How computer compare two number in bottom layer?

Question

I have written code like follow:

int a = -1;
unsigned int b = 0xffffffff;

if (a == b)
    printf("a == b\n");
else
    printf("a != b\n");

printf("a = %x b = %x\n", a, b);
return 0;

And the result is as follow:

It shows that a and b are equal. So I want to know how the computer make this judgement?

Are you asking how a computer does integer comparisons or how `-1` is the same as `0xffffffff` (in this case)? — OMGtechy, Mar 08 '16 at 11:24
Possible duplicate of [Why the address number is not equal in dec and in hex?](http://stackoverflow.com/questions/16878650/why-the-address-number-is-not-equal-in-dec-and-in-hex) — Jongware, Mar 08 '16 at 11:25
Take a look [HERE](http://stackoverflow.com/a/2085031/3436922) — LPs, Mar 08 '16 at 11:26

Johann Gerell · Accepted Answer · 2016-03-08T12:07:31.480

5

In any arithmetic operation with a signed integer a and unsigned integer b as operands, a will be implicitly cast to unsigned. Since -1 signed in this case is 0xffffffff unsigned, a and b compares equal.

edited Mar 08 '16 at 12:07

answered Mar 08 '16 at 11:24

Johann Gerell

24,991
10
72
122

score 1 · Answer 2 · answered Mar 08 '16 at 11:24

The machine representation of your two values a and b is the same bit pattern (on your particular computer and implementation), so the a == b test is true.

BTW, you should enable all warnings and debug info when compiling (e.g. compile with gcc -Wall -Wextra -g if using GCC...). You'll probably get some warnings, because you probably has hit some undefined behavior. And you could run your code step by step in your debugger (e.g. gdb) and query the values (and their machine representations).

How computer compare two number in bottom layer?

2 Answers2