Why Is Comparing if an Unsigned Int >= 0 a “Pointless Comparison”?

Question

I got warning:

Pe186 \"Pointless comparison of unsigned int with zero\"

when I tried to compile the following code:

Answer 1

clLoop >= 0 is always true. It doesn't matter whether you pre-decrement or post-decrement, an unsigned value is at least 0. When you decrement 0 you get UINT_MAX.

The compiler figures that you probably don't mean to loop forever (or you'd have used a different construct, that more obviously loops forever), hence the warning.

Answer 2

You check whether the unsigned int is greater than or equal (>=) zero. This expression will always be true, because unsigned integers will never be less than zero.

The compiler tries to warn you that you are about to program an infinite loop.

Answer 3

An unsigned integer never falls below 0 even after decrementing infinitely (i.e. clLoop >= 0 will always be true), which makes the comparison pointless.

Answer 4

As already stated by others unsigned int is always >= 0 i.e. pointless comparison. Mayank Jindal posted for me the starting point for a solution.

To document the "fix" as a whole: do the following instead, this works as intended:

unsigned int cpLoopStart = 23U;
unsigned int clLoop = 0U;
for(clLoop = cpLoopStart; clLoop > 0U; clLoop--}
{
    //Do something
}

clLoop will start at 23 and end at 1 then exits the loop. I hope it was intended that clLoop will never be 0 inside the loop.

I also prefer to append "U" for unsigned typed values, this makes is clearer also when comparing values and variables.

Answer 5

gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-11) in Centos 7 x86_64 does not give the warning.

But when loop index is decremented to -1, loop index is implicitly converted to a value which is equal to UINT_MAX (limits.h)

UINT_MAX + 1u is equal to 0. 0 - 1 is equal to UINX_MAX.

limits.h Various platform-dependent constants proposed by ANSI

One of alternative solution is:

unsigned int clLoop, i;

for(i = cpLoopStart+1, clLoop = i-1; i > 0; i--, clLoop = i-1)                                  
{
    //Do something
}

i will change in the range [1, cpLoopStart+1]

clLoop will change in the range [0, cpLoopStart]

Answer 6

You are checking if an unsigned int is equal or greater than 0. Which is always true.