Why does this output of the same expression from printf differ from cout?

Question

I'm using Visual C++ 2012 and compiling from the command line the following files:

#include <stdio.h> int main() {     printf("%.5f", 18/4+18%4);     return 0; }  

Linking with MSVCRT.LIB rather than LIBCMT to avoid runtime error R6002.
The value that is output is 0.00000 for this program.

However, if I perform the exact same thing in C++

 #include <iostream>  using namespace std;  int main()  {       cout << 18/4+18%4 << endl;       return 0;  } 

Now, it prints out 6, like it should.

What's the difference? Is it to do with the languages themselves (C vs C++) or the output methods (cout vs printf), or is it just a quirk with MSVC?

Answer 1

The expression 18/4+18%4 evaluates to an int, and you are requesting a float. You should always compile with warnings enabled, and pay attention to them (they say a warning is a bug waiting to happen, and they are right).

This is what my compiler (GCC 4.8.1) tells me (and even without enforcing -Wall):

warning: format ‘%.5f’ expects type ‘double’, but argument 2 has type ‘int’ 

On the other hand, the std::cout<< operation is able to deduce the type of your expression and correctly stream it to your screen.