Conditional Statements

if-else Constructs

Key points when using if-else:

• Avoid adding semicolons directly after if statements
• Use braces when if or else controls multiple statements
• Nested if: else can combine with another if to form multiple conditions
• Dangling else ambgiuity: else pairs with the nearest preceding if
• An if-else pair counts as a single statement

Relational Operators

• Best practice: place constants on the left side when comparing with variables—this helps catch accidental assignment mistakes early
• Available operators: >, <, >=, <=, ==, !=
• The equality operator == differs from the assignment operator =—confusing them produces bugs
• Avoid chaining relational operators; use logical operators like && instead

Conditional Operator

The ternary operator format: condition ? expression1 : expression2

Logic: if condition evaluates to true, expression1 executes and becomes the result; otherwise expression2 executes. This provides a compact alternative to simple if-else blocks.

int max = (a > b) ? a : b;

Logical Operators

• !: logical NOT—reverses the truth value of an expression
• &&: logical AND—both operands must be true for the result to be true
• ||: logical OR—at least one operand must be true for the result to be true
• Short-circuit evaluation: if the left operand determines the result, the right operand is not evaluated

Example: determining leap years

#include <stdio.h>

int main() {
    int year;
    scanf("%d", &year);
    if ((year % 4 == 0 && year % 100 != 0) || (year % 400 == 0))
        printf("Leap year\n");
    return 0;
}

switch Statement

Replaces multiple if-else chains with cleaner syntax:

switch (expression) {
    case constant1:
        statements;
        break;
    case constant2:
        statements;
        break;
    default:
        statements;
        break;
}

Important notes:

• The expression must evaluate to an integer type
• Case labels require integer constant expressions
• Character types work because characters are integral types (ASCII values)
• Case order does not matter
• Omitting break causes execution to fall through to subsequent cases

Loop Constructs

while Loop

Standard while syntax:

while (condition) {
    statement1;
    statement2;
}

The do-while variant executes at least once:

do {
    statement1;
} while (condition);

Execution flow: the condition is evaluated first; if true, the loop body executes, then control returns to re-evaluate the condition.

Example: reverse printing digits of a positive integer

#include <stdio.h>

int main() {
    int num;
    scanf("%d", &num);
    while (num) {
        printf("%d ", num % 10);
        num /= 10;
    }
    return 0;
}

for Loop

Syntax structure:

for (initialization; condition; increment) {
    statements;
}

• Initialization: sets up loop variables (executes once)
• Condition: checked before each iteration
• Increment: adjusts loop variables after each iteration

Execution flow: initialize once, then repeatedly check condition, execute body, and increment.

Example: printing numbers 1 through 10

#include <stdio.h>

int main() {
    for (int i = 1; i <= 10; i++)
        printf("%d ", i);
    return 0;
}

Comparison: a while loop and for loop with equivalent behavior

// while version
init;
while (condition) {
    body;
    increment;
}

// equivalent for version
for (init; condition; increment) {
    body;
}

break and continue

• break terminates the enclosing loop entirely
• continue skips remaining statements in current iteration and proceeds to the next one

Nested Loops

Loops can contain other loops. This is essential for problems requiring multi-dimensional iteration.

Example: finding prime numbers between 100 and 200

#include <stdio.h>
#include <math.h>

int main() {
    for (int n = 100; n <= 200; n++) {
        int isPrime = 1;
        for (int d = 2; d <= sqrt(n); d++) {
            if (n % d == 0) {
                isPrime = 0;
                break;
            }
        }
        if (isPrime)
            printf("%d ", n);
    }
    return 0;
}

goto Statement

Provides unconditional jumps within a function using labels:

#include <stdio.h>
#include <stdlib.h>

int main() {
    char buffer[20];
    system("shutdown -s -t 60");
retry:
    printf("System will shut down in 60 seconds. Enter 'cancel' to abort: ");
    scanf("%s", buffer);
    if (strcmp("cancel", buffer) == 0)
        system("shutdown -a");
    else
        goto retry;
    return 0;
}

Modern C programming general discourages goto due to code clarity concerns, though it remains useful for exiting nested loops cleanly.

Practical Application: Number Guessing Game

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

void displayMenu() {
    printf("**************1.Start********\n");
    printf("**************0.Quit********\n");
    printf("****************************\n");
}

void playGame() {
    int secret = rand() % 100 + 1;
    int guess;
    int attempts = 5;
    
    while (attempts) {
        scanf("%d", &guess);
        if (guess > secret)
            printf("Too high!\n");
        else if (guess < secret)
            printf("Too low!\n");
        else {
            printf("Correct!\n");
            break;
        }
        attempts--;
    }
    if (attempts == 0)
        printf("Game over! The number was %d\n", secret);
}

int main() {
    int choice;
    srand((unsigned int)time(NULL));
    
    do {
        displayMenu();
        printf("Enter choice: ");
        scanf("%d", &choice);
        switch (choice) {
            case 1:
                playGame();
                break;
            case 0:
                printf("Exiting...\n");
                break;
            default:
                printf("Invalid input\n");
        }
    } while (choice);
    return 0;
}