c++ different print behavior between values and variables

Question

I tried the following code on mac, the output is kind of strange to me.

float a = 2.225;
float b = 123.235;
printf("round: %.2f %.2f \n", a, b);
printf("round: %.2f %.2f \n", 2.225, 123.235);

output:

round: 2.22 123.24 
round: 2.23 123.23 

g++ --version

Configured with: --prefix=/Library/Developer/CommandLineTools/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 7.3.0 (clang-703.0.31)
Target: x86_64-apple-darwin15.5.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

What are the rounding rules in printing? How I can get same printing result in these two situations? Many thanks.

Answer 1

The first printf() converts the float values to double and then prints the results. Note that the initializations convert the double constants to float. The second passes the double values directly. You can specify float constants with the suffix F — but the rules of C still convert those float constants to double before actually calling printf() because of default argument promotion rules.

#include <stdio.h>

int main(void)
{
    float a = 2.225;
    float b = 123.235;
    printf("round: %.2f %.2f\n", a, b);
    printf("round: %.2f %.2f\n", 2.225, 123.235);
    printf("round: %.2f %.2f\n", 2.225F, 123.235F);

    return 0;
}

Output:

round: 2.22 123.24 
round: 2.23 123.23 
round: 2.22 123.24