为什么浮点比较在不同的编译器上给出不同的输出结果？

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问题描述

我正在阅读

I was reading this. It contains following C program.

#include<stdio.h>
int main()
{
    float x = 0.1;
    if (x == 0.1)
        printf("IF");
    else if (x == 0.1f)
        printf("ELSE IF");
    else
        printf("ELSE");
}

The article says that

But I tried it on different compilers & getting different outputs:

Here is the outputs on various IDEs.

1) Orwell Dev C++: ELSE

2) Code Blocks 13.12: ELSE IF but it gives following warnings during compilation.

Why this comparison is unsafe?

3) Ideone.com: ELSE IF (see run: http://ideone.com/VOE3E0)

4) TDM GCC 32 bit: ELSE IF

5) MSVS 2010: ELSE IF but compiles with warning

What is exactly happening here? What's wrong with the program? Is it implementation defined behavior occurring?

Please help me.

解决方案

A floating point number maybe represented in the following form:

So there will be rounding or relative errors when the space is less in memory.

From wiki:

The IEEE 754 standard specifies a binary32 as having:

Sign bit: 1 bit
Exponent width: 8 bits
Significand precision: 24 bits (23 explicitly stored)

Larger (more bits) floating point representations allow for greater precision.

Floating point math is not exact. Simple values like 0.1 cannot be precisely represented using binary floating point numbers, and the limited precision of floating point numbers means that slight changes in the order of operations can change the result. A must read:

What Every Computer Scientist Should Know About Floating-Point Arithmetic

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