CODEWITHSUNDEEP
carrays
Andre
Andre

Reputation: 23

In C what is the difference between a normal array and one declared with #define?

I just started learning C and I was wondering if there is a difference between an array declared like this one: int m[10][5]; and an array declared like this:

#define NL 10
#define NC 5
int m[NL][NC];

I tought that the memory allocated might be different,but i'm not really sure.

Upvotes: 1

Views: 86

Answers (2)

zanedp
zanedp

Reputation: 427

The #define's are "preprocessor directives". The C preprocessor (CPP) is basically a program that is run by your compiler before actually processing your C code.

For your #define's, the preprocessor basically does a "search and replace" for NL and NC, replacing with 10 and 5, respectively. It does this without much understanding of C. (In fact, you could run the C preprocessor on any kind of text file and have it do the same search and replace.)

To get gcc or clang to stop after running the preprocessor, use the -E option and it'll write the preprocessor results to stdout.

Here's an example of having GCC stop after running the preprocessor on your source code:

$ gcc -E andre.c
# 1 "andre.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "andre.c"


int m[10][5];

As you can see, after running the preprocessor, NL and NC have been replaced with the text you supplied in their #define's.

If you're using a compiler other than gcc or clang, check its documentation for how to stop processing after running the preprocessor. Often, preprocessed source code has the extension .i.

(On my machine, cpp andre.c does the exact same thing as the call to gcc above.)

Upvotes: 1

Mohamed Hassan
Mohamed Hassan

Reputation: 11

The C/C++ macros will be replaced by the compiler with inline code and there is no difference between declaration with #define and the inline code.

Upvotes: 1

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