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Practical C programs demonstrating input/output operations: calculators, temperature converters, pattern printing, menu-driven programs, and formatted output. Complete I/O examples with explanations.
The best way to solidify your understanding of C input/output is through practice. This collection of practical programs demonstrates how to combine printf(), scanf(), fgets(), getchar(), and other I/O functions to build real, useful programs. Each example is complete and ready to compile.
Program 1: Simple Calculator
Example
#include <stdio.h>
int main() {
float num1, num2, result;
char operator;
printf("========== Simple Calculator ==========\n");
printf("Enter expression (e.g., 5 + 3): ");
scanf("%f %c %f", &num1, &operator, &num2);
switch (operator) {
case '+': result = num1 + num2; break;
case '-': result = num1 - num2; break;
case '*': result = num1 * num2; break;
case '/':
if (num2 == 0) {
printf("Error: Division by zero!\n");
return 1;
}
result = num1 / num2;
break;
case '%':
result = (int)num1 % (int)num2;
break;
default:
printf("Error: Invalid operator '%c'\n", operator);
return 1;
}
printf("%.2f %c %.2f = %.2f\n", num1, operator, num2, result);
return 0;
}
Output
========== Simple Calculator ========== Enter expression (e.g., 5 + 3): 15.5 * 3.2 15.50 * 3.20 = 49.60
Program 2: Temperature Converter
Example
#include <stdio.h>
int main() {
float temp;
int choice;
printf("===== Temperature Converter =====\n");
printf("1. Celsius to Fahrenheit\n");
printf("2. Fahrenheit to Celsius\n");
printf("3. Celsius to Kelvin\n");
printf("Choose conversion (1-3): ");
scanf("%d", &choice);
printf("Enter temperature: ");
scanf("%f", &temp);
switch (choice) {
case 1:
printf("%.2f°C = %.2f°F\n", temp, (temp * 9.0/5.0) + 32);
break;
case 2:
printf("%.2f°F = %.2f°C\n", temp, (temp - 32) * 5.0/9.0);
break;
case 3:
printf("%.2f°C = %.2f K\n", temp, temp + 273.15);
break;
default:
printf("Invalid choice!\n");
}
return 0;
}
Output
===== Temperature Converter ===== 1. Celsius to Fahrenheit 2. Fahrenheit to Celsius 3. Celsius to Kelvin Choose conversion (1-3): 1 Enter temperature: 100 100.00°C = 212.00°F
Program 3: Student Report Card
Example
#include <stdio.h>
int main() {
char name[50];
int roll;
float marks[5];
float total = 0, percentage;
char *subjects[] = {"Math", "Science", "English", "History", "Computer"};
printf("===== Student Report Card =====\n\n");
printf("Enter student name: ");
scanf(" %[^\n]", name);
printf("Enter roll number: ");
scanf("%d", &roll);
printf("\nEnter marks (out of 100):\n");
for (int i = 0; i < 5; i++) {
printf(" %s: ", subjects[i]);
scanf("%f", &marks[i]);
total += marks[i];
}
percentage = total / 5;
printf("\n╔══════════════════════════════════╗\n");
printf("║ REPORT CARD ║\n");
printf("╠══════════════════════════════════╣\n");
printf("║ Name: %-25s ║\n", name);
printf("║ Roll: %-25d ║\n", roll);
printf("╠══════════════════════════════════╣\n");
printf("║ %-10s %10s %10s ║\n", "Subject", "Marks", "Grade");
printf("║ %-10s %10s %10s ║\n", "-------", "-----", "-----");
for (int i = 0; i < 5; i++) {
char grade;
if (marks[i] >= 90) grade = 'A';
else if (marks[i] >= 80) grade = 'B';
else if (marks[i] >= 70) grade = 'C';
else if (marks[i] >= 60) grade = 'D';
else grade = 'F';
printf("║ %-10s %10.1f %10c ║\n", subjects[i], marks[i], grade);
}
printf("╠══════════════════════════════════╣\n");
printf("║ Total: %8.1f / 500 ║\n", total);
printf("║ Percent: %8.2f%% ║\n", percentage);
printf("║ Result: %-8s ║\n",
(percentage >= 40) ? "PASS" : "FAIL");
printf("╚══════════════════════════════════╝\n");
return 0;
}
Output
