Unix – configure.in or configure.ac

In practice, the difference is in the location where the preprocessor searches for the included file.

For #include <filename> the preprocessor searches in an implementation dependent manner, normally in search directories pre-designated by the compiler/IDE. This method is normally used to include standard library header files.

For #include "filename" the preprocessor searches first in the same directory as the file containing the directive, and then follows the search path used for the #include <filename> form. This method is normally used to include programmer-defined header files.

A more complete description is available in the GCC documentation on search paths.

• ++i will increment the value of i, and then return the incremented value.

   i = 1;
   j = ++i;
   (i is 2, j is 2)
  

• i++ will increment the value of i, but return the original value that i held before being incremented.

   i = 1;
   j = i++;
   (i is 2, j is 1)
  

For a for loop, either works. ++i seems more common, perhaps because that is what is used in K&R.

In any case, follow the guideline "prefer ++i over i++" and you won't go wrong.

There's a couple of comments regarding the efficiency of ++i and i++. In any non-student-project compiler, there will be no performance difference. You can verify this by looking at the generated code, which will be identical.

The efficiency question is interesting... here's my attempt at an answer: Is there a performance difference between i++ and ++i in C?

As @OnFreund notes, it's different for a C++ object, since operator++() is a function and the compiler can't know to optimize away the creation of a temporary object to hold the intermediate value.