Difference between preprocessor directive #if and normal if

Question

What is difference between preprocessor directive #if and normal if in C? I'm new to C.

Answer 1

Statements with # in front of them are called preprocessor directives. They are processed by a parser before the code is actually compiled. From the first search hit using Google (http://www.cplusplus.com/doc/tutorial/preprocessor/):

Preprocessor directives are lines included in the code of our programs that are not program statements but directives for the preprocessor. These lines are always preceded by a hash sign (#). The preprocessor is executed before the actual compilation of code begins, therefore the preprocessor digests all these directives before any code is generated by the statements.

So a #if will be decided at compile time, a "normal" if will be decided at run time. In other words,

#define TEST 1 #if TEST printf("%d", TEST); #endif 

Will compile as

printf("%d", 1); 

If instead you wrote

#define TEST 1 if(TEST) printf("%d", TEST); 

The program would actually compile as

if(1) printf("%d", 1);