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Control Structure in C

Control structures determine how a program runs its instructions. They allow the program to make decisions, repeat steps, or jump to different parts of the code based on conditions. A control structure is a combination of:
  • Decision-making statements — if, else, switch
  • Looping statements — for, while, do-while
  • Jumping statements — break, continue, goto, return
Control Structure diagram showing three branches: Decision-Making (if, if-else, switch-case), Looping (for, while, do-while), and Jumping (break, continue, goto, return)

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Decision-Making Statements

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1. if Statement

Executes a block only when the condition is true. 
if (condition) {
    // executes if condition is true
}
Example: 
#include <stdio.h>
int main() {
    int age = 18;
    if (age >= 18) {
        printf("Eligible to Vote\n");
        printf("Welcome\n");
    }
    return 0;
}
Output: Eligible to Vote and Welcome
Without curly braces, only the immediately following line is part of the if. The rest always executes.

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2. if-else Statement

if (condition) {
    // true block
} else {
    // false block
}
Example: 
int num = 5;
if (num % 2 == 0) {
    printf("Even number");
} else {
    printf("Odd number");
}
// Output: Odd number

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3. if-else if-else Ladder

Check multiple conditions in sequence: 
int marks = 72;
if (marks >= 90) {
    printf("Grade A\n");
} else if (marks >= 75) {
    printf("Grade B\n");
} else if (marks >= 50) {
    printf("Grade C\n");
} else {
    printf("Fail\n");
}
// Output: Grade C

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4. Nested if

An if inside another if: 
int age = 25;
char citizen = 'Y';
if (age >= 18) {
    if (citizen == 'Y') {
        printf("You can vote in India.\n");
    }
}

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5. switch-case

Use when choosing one option from many based on a single value: 
switch (expression) {
    case constant1:
        // code
        break;
    case constant2:
        // code
        break;
    default:
        // runs if no case matches
}
Rules:
  • Expression must be int, char, or enum (NOT float)
  • Case labels must be constants
  • break stops the switch from falling through to the next case
  • default is optional
Example: 
#include <stdio.h>
int main() {
    int day = 3;
    switch (day) {
        case 1: printf("Monday\n");    break;
        case 2: printf("Tuesday\n");   break;
        case 3: printf("Wednesday\n"); break;
        case 4: printf("Thursday\n");  break;
        case 5: printf("Friday\n");    break;
        default: printf("Weekend\n");
    }
    return 0;
}
// Output: Wednesday
Fall-through (no break): 
int x = 2;
switch (x) {
    case 1: printf("One\n");
    case 2: printf("Two\n");   // no break → continues
    case 3: printf("Three\n"); break;
}
// Output:
// Two
// Three

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Looping Statements

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while Loop

Checks condition before executing the body: 
int i = 1;
while (i <= 5) {
    printf("%d\n", i);
    i++;
}

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do-while Loop

Executes the body at least once, then checks condition: 
int i = 1;
do {
    printf("%d\n", i);
    i++;
} while (i <= 5);

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for Loop

Best when the number of iterations is known: 
for (int i = 1; i <= 5; i++) {
    printf("%d\n", i);
}

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Nested Loops

for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 3; j++) {
        printf("%d %d\n", i, j);
    }
}

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Infinite Loop

while (1) { printf("Running...\n"); }
for (;;) { printf("Running...\n"); }

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Jumping Statements

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break

Exits a loop or switch immediately: 
for (int i = 1; i <= 5; i++) {
    if (i == 3) break;
    printf("%d\n", i);
}
// Output: 1  2

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continue

Skips the rest of the current iteration: 
for (int i = 1; i <= 5; i++) {
    if (i == 3) continue;
    printf("%d\n", i);
}
// Output: 1  2  4  5

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goto

Transfers control to a labeled statement (use sparingly): 
#include <stdio.h>
int main() {
    int num = 2;
    if (num == 2) goto label;
    printf("This will be skipped\n");
label:
    printf("Jumped here using goto\n");
    return 0;
}
// Output: Jumped here using goto

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return

Exits a function and optionally returns a value: 
int add(int a, int b) {
    return a + b;
}
int main() {
    printf("%d\n", add(3, 4));  // Output: 7
    return 0;
}

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exit()

Terminates the entire program immediately. Requires #include <stdlib.h>: 
#include <stdio.h>
#include <stdlib.h>
int main() {
    printf("Start\n");
    exit(0);
    printf("This will not execute\n");
    return 0;
}
// Output: Start