===== Student Report Card ===== Enter student name: John Smith Enter roll number: 101 Enter marks (out of 100): Math: 85 Science: 92 English: 78 History: 65 Computer: 95 ╔══════════════════════════════════╗ ║ REPORT CARD ║ ╠══════════════════════════════════╣ ║ Name: John Smith ║ ║ Roll: 101 ║ ╠══════════════════════════════════╣ ║ Subject Marks Grade ║ ║ ------- ----- ----- ║ ║ Math 85.0 B ║ ║ Science 92.0 A ║ ║ English 78.0 C ║ ║ History 65.0 D ║ ║ Computer 95.0 A ║ ╠══════════════════════════════════╣ ║ Total: 415.0 / 500 ║ ║ Percent: 83.00% ║ ║ Result: PASS ║ ╚══════════════════════════════════╝
Program 4: Number System Converter
Example
#include <stdio.h>
int main() {
int num;
printf("Enter a decimal number: ");
scanf("%d", &num);
printf("\n=== Number System Conversions ===\n");
printf("Decimal: %d\n", num);
printf("Octal: %o\n", num);
printf("Hexadecimal: %X\n", num);
// Binary conversion
printf("Binary: ");
if (num == 0) {
printf("0");
} else {
int binary[32];
int i = 0;
int temp = num;
while (temp > 0) {
binary[i++] = temp % 2;
temp /= 2;
}
for (int j = i - 1; j >= 0; j--) {
printf("%d", binary[j]);
}
}
printf("\n");
printf("\nFormatted outputs:\n");
printf("With prefix: 0%o (octal), 0x%X (hex)\n", num, num);
printf("Padded hex: %08X\n", num);
return 0;
}
Output
Enter a decimal number: 255 === Number System Conversions === Decimal: 255 Octal: 377 Hexadecimal: FF Binary: 11111111 Formatted outputs: With prefix: 0377 (octal), 0xFF (hex) Padded hex: 000000FF
Program 5: Pattern Printing with Formatting
Example
#include <stdio.h>
int main() {
int n;
printf("Enter number of rows: ");
scanf("%d", &n);
// Right-aligned number triangle
printf("\n--- Right Triangle ---\n");
for (int i = 1; i <= n; i++) {
for (int j = 1; j <= n - i; j++) {
printf(" ");
}
for (int j = 1; j <= i; j++) {
printf("%2d", j);
}
printf("\n");
}
// Multiplication table (formatted)
printf("\n--- Multiplication Table (1-%d) ---\n", n);
printf("%4s", "");
for (int i = 1; i <= n; i++) {
printf("%4d", i);
}
printf("\n ");
for (int i = 1; i <= n; i++) {
printf("----");
}
printf("\n");
for (int i = 1; i <= n; i++) {
printf("%2d |", i);
for (int j = 1; j <= n; j++) {
printf("%4d", i * j);
}
printf("\n");
}
return 0;
}
Output
Enter number of rows: 5
--- Right Triangle ---
1
1 2
1 2 3
1 2 3 4
1 2 3 4 5
--- Multiplication Table (1-5) ---
1 2 3 4 5
--------------------
1 | 1 2 3 4 5
2 | 2 4 6 8 10
3 | 3 6 9 12 15
4 | 4 8 12 16 20
5 | 5 10 15 20 25Program 6: Interactive Menu-Driven Program
Example
#include <stdio.h>
#include <string.h>
int main() {
int choice;
char input[100];
do {
printf("\n===== Text Utility Menu =====\n");
printf("1. Count characters\n");
printf("2. Convert to uppercase\n");
printf("3. Reverse string\n");
printf("4. Check palindrome\n");
printf("0. Exit\n");
printf("Choice: ");
scanf("%d", &choice);
while (getchar() != '\n'); // Clear buffer
if (choice == 0) break;
if (choice >= 1 && choice <= 4) {
printf("Enter text: ");
fgets(input, sizeof(input), stdin);
input[strcspn(input, "\n")] = '\0';
}
switch (choice) {
case 1:
printf("Length: %lu characters\n", strlen(input));
break;
case 2:
printf("Uppercase: ");
for (int i = 0; input[i]; i++) {
if (input[i] >= 'a' && input[i] <= 'z')
putchar(input[i] - 32);
else
putchar(input[i]);
}
putchar('\n');
break;
case 3:
printf("Reversed: ");
for (int i = strlen(input) - 1; i >= 0; i--) {
putchar(input[i]);
}
putchar('\n');
break;
case 4: {
int len = strlen(input);
int isPalin = 1;
for (int i = 0; i < len / 2; i++) {
if (input[i] != input[len - 1 - i]) {
isPalin = 0;
break;
}
}
printf("\"%s\" is %sa palindrome\n", input, isPalin ? "" : "NOT ");
break;
}
default:
printf("Invalid choice!\n");
}
} while (choice != 0);
printf("Goodbye!\n");
return 0;
}
Output
===== Text Utility Menu ===== 1. Count characters 2. Convert to uppercase 3. Reverse string 4. Check palindrome 0. Exit Choice: 2 Enter text: hello world Uppercase: HELLO WORLD ===== Text Utility Menu ===== 1. Count characters 2. Convert to uppercase 3. Reverse string 4. Check palindrome 0. Exit Choice: 4 Enter text: racecar "racecar" is a palindrome ===== Text Utility Menu ===== Choice: 0 Goodbye!
