• Open a terminal
• Create your file by writing the following command

vim ex1.c

• Enter insertion mode by pressing Insert
• Write the lines below

#include <stdio.h>
 
int main(){
 
	printf("Hello world !\n");
 
	return 0 ;
}

• Then press Esc key
• Write :wq
• Compile your code with the following line

gcc -Wall ex1.c -o ex1

• Then run your programm by typing “./ex1” in the terminal

• Open a terminal
• Copy the content of the first exercice to the second by writing the following command

cp ex1.c ex2.c

• Open ex2.c with vim

vim ex2.c

• Edit your file so it looks like this :

#include <stdio.h>
 
int main(){
#ifdef DEBUG
	printf("Hello world !\n");
#endif
 
	return 0 ;
}

• Compile your code with gcc

gcc -Wall ex2.c -o ex2

• Run it with “./ex2”
• Nothing should happen, it's normal because you didn't told the compiler to use the preprocessor part. which is defined by the #ifdef #endif quotes
• now recompile your program with the following line

gcc -Wall -DDEBUG ex2.c -o ex2

• Run it again
• Hello world !