Question 53558 in Javascript Question posted in
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Multiplying by -1 not working to return a negative number – Javascript

The following line in the swapDigits function below does not seem to work to convert the number a negative number.

swappedNumber = Number(numberStringArray.join("")) * -1;

If I multiply a positive number by -1 in the console it correctly makes the number negative but that does not seem to work in the code below. How can I make the swappedNumber negative? Thank you.

// swapDigits 

const swapDigits = (number) => {
  let swappedNumber = 0;

  if (number < 0) {
    number = Math.abs(number);

    const numberString = number.toString();
    console.log(numberString);
    const numberStringArray = [];

    if (numberString.length % 2 == 0) {
      for (let i = 0; i < numberString.length; i = i + 2) {
        numberStringArray.push(numberString[i + 1]);
        numberStringArray.push(numberString[i]);
      }
      swappedNumber = Number(numberStringArray.join(""));
    } else {
      numberStringArray.push(numberString[0]);
      for (let i = 1; i < numberString.length; i = i + 2) {
        numberStringArray.push(numberString[i + 1]);
        numberStringArray.push(numberString[i]);
      }
      swappedNumber = Number(numberStringArray.join("")) * -1;
    }
  } else {
    const numberString = number.toString();
    console.log(numberString);
    const numberStringArray = [];

    if (numberString.length % 2 == 0) {
      for (let i = 0; i < numberString.length; i = i + 2) {
        numberStringArray.push(numberString[i + 1]);
        numberStringArray.push(numberString[i]);
      }
      swappedNumber = Number(numberStringArray.join(""));
    } else {
      numberStringArray.push(numberString[0]);
      for (let i = 1; i < numberString.length; i = i + 2) {
        numberStringArray.push(numberString[i + 1]);
        numberStringArray.push(numberString[i]);
      }
      swappedNumber = Number(numberStringArray.join(""));
    }
  }

  return swappedNumber;
};


const even = swapDigits(-564783);
console.log(even); // logs 657438 instead of -657438

const odd = swapDigits(512364783);
console.log(odd);
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2

Answers


  1. 0

    The first branch of your "if" doesn’t multiply the number by -1:

    if (numberString.length % 2 == 0) {
      for (let i = 0; i < numberString.length; i = i + 2) {
        numberStringArray.push(numberString[i + 1]);
        numberStringArray.push(numberString[i]);
      }
      swappedNumber = Number(numberStringArray.join(""));
}

    So if there’s an even number of digits, you won’t negate the number.

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  2. 0

    You can simplify your logic by capturing the sign, before calling absolute value.

    You can also swap without a temporary variable by using an XOR swap.

    const swapDigits = (n) => {
  const
    sign = n < 0 ? -1 : 1,
    digits = Math.abs(n).toString(10).split('').map(d => parseInt(d, 10));
  for (let i = 0; i < digits.length; i+=2) {
    if (digits[i + 1]) {
      digits[i]     ^= digits[i + 1];
      digits[i + 1] ^= digits[i];
      digits[i]     ^= digits[i + 1];
    }
  }
  return sign * parseInt(digits.join(''), 10);
};

console.log(swapDigits(-564783));   // even = -657438
console.log(swapDigits(512364783)); // odd  = 1532468703
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