while(x>0){
        a = a * 10 + x%10; 
        // 0 = 0 *10 + 123%10 = 3
        // a=3
        // 0 = 0 *10 + 12%10 = 2
        // a=2
        // 0 = 0*10 + 1%10 = 1
        // a=1
        x = Math.floor(x/10)
         //  x = 123/10 = 12
        //  x = 12/10 =1
    }
    console.log(a);
    return a;
}
var x = -123;
reverse(x)

while (x > 0) will not really work well if x is negative. (note that the reverse of a negative x is simply -reverse(-x)) — user5734311
– user5734311, Commented Apr 16, 2022 at 7:40
if you didn't want to change your logic you could use abs on top of your input and deal with the negative sign at the end — Diego D
– Diego D, Commented Apr 16, 2022 at 7:43
convert negative integer to positive by multiplying with -1, then reverse then convert it back to negative. — Stan Mayan
– Stan Mayan, Commented Apr 16, 2022 at 7:44

Chetan Ahirrao · Accepted Answer · 2022-04-16 07:46:00Z

3

Just Preserve the sign of an integer, like

var reverse = function(x){
    let sign = x<0?-1:1;
    let a = 0;
    x=Math.abs(x);
    while(x>0){
        a = a * 10 + x%10; 
        // 0 = 0 *10 + 123%10 = 3
        // a=3
        // 0 = 0 *10 + 12%10 = 2
        // a=2
        // 0 = 0*10 + 1%10 = 1
        // a=1
        x = Math.floor(x/10)
         //  x = 123/10 = 12
        //  x = 12/10 =1
    }
    console.log(sign*a);
    return a;
}
var x = -123;
reverse(x)

answered Apr 16, 2022 at 7:46

Chetan Ahirrao

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Comments

Stan Mayan · Accepted Answer · 2022-04-16 07:46:44Z

1

Convert negative integer to positive by multiplying with -1, then reverse then convert it back to negative

var reverse = function(x){
    let isNegative=x<0;
    x=Math.abs(x);
    let a = 0;
    while(x>0){
        a = a * 10 + x%10; 
        // 0 = 0 *10 + 123%10 = 3
        // a=3
        // 0 = 0 *10 + 12%10 = 2
        // a=2
        // 0 = 0*10 + 1%10 = 1
        // a=1
        x = Math.floor(x/10)
         //  x = 123/10 = 12
        //  x = 12/10 =1
    }
    console.log(a);
    return isNegative ? a* -1 : a;
}

answered Apr 16, 2022 at 7:46

Stan Mayan

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Comments

Livin Sunny · Accepted Answer · 2022-04-16 08:03:30Z

let reverse = function(x){
    let a = 0;
    while(true){
        a = a * 10 + x%10; 
        // 0 = 0 *10 + 123%10 = 3
        // a=3
        // 0 = 0 *10 + 12%10 = 2
        // a=2
        // 0 = 0*10 + 1%10 = 1
        // a=1
        x = x > 0 ? Math.floor(x/10) : Math.ceil(x/10);
        //  x = 123/10 = 12
        //  x = 12/10 =1
      if(x === 0){
        break;
      }
    }
    console.log(a);
    return a;
}
let x = -123;
reverse(x);

Explanation :

Changed while loop condition from x>0 to true to enable entering into loop when the argument is less than or equal to zero
Math.floor() round the number to the nearest smaller integer. Which means

Math.floor(10.5); Console output : 10

Math.floor(-10.5); Console output : -11

Incorporated the logic using Math.ceil() function when the number is less than zero.

zoe bluberry · Accepted Answer · 2022-04-16 08:03:54Z

0

parse to string -> reverse -> parse to number back

function reverse(num){
  return num < 0 ? 
    ((num * -1)+'').split('').reverse().join('') *-1 
    : (num+'').split('').reverse().join('') *1
}

answered Apr 16, 2022 at 8:03

zoe bluberry

215 bronze badges

Comments

Siva · Accepted Answer · 2022-04-16 08:56:20Z

0

var negativeNumber = -9813;

function convertToPositive(number) {
  if (number < 0) {
    var positiveNum = Math.abs(number);
    var stringNum = positiveNum.toString();
    var numArray = stringNum.split('');
    var reverseNum = numArray.reverse();
    var reverseString = reverseNum.join('');
    var reverseNum = parseInt(reverseString, 10);
    var negativeReverseNum = reverseNum * -1;
    return negativeReverseNum;
  }
}

convertToPositive(negativeNumber);

answered Apr 16, 2022 at 8:56

Siva

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