Wrong result when comparing 0.55 and 0.55f

Question

In C, 0.55 == 0.55f is false while 0.5 == 0.5f is true. Why is it different?

1. Comparing 0.55:

   #include <stdio.h>
   int main() {
       if (0.55 == 0.55f)
           printf("Hi");
       else
           printf("Hello");
   }
   

   Outputs Hello.

2. Comparing 0.5:

   #include <stdio.h>
   int main() {
       if (0.5 == 0.5f)
           printf("Hi");
       else
           printf("Hello");
   }
   

   Outputs Hi.

For both the code snippets, I expected Hello.
Why this difference?

Answer 1

0.5 is a dyadic rational and of an appropriate magnitude so 0.5 is exactly one-half either as a float or a double.

The same cannot be said for 0.55. A double will store that number with no less precision than a float, and most likely more.

In both cases, the float is implicitly converted to a double prior to ==, but by then any truncation has taken place.