Comparison operation on unsigned and signed integers

Question

See this code snippet

int main()
{ 
 unsigned int a = 1000;
 int b = -1;
 if (a>b) printf(\"A is BIG! %d\\n\", a-b);
 else printf(\"a is SMALL! %d\\n\", a         


        

Answer 1

You are doing unsigned comparison, i.e. comparing 1000 to 2^32 - 1.

The output is signed because of %d in printf.

N.B. sometimes the behavior when you mix signed and unsigned operands is compiler-specific. I think it's best to avoid them and do casts when in doubt.

Answer 2

On a typical implementation where int is 32-bit, -1 when converted to an unsigned int is 4,294,967,295 which is indeed ≥ 1000.

Even if you treat the subtraction in an unsigned world, 1000 - (4,294,967,295) = -4,294,966,295 = 1,001 which is what you get.

That's why gcc will spit a warning when you compare unsigned with signed. (If you don't see a warning, pass the -Wsign-compare flag.)

Answer 3

 #include<stdio.h>
 int main()
 {
   int a = 1000;
   signed int b = -1, c = -2;
   printf("%d",(unsigned int)b);
   printf("%d\n",(unsigned int)c);
   printf("%d\n",(unsigned int)a);

   if(1000>-1){
      printf("\ntrue");
   }
   else 
     printf("\nfalse");
     return 0;
 }

For this you need to understand the precedence of operators

1. Relational Operators works left to right ... so when it comes

   if(1000>-1)

then first of all it will change -1 to unsigned integer because int is by default treated as unsigned number and it range it greater than the signed number

-1 will change into the unsigned number ,it changes into a very big number

Answer 4

The hardware is designed to compare signed to signed and unsigned to unsigned.

If you want the arithmetic result, convert the unsigned value to a larger signed type first. Otherwise the compiler wil assume that the comparison is really between unsigned values.

And -1 is represented as 1111..1111, so it a very big quantity ... The biggest ... When interpreted as unsigned.

Answer 5

while comparing a>b where a is unsigned int type and b is int type, b is type casted to unsigned int so, signed int value -1 is converted into MAX value of unsigned**(range: 0 to (2^32)-1 )** Thus, a>b i.e., (1000>4294967296) becomes false. Hence else loop printf("a is SMALL! %d\n", a-b); executed.

Answer 6

Find a easy way to compare, maybe useful when you can not get rid of unsigned declaration, (for example, [NSArray count]), just force the "unsigned int" to an "int".

Please correct me if I am wrong.

if (((int)a)>b) {
    ....
}