While, smallest floating point number

In addition to the problems that Griff mentioned, the final output wouldn't work anyway. If last is 0, multiplying it by 2 will still give you 0.

You'll need to use two variables so that you can keep a copy of the previous step.

float last = 1.0;
float temp = last / 2.0;
while (temp > 0.0)
{
    last = temp;
    temp = last / 2.0;
}

printf("%f", last);

Quote:

i don't know, why its like that, why it doesn't work...

Your code do not behave the way you expect, or you don't understand why !

There is an almost universal solution: Run your code on debugger step by step, inspect variables.
The debugger is here to show you what your code is doing and your task is to compare with what it should do.
There is no magic in the debugger, it don't know what your code is supposed to do, it don't find bugs, it just help you to by showing you what is going on. When the code don't do what is expected, you are close to a bug.
To see what your code is doing: Just set a breakpoint and see your code performing, the debugger allow you to execute lines 1 by 1 and to inspect variables as it execute.

Debugger - Wikipedia, the free encyclopedia[^]

Mastering Debugging in Visual Studio 2010 - A Beginner's Guide[^]
Basic Debugging with Visual Studio 2010 - YouTube[^]

1.11 — Debugging your program (stepping and breakpoints) | Learn C++[^]

The debugger is here to only show you what your code is doing and your task is to compare with what it should do.

float x = 1.f;

while (x / 2.f > 0.f)
  x /= 2.f;

printf("%g\n", x);