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Keywords, Variables and Constants

Keywords

Keywords in C (and any other programming languages) are special reserved tokens that represent a particular functionality or meaning. There are a total of 32 keywords in C.

Keywords
auto break case char const continue default do
double else enum extern float for goto if
int long register return short signed sizeof static
struct switch typedef union unsigned void volatile while

Each Keyword here has a significance of it's own and cannot be used in any other context.

Variables

Variables are user defined tokens that acts as a memory address for to store and fetch data in a program. Variables can be re-assigned to some other data value of the same type during the runtime.

Declaring a variable

// declaration and assignment seperately:
int a;
a = 10;
// declarating and assigning in one line:
int b = 20;

A variable declaration consists of 3 parts: The data type, the variable name and the value of variable.

Rules for naming a variable

  • Name of a variable can contain alphabets (a-z, A-Z), digits (0-9) and underscore (_)
  • Name of a variable must start with an alphabet or underscore
  • A keyword cannot be used as a variable name
  • There should be no space between the characters of the variable name

Declaring and Assigning Seperately v/s During Declaration

A variable can be defined initially with no value set to it to edit later. It is usually used when the program involves input from the user or the value is assigned between the program. For example: 

#include<stdio.h>

void main(){
    // declaration
    int a;
    printf("enter value of a:");
    // reading and assignment
    scanf("%d", &a);
}

The scanf function will be explained in further articles.

Constants

Constant are tokens that act as a memory address that store a value and can be used to fetch it. They are similar to variables except for that they can not be modified once assigned to a value. The naming constraints for constants are same as variable's.

Declaring a constant

Constants can be defined using the const keyword. For example 

const int year = 2024;

The value year here is an constant and can not be changed or reassigned. The compiler shows the following error when we try to reassign it:

Code (const.c) 

#include<stdio.h>

int main(){
    const int year = 2023;
    year = 2024;
    return 0;
}

Error 

sarthak@sarthak ~/D/c_cpp (main)> gcc const.c 
const.c: In function ‘main’:
const.c:5:10: error: assignment of read-only variable ‘year’
    5 |     year = 2025;
      |          ^