Checking whether a number is positive or negative

问题:

I can check whether a number is odd/even using bitwise operators. Can I check whether a number is positive/zero/negative without using any conditional statements/operators like if/ternary etc.

Can the same be done using bitwise operators and some trick in C or in C++?

回答1:

Can I check whether a number is positive/zero/negative without using any conditional statements/operators like if/ternary etc.

Of course:

bool is_positive = number > 0;
bool is_negative = number < 0;
bool is_zero = number == 0;

回答2:

If the high bit is set on a signed integer (byte, long, etc., but not a floating point number), that number is negative.

int x = -2300;  // assuming a 32-bit int

if ((x & 0x80000000) != 0)
{
    // number is negative
}

ADDED:

You said that you don't want to use any conditionals. I suppose you could do this:

int isNegative = (x & 0x80000000);

And at some later time you can test it with if (isNegative).

回答3:

There is a detailed discussion on the Bit Twiddling Hacks page.

int v;      // we want to find the sign of v
int sign;   // the result goes here 

// CHAR_BIT is the number of bits per byte (normally 8).
sign = -(v < 0);  // if v < 0 then -1, else 0. 
// or, to avoid branching on CPUs with flag registers (IA32):
sign = -(int)((unsigned int)((int)v) >> (sizeof(int) * CHAR_BIT - 1));
// or, for one less instruction (but not portable):
sign = v >> (sizeof(int) * CHAR_BIT - 1); 

// The last expression above evaluates to sign = v >> 31 for 32-bit integers.
// This is one operation faster than the obvious way, sign = -(v < 0). This
// trick works because when signed integers are shifted right, the value of the
// far left bit is copied to the other bits. The far left bit is 1 when the value
// is negative and 0 otherwise; all 1 bits gives -1. Unfortunately, this behavior
// is architecture-specific.

// Alternatively, if you prefer the result be either -1 or +1, then use:

sign = +1 | (v >> (sizeof(int) * CHAR_BIT - 1));  // if v < 0 then -1, else +1

// On the other hand, if you prefer the result be either -1, 0, or +1, then use:

sign = (v != 0) | -(int)((unsigned int)((int)v) >> (sizeof(int) * CHAR_BIT - 1));
// Or, for more speed but less portability:
sign = (v != 0) | (v >> (sizeof(int) * CHAR_BIT - 1));  // -1, 0, or +1
// Or, for portability, brevity, and (perhaps) speed:
sign = (v > 0) - (v < 0); // -1, 0, or +1

// If instead you want to know if something is non-negative, resulting in +1
// or else 0, then use:

sign = 1 ^ ((unsigned int)v >> (sizeof(int) * CHAR_BIT - 1)); // if v < 0 then 0, else 1

// Caveat: On March 7, 2003, Angus Duggan pointed out that the 1989 ANSI C
// specification leaves the result of signed right-shift implementation-defined,
// so on some systems this hack might not work. For greater portability, Toby
// Speight suggested on September 28, 2005 that CHAR_BIT be used here and
// throughout rather than assuming bytes were 8 bits long. Angus recommended
// the more portable versions above, involving casting on March 4, 2006.
// Rohit Garg suggested the version for non-negative integers on September 12, 2009. 

回答4:

Or, you could use signbit() and the work's done for you.

I'm assuming that under the hood, the math.h implementation is an efficient bitwise check (possibly solving your original goal).

Reference: http://en.cppreference.com/w/cpp/numeric/math/signbit

回答5:

#include<stdio.h>

void main()
{
    int n;  // assuming int to be 32 bit long

    //shift it right 31 times so that MSB comes to LSB's position
    //and then and it with 0x1
    if ((n>>31) & 0x1 == 1) {
        printf("negative number\n");
    } else {
        printf("positive number\n");
    }

    getch();
}

回答6:

Signed integers and floating points normally use the most significant bit for storing the sign so if you know the size you could extract the info from the most significant bit.

There is generally little benefit in doing this this since some sort of comparison will need to be made to use this information and it is just as easy for a processor to tests whether something is negative as it is to test whether it is not zero. If fact on ARM processors, checking the most significant bit will be normally MORE expensive than checking whether it is negative up front.

回答7:

It is quite simple

It can be easily done by

return ((!!x) | (x >> 31));

it returns

• 1 for a positive number,
• -1 for a negative, and
• 0 for zero

回答8:

// if (x < 0) return -1
// else if (x == 0) return 0
// else return 1
int sign(int x) {
  // x_is_not_zero = 0 if x is 0 else x_is_not_zero = 1
  int x_is_not_zero = (( x | (~x + 1)) >> 31) & 0x1;
  return (x & 0x01 << 31) >> 31 | x_is_not_zero; // for minux x, don't care the last operand 
}

Here's exactly what you waht!

回答9:

Here is an update related to C++11 for this old question. It is also worth considering std::signbit.

On Compiler Explorer using gcc 7.3 64bit with -O3 optimization, this code

bool s1(double d)
{
    return d < 0.0;
}

generates

s1(double):
  pxor xmm1, xmm1
  ucomisd xmm1, xmm0
  seta al
  ret

And this code

bool s2(double d)
{
    return std::signbit(d);
}

generates

s2(double):
  movmskpd eax, xmm0
  and eax, 1
  ret

You would need to profile to ensure that there is any speed difference, but the signbit version does use 1 less opcode.

回答10:

This can not be done in a portable way with bit operations in C. The representations for signed integer types that the standard allows can be much weirder than you might suspect. In particular the value with sign bit on and otherwise zero need not be a permissible value for the signed type nor the unsigned type, but a so-called trap representation for both types.

All computations with bit operators that you can thus do might have a result that leads to undefined behavior.

---

In any case as some of the other answers suggest, this is not really necessary and comparison with < or > should suffice in any practical context, is more efficient, easier to read... so just do it that way.

回答11:

if( (num>>sizeof(int)*8 - 1) == 0 )
    // number is positive
else
   // number is negative

If value is 0 then number is positive else negative

回答12:

A simpler way to find out if a number is positive or negative: Let the number be x check if [x * (-1)] > x. if true x is negative else positive.

回答13:

You can differentiate between negative/non-negative by looking at the most significant bit. In all representations for signed integers, that bit will be set to 1 if the number is negative.

There is no test to differentiate between zero and positive, except for a direct test against 0.

To test for negative, you could use

#define IS_NEGATIVE(x) ((x) & (1U << ((sizeof(x)*CHAR_BIT)-1)))

回答14:

Suppose your number is a=10 (positive). If you shift a a times it will give zero.

i.e:

10>>10 == 0

So you can check if the number is positive, but in case a=-10 (negative):

-10>>-10 == -1

So you can combine those in an if:

if(!(a>>a))
   print number is positive
else 
   print no. is negative 

回答15:

When you're sure about the size of an integer (assuming 16-bit int):

bool is_negative = (unsigned) signed_int_value >> 15;

When you are unsure of the size of integers:

bool is_negative = (unsigned) signed_int_value >> (sizeof(int)*8)-1; //where 8 is bits

The unsigned keyword is optional.