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I am doing a test and the task is to write a function that takes an integer as input, and returns the number of bits that are equal to one in the binary representation of that number. You can guarantee that input is non-negative.

So i have the following problem : while my code runs ok for the most part and passes all tests when the inputs are 9843520790 and 8989787640 I am getting wrong results. So I hoped in the calc and made this numbers binary and tried couple of console.log(bNum) and what I noticed is that I loose the high byte during the conversion. Meaning that after bNum=(n>>>0).toString(2); the console.log(bNum) returns 11001001100011110010110101110011 for 7676570995 when it should return 111001001100011110010110101110011.

 var countBits = function(n) {
  let bNum=(n>>>0).toString(2);//converts int to base 2 
  console.log(bNum);
  let sum=0;
  for(let i=0;i<=bNum.length-1;i++)
    {
       if(bNum[i]==1)
         sum+=1;
    }
  return sum;
};

I am not asking for a coding solution just someone to explain why this is happening so I can find a wayto fix it.Thanks in advance :)

Κωνσταντίνος Τσιλιλής
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1 Answers1

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I think your issue is n>>>0. I not to familiar with the >>>0 but as far as I know it converts the input to a 32bit unsigned in. The max value for an uint is 4294967295 so those numbers exceed it. Therefore I think some information is lost in the conversion.

 var countBitsOld = function(n) {
  let bNum=(n>>>0).toString(2);//converts int to base 2 
  let sum=0;
  for(let i=0;i<=bNum.length-1;i++)
    {
       if(bNum[i]==1)
         sum+=1;
    }
  return sum;
};
 var countBitsNew = function(n) {
  let bNum=(n).toString(2);//converts int to base 2 
  let sum=0;
  for(let i=0;i<=bNum.length-1;i++)
    {
       if(bNum[i]==1)
         sum+=1;
    }
  return sum;
};

const test = (n) => console.log({old: countBitsOld(n), new: countBitsNew(n)});
test(9843520790);
test(8989787640);
test(76765709950);

.

Vincent Menzel
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  • I'll be damned. That did it. Could you please tell me the difference between (n).toString and (n>>>0) ? – Κωνσταντίνος Τσιλιλής Jul 08 '22 at 07:01
  • You can find the explanation in the thread below. In short the `>>>` operator converts the input into an unsigned int32. If your number is larger than 4294967295 you will have issues. lager https://stackoverflow.com/questions/1822350/what-is-the-javascript-operator-and-how-do-you-use-it – Vincent Menzel Jul 08 '22 at 07:33