A variable is like a box in which you can keep something, and that “thing” is called, the value.

Imagine a program that prints a char(an alphabet letter) in several different ways:

#include <stdio.h>

int main(int argc, char *argv[]){

  printf("The ASCII number for %c is %d.\n", 'A', 'A');

  printf("Soon you'll see how %c can be replaced with another letter!\n", 'A');

  return 0;
}

Personally I use variables only if:

• I need to “recall” the same value more than twice AND the value may get changed.
• Dynamic memory allocations. (Do NOT read about malloc(), it’s not the time yet!)

For declaring(or defining or creating) a variable, we write its data type[1] followed by its name:

#include <stdio.h>

int main(int argc, char *argv[]){

  int my_number;

  return 0;
}

TIP: It’s a good practice to (read it: YOU MUST)set an initial value for your variables.[2] Try this example:

#include <stdio.h>

int main(int argc, char *argv[]){

  int my_monster;
  int my_number = 0;

  printf("my_number is %d while my_monster is %d?!"
    "Run this program again to wonder more!\n", my_number, my_monster);

  return 0;
}

Oops! Welcome to the quantum world of C! probability and unpredictable-ity![3]

After all, variables are called “variables” because their value may vary:

#include <stdio.h>

int main(int argc, char *argv[]){

  int my_number = 0;

  printf("my_number is just created and it holds %d inside.\n", my_number);

  my_number = 5;

  printf("Now it's %d.\n", my_number);

  my_number = 11;

  printf("And here it's %d.\n", my_number);

  return 0;
}

Same story for the characters and you can declare them using the char keyword:

#include <stdio.h>

int main(int argc, char *argv[]){

  char my_character = 'A';

  printf("my_character is just created and it holds %c inside.\n",
    my_character);

  printf("%c is the first letter in alphabet.\n", my_character);

  my_character = 'B';

  printf("my_character is now holding %c.\n", my_character);

  return 0;
}

Rule #1: You can only use alphabet letters(both uppercase and lowercase)

and underscores, but not hyphens.

Rule #2: Variable names can NOT be started with numbers but they may

have some digits in them. (wat?)

Also, when naming your variable, please consider the following “terms”:

• Make some sense.
• Keep it short and meaningful.
• Use plural form of the words whenever it’s needed.
• Follow the same naming convention. (lowercase_with_underscores, camelCase, PascalCase, etc.)

Example of a valid name which is just stupid:

#include <stdio.h>

int main(int argc, char *argv[]){

  int my_number_9 = 127;

  printf("my_number_9 is %d. Such a revelation!\n", my_number_9);

  return 0;
}

While it’s perfectly legal and allowed, it’s a terrible idea and makes no sense. DO NOT DO IT!

Another BAD EXAMPLE of variable naming is using long_long_stupid_names_which_makes_absolutely_no_sense:

#include <stdio.h>

int main(int argc, char *argv[]){

  int i_guess_i_will_use_this_number_somewhere = 15;

  printf("You see? Here I have my lovely %d!\n",
    i_guess_i_will_use_this_number_somewhere);

  return 0;
}

I guess it’s enough of the theory and you gotta try some code on your own. Good luck!

Hint: It’s always a good idea to have a look at how other people are doing it, and personally, I highly recommend the source code of the OpenBSD project or nginx or redis.

[1] To tell the compiler what sort of value we’re going to store in it.

[2] Simply because C does NOT “wipe” or “clean” the old value from your computer’s memory upon the allocation.

[3] To be honest, I see it as a feature, since it does NOT do what I haven’t asked for. Did I ever say “clear that area of RAM which had some values in it before” somewhere in my code? Of course not! :D