Program 7: Formatted Receipt Generator
Example
#include <stdio.h>
int main() {
char items[5][30];
float prices[5];
int quantities[5];
int n;
printf("How many items (max 5)? ");
scanf("%d", &n);
while (getchar() != '\n');
for (int i = 0; i < n; i++) {
printf("\nItem %d name: ", i + 1);
fgets(items[i], sizeof(items[i]), stdin);
items[i][strcspn(items[i], "\n")] = '\0';
printf("Price: $");
scanf("%f", &prices[i]);
printf("Quantity: ");
scanf("%d", &quantities[i]);
while (getchar() != '\n');
}
// Print receipt
float subtotal = 0;
printf("\n");
printf("┌────────────────────────────────────────┐\n");
printf("│ SHOPPING RECEIPT │\n");
printf("├──────────────────┬─────┬───────┬───────┤\n");
printf("│ %-16s │ Qty │ Price │ Total │\n", "Item");
printf("├──────────────────┼─────┼───────┼───────┤\n");
for (int i = 0; i < n; i++) {
float total = prices[i] * quantities[i];
subtotal += total;
printf("│ %-16.16s │ %3d │ %5.2f │%6.2f │\n",
items[i], quantities[i], prices[i], total);
}
float tax = subtotal * 0.08;
printf("├──────────────────┴─────┴───────┼───────┤\n");
printf("│ Subtotal: │%6.2f │\n", subtotal);
printf("│ Tax (8%%): │%6.2f │\n", tax);
printf("│ TOTAL: │%6.2f │\n", subtotal + tax);
printf("└────────────────────────────────┴───────┘\n");
return 0;
}
Output
How many items (max 5)? 3 Item 1 name: Laptop Price: $999.99 Quantity: 1 Item 2 name: Mouse Price: $29.99 Quantity: 2 Item 3 name: USB Cable Price: $9.99 Quantity: 3 ┌────────────────────────────────────────┐ │ SHOPPING RECEIPT │ ├──────────────────┬─────┬───────┬───────┤ │ Item │ Qty │ Price │ Total │ ├──────────────────┼─────┼───────┼───────┤ │ Laptop │ 1 │999.99 │999.99 │ │ Mouse │ 2 │ 29.99 │ 59.98 │ │ USB Cable │ 3 │ 9.99 │ 29.97 │ ├──────────────────┴─────┴───────┼───────┤ │ Subtotal: │1089.94│ │ Tax (8%): │ 87.20 │ │ TOTAL: │1177.14│ └────────────────────────────────┴───────┘
Program 8: Input Validation Example
Example
#include <stdio.h>
#include <string.h>
int main() {
char buffer[100];
int age;
int valid;
// Robust input validation
do {
printf("Enter your age (1-150): ");
fgets(buffer, sizeof(buffer), stdin);
valid = sscanf(buffer, "%d", &age);
if (valid != 1) {
printf("Error: Please enter a number.\n");
} else if (age < 1 || age > 150) {
printf("Error: Age must be between 1 and 150.\n");
valid = 0;
}
} while (!valid);
printf("Your age is: %d\n", age);
if (age < 13) printf("Category: Child\n");
else if (age < 18) printf("Category: Teenager\n");
else if (age < 60) printf("Category: Adult\n");
else printf("Category: Senior\n");
return 0;
}
Output
Enter your age (1-150): abc Error: Please enter a number. Enter your age (1-150): 200 Error: Age must be between 1 and 150. Enter your age (1-150): 25 Your age is: 25 Category: Adult
Interview Questions
Q1: How would you read a full line of input that may contain spaces?
Use fgets(buffer, size, stdin) or scanf(" %[^\n]", buffer). The %[^\n] scanset reads everything until a newline. Never use gets() — it's unsafe and removed from C11. After fgets(), remove the trailing newline with buffer[strcspn(buffer, "\n")] = '\0'.
Q2: How do you handle invalid input gracefully in C?
Check scanf()'s return value — it returns the number of items successfully read. If it returns 0, the input didn't match. Use fgets() + sscanf() for more robust handling: read the entire line first, then parse it. Always clear the input buffer after failed reads with while(getchar() != '\n');.
Q3: How do you create aligned table output in C?
Use width specifiers in printf(): %-15s for left-aligned strings in a 15-char field, %8.2f for right-aligned floats with 2 decimal places in an 8-char field. Combