Data Types, Variables, Operators and printf/scanf in C

In C, every variable has a type that decides how many bytes it uses and what values it can hold.

1. Declaring variables

#include <stdio.h>

int main(void) {
    char grade = 'A';
    int age = 21;
    float price = 99.5f;
    double pi = 3.14159265;
    printf("%c %d %.1f %.4f\n", grade, age, price, pi);
    printf("char=%zu int=%zu double=%zu\n", sizeof(char), sizeof(int), sizeof(double));
    return 0;
}

A 21 99.5 3.1416
char=1 int=4 double=8

• sizeof gives the size in bytes (%zu prints it).
• Uninitialised local variables contain garbage — always give a starting value.

Format specifiers

%.2f = 2 decimal places, %5d = width 5 (right-aligned), %-5d = left-aligned.

2. Reading input with scanf

#include <stdio.h>

int main(void) {
    // Input: 12 30
    int a, b;
    scanf("%d %d", &a, &b);
    printf("sum = %d\n", a + b);
    return 0;
}

sum = 42

Note the & before each variable: scanf needs the address where it should store the value. Forgetting & is one of the most common C bugs (strings are the exception — covered later).

3. Arithmetic and integer division

#include <stdio.h>

int main(void) {
    printf("%d %d %d\n", 17 / 5, 17 % 5, -17 / 5);
    printf("%.2f\n", 17 / 5.0);
    printf("%.2f\n", (float)17 / 5);
    printf("%d\n", 7 / 2 * 2);
    return 0;
}

3 2 -3
3.40
3.40
6

• int / int cuts the decimal part (towards zero: −17/5 = −3).
• Make one side a float (5.0 or a cast) to get decimals.
• 7 / 2 * 2 = 3 * 2 = 6, not 7.

4. Increment and decrement

#include <stdio.h>

int main(void) {
    int i = 5;
    int a = i++;   /* use 5, then i becomes 6 */
    int b = ++i;   /* i becomes 7, then use 7 */
    printf("%d %d %d\n", a, b, i);
    return 0;
}

5 7 7

Never write i = i++ or use i++ twice in one expression (i++ + i++). That's undefined behaviour in C — different compilers give different answers.

5. Relational, logical and other operators

• Relational: == != > < >= <= → result is 1 (true) or 0 (false).
• Logical: &&, ||, ! — with short-circuit: in a && b, if a is false, b is never evaluated.
• Assignment: += -= *= /= %=
• Ternary: cond ? x : y
• Bitwise: & | ^ ~ << >> (x << 1 doubles x).

#include <stdio.h>

int main(void) {
    int x = 5;
    printf("%d %d %d\n", x > 3, x == 4, !x);
    printf("%d %d %d\n", 6 & 3, 6 | 3, 6 ^ 3);
    printf("%d %d\n", 5 << 2, 20 >> 1);
    int y = 0;
    if (x > 10 && ++y) { }
    printf("y = %d\n", y);
    return 0;
}

1 0 0
2 7 5
20 10
y = 0

(++y never ran because x > 10 was false — short-circuit.)

6. Constants and type casting

#define PI 3.14159      /* macro: text replaced before compiling */
const int MAX = 100;    /* a read-only variable */
int r = (int)9.99;      /* explicit cast → 9 */

Common mistakes

• Forgetting & in scanf.
• Expecting 5 / 2 to be 2.5.
• Using %d to print a float (garbage output).
• Using = instead of == in conditions (if (x = 5) is always true!).

Quick recap

• Types: char (1), int (4), float (4), double (8) bytes typically; check with sizeof.
• printf/scanf use format specifiers; scanf needs &.
• int/int truncates; % gives the remainder.
• i++ vs ++i; never modify a variable twice in one expression.
• Relational results are 1 or 0; && and || short-circuit.