Check if a number x is positive (x>0) by ONLY using bitwise operators in C.

Question

isPositive - return 1 if x > 0, return 0 otherwise

Example: isPositive(-1) = 0.

Legal ops: ! ~ & ^ | + << >>(arithmetic shift)

Max ops: 8

Note: No conditional statements are allowed. size of int is a word(4 bytes). It is a signed representation using two's compliment.

int isPositive(int x) {
  return ???;
}

Answer 1

if your working with a number system that uses the MSB as the signage bit, you can do:

int IsPositive(int x)
{
    return (((x >> 31) & 1) ^ 1) ^ !x;
}

Answer 2

you can do:

int isPositive(int x) {
   return !((x&(1<<31)) | !x);
}

Answer 3

Haven't done assembler for quite a while, but as far as I remember first digit in the word represents negative value e.g. 1000 is -8, hence if most significant bit is 1 the number is negative. So the answer is !(x>>31)

Answer 4

Assuming a two’s complement representation (not always the case!), this can be achieved by testing whether the most significant bit is set (in which case the number is negative).

Notice that the following code uses illegal operations (+, * and -) but these are for clarity and platform independence only. If you know more about your particular platform, e.g. that int is a 32 bit number, the relevant constants can be replaced by their numeric value.

// Will be 1 iff x < 0.
int is_neg = (x & (INT_MAX + 1)) >> (CHAR_BIT * sizeof(int) - 1);
// Will be 1 iff x != 0.
int is_not_zero = !!x;
return !is_neg & is_not_zero;

Answer 5

int isPositive(int x)
{
 return (!(x & 0x80000000) & !!x); 
}

Answer 6

return !((x & 0x80000000) >> 31 | !x);

Answer 7

Let's play with the sign bit: sign(~n) : 1 if n >= 0

To get rid of the case when n is 0: sign(~n + 1) : 1 if n > 0 or n = MIN_INT

So, we want the case when both functions return 1:

return ((~n & (~n + 1)) >> 31) & 1;

Answer 8

int isPositive(int x)

{

 return ( ! (x & ( 1 << 31 ) ) );

}

It will return 1 if given no is +ve and return 0 if given no is -ve

in this function we got sign bit if that is 1 it means no is -ve so we return 0 and if sign bit is 0 it means number is +ve so we return 1 .