C provides four fundamental data types for storing different kinds of values:

Choosing the Right Data Type

• Use int when storing whole numbers like age, quantity, or count
• Use float or double when storing numbers with decimals like price or temperature
• Use char when storing single characters like grades or initials

A constant is a value that cannot be changed after it is set. We define constants using either the const keyword or the #define macro.

Using the const Keyword

const int MAX_SCORE = 100;    // Cannot be changed later
const float PI = 3.1415;       // Value of PI is locked

Using #define Macro

#define PI 3.1415    // Defines a constant without using memory

Constants are useful for values that should never change, such as the value of Pi or maximum allowed scores.

An operator is a symbol that tells the compiler to perform specific mathematical or logical operations. An expression combines variables, constants, and operators to produce a value.

Arithmetic Operators

Arithmetic operators perform mathematical calculations:

Relational Operators

Relational operators compare two values and return true (1) or false (0):

Logical Operators

Logical operators combine multiple conditions:

Assignment Operators

Assignment operators assign values to variables and can combine with arithmetic:

Increment and Decrement Operators

These operators increase or decrease a variable's value by 1:

int x = 5;
x++;    // x is now 6 (postfix increment)
++x;    // x is now 7 (prefix increment)
x--;    // x is now 6 (postfix decrement)
--x;    // x is now 5 (prefix decrement)

When an expression has multiple operators, C follows precedence rules to determine which operation happens first. Multiplication and division (*, /, %) have higher precedence than addition and subtraction (+, -).

int result = 8 + 4 * 3;   // Result is 20, not 36
                            // Because 4 * 3 is calculated first

Use parentheses () to force a specific order:

int result = (8 + 4) * 3;  // Result is 36

The following program demonstrates variables, data types, constants, and operators working together:

#include <stdio.h>
#define PI 3.1416    // Defining a constant using #define

int main() {
    // Declaring and initialising variables
    int age = 16;
    float height = 5.9;
    char grade = 'A';
    const int MAX_SCORE = 100;    // Constant using 'const'

    // Using arithmetic operators
    int sum = age + 10;           // Addition
    int product = age * 2;        // Multiplication

    // Using relational operators
    if (age >= 18) {
        printf("Adult\n");
    } else {
        printf("Minor\n");
    }

    // Printing output
    printf("Age: %d\n", age);
    printf("Height: %.1f\n", height);
    printf("Grade: %c\n", grade);
    printf("Max Score: %d\n", MAX_SCORE);
    printf("Sum: %d\n", sum);
    printf("Product: %d\n", product);

    return 0;
}

Output:

Minor
Age: 16
Height: 5.9
Grade: A
Max Score: 100
Sum: 26
Product: 32

In everyday life, variables and data types are used in many practical situations in Tanzania. For example, when a shopkeeper uses a simple program to calculate the total price of items, they use int variables for quantities and float variables for prices in Tanzanian shillings (TZS). The program might calculate the total using arithmetic operators like multiplication (*) for quantities and addition (+) for multiple items, then use relational operators to check if the customer gets a discount for purchases above TZS 50,000. This same principle applies to mobile money transactions, school grade calculations, and market inventory management.