零值上的一元运算符 - （） - c ++

Question

I wrote this code to overload the unary operator- on a matrix class:

const RegMatrix RegMatrix::operator-()const{
    RegMatrix result(numRow,numCol);
    int i,j;
    for(i=0;i<numRow;++i)
        for(j=0;j<numCol;++j){
            result.setElement(i,j,(-_matrix[i][j]));
        }

        return result;
}

When i ran my program with debugger in visual studio, it showed me that when the operation is done on a double equals zero, it inserts the result matrix the number -0.00000. Is it some weird VS-display feature, or is it something i should handle carefully?

Answer 1

Signed zero is zero with an associated sign. In ordinary arithmetic, −0 = +0 = 0. However, in computing, some number representations allow for the existence of two zeros, often denoted by −0 (negative zero) and +0 (positive zero). This occurs in some signed number representations for integers, and in most floating point number representations. The number 0 is usually encoded as +0, however it can be represented by either +0 or −0.

The IEEE 754 standard for floating point arithmetic (presently used by most computers and programming languages that support floating point numbers) requires both +0 and −0. The zeroes can be considered as a variant of the extended real number line such that 1/−0 = −∞ and 1/+0 = +∞, division by zero is only undefined for ±0/±0.

Negatively signed zero echoes the mathematical analysis concept of approaching 0 from below as a one-sided limit, which may be denoted by x → 0−, x → 0−, or x → ↑0. The notation "−0" may be used informally to denote a small negative number that has been rounded to zero. The concept of negative zero also has some theoretical applications in statistical mechanics and other disciplines.

It is claimed that the inclusion of signed zero in IEEE 754 makes it much easier to achieve numerical accuracy in some critical problems, 1 in particular when computing with complex elementary functions.[2] On the other hand, the concept of signed zero runs contrary to the general assumption made in most mathematical fields (and in most mathematics courses) that negative zero is the same thing as zero. Representations that allow negative zero can be a source of errors in programs, as software developers do not realize (or may forget) that, while the two zero representations behave as equal under numeric comparisons, they are different bit patterns and yield different results in some operations.

For more information see Signed Zero wiki page.

Answer 2

使用double（ IEEE754 ），定义了正负零。

Answer 3

Well for doubles actually have different values for '0.0' and '-0.0' I think it makes perfect sense....

What different result did you expect?

Answer 4

As ereOn said, you've got a negative zero:

#include <stdio.h>
int main()
{
    printf("%f\n", -0.0);
}

Answer 5

-0 and 0 are the same thing, and it is nothing to worry about. Floating point numbers have the capability to have both a positive and negative 0, for math reasons. But -0 is interpreted the same way as 0 in C/C++ arithmetic.