Variables and Data Types

In C every variable has a fixed type that you write when you declare it. The type tells the compiler how much memory to reserve and how to interpret the bits stored there.

#include <stdio.h>

int main(void) {
    int students = 42;
    float height = 1.68f;
    double pi = 3.14159265;
    char grade = 'A';

    printf("students = %d\n", students);
    printf("height = %.2f m\n", height);
    printf("pi = %.5f\n", pi);
    printf("grade = %c\n", grade);
    return 0;
}

students = 42
height = 1.68 m
pi = 3.14159
grade = A

Common types

• int holds whole numbers, usually from about minus two billion to plus two billion.
• float holds decimals with about 7 digits of precision.
• double holds decimals with about 15 digits of precision, and is the usual choice.
• char holds a single character, written in single quotes like 'A'.

You can check how many bytes a type uses with sizeof. The exact sizes depend on your machine, but these are typical on a modern computer:

#include <stdio.h>

int main(void) {
    printf("int: %zu bytes\n", sizeof(int));
    printf("double: %zu bytes\n", sizeof(double));
    printf("char: %zu byte\n", sizeof(char));
    return 0;
}

int: 4 bytes
double: 8 bytes
char: 1 byte

Integer division

When both sides of / are integers, C throws away the fractional part. Make at least one side a decimal to get a decimal result.

#include <stdio.h>

int main(void) {
    int a = 7, b = 2;
    printf("%d / %d = %d\n", a, b, a / b);
    printf("%d %% %d = %d\n", a, b, a % b);
    printf("as double: %.1f\n", (double)a / b);
    return 0;
}

7 / 2 = 3
7 % 2 = 1
as double: 3.5

Constants

Add const to a declaration when a value should never change. The compiler will refuse any later attempt to modify it.

#include <stdio.h>

int main(void) {
    const int DAYS_IN_WEEK = 7;
    int weeks = 3;
    printf("%d weeks = %d days\n", weeks, weeks * DAYS_IN_WEEK);
    return 0;
}

3 weeks = 21 days

GATE-style practice questions

#include <stdio.h>
int main(void) {
    int a = 7, b = 2;
    float c = a / b;
    float d = (float)a / b;
    printf("%.1f %.1f\n", c, d);
    return 0;
}

#include <stdio.h>
int main(void) {
    unsigned char c = 250;
    c = c + 10;
    printf("%d\n", c);
    return 0;
}

#include <stdio.h>
int main(void) {
    char c = 'A';
    printf("%zu %zu %zu\n", sizeof(c), sizeof('A'), sizeof(c + 1));
    return 0;
}