function sumDigits; How do I get this function to work with a negative number?

Question

Question:

Write a function called sumDigits.

Given a number, sumDigits returns the sum of all its digits.

var output = sumDigits(1148);
console.log(output); // --> 14

If the number is negative, the first digit should count as negative.

var output = sumDigits(-316);
console.log(output); // --> 4

This is what I currently have coded and it works for positive values but I can't wrap my head around how to tackle the problem when given a negative value. When -316 is put into the function, NaN is returned and I understand that when I toString().split('') the number, this is what is returned: ['-', '3', '1', '6']. How do I deal with combining index 0 and 1?

function sumDigits(num) {
  var total = 0;
  var newString = num.toString().split('');
  for (var i = 0; i < newString.length; i ++) {
    var converted = parseInt(newString[i]);
    total += converted;
  }
  return total;
}

sumDigits(1148);

Any hints on what methods I should be using? and is there a smarter way to even look at this?

Answer 1

This should do it:

    function sumDigits(num) {
      var total = 0;
      var newString = num.toString().split('');
      for (var i = 0; i < newString.length; i ++) {
        if(newString[i]==='-') { //check to see if the first char is -
          i++; //if it is, lets move to the negative number
          var converted =  parseInt(newString[i]); // parse negative number
          total -= converted; // subtract value from total
          continue; // move to the next item in the loop
        }
        var converted = parseInt(newString[i]);
        total += converted;
      }
      return total;
    }

console.log(sumDigits(-316));

Answer 2

One way to do this, is to do a split that will keep the minus and the first digit together, not split.

You can do that with a regular expression, and use match instead of split:

var newString = num.toString().match(/-?\d/g);

function sumDigits(num) {
  var total = 0;
  var newString = num.toString().match(/-?\d/g);
  for (var i = 0; i < newString.length; i++) {
    var converted = parseInt(newString[i]);
    total += converted;
  }
  return total;
}

var result = sumDigits(-316);
console.log(result);

In a bit shorter version, you could use map and reduce, like this:

function sumDigits(num) {
  return String(num).match(/-?\d/g).map(Number).reduce( (a, b) => a+b );
}

console.log(sumDigits(-316));

Answer 3

You could always use String#replace with a function as a parameter:

function sumDigits (n) {
  var total = 0
  n.toFixed().replace(/-?\d/g, function (d) {
    total += +d
  })
  return total
}

console.log(sumDigits(-1148)) //=> 14

Answer 4

Is there a smarter way to even look at this?

You can avoid the conversion from number to string and back by using the modulo operator to extract the last digit. Repeat this step until you got all digits:

function sumDigits(num) {
  let total = 0, digit = 0;
  while (num != 0) {
    total += digit = num % 10;
    num = (num - digit) * 0.1;
  }
  return total < 0 ? digit + digit - total : total;
}

console.log(sumDigits(-316));                    // 4
console.log(sumDigits(1148));                    // 14
console.log(sumDigits(Number.MAX_SAFE_INTEGER)); // 76

Answer 5

function sumDigits(num) {
  let string = num.toString();
  let zero = 0;
  let total = 0;
  for (var i = 0; i < string.length; i++) {
    if (Math.sign(num) === 1) {
      total = zero += Number(string[i]);
    } else {
            for (var i = 2; i < string.length; i++) {
            total = (zero += Number(string[i])) - Number(string[1]);
      }
            }
  }
  return total;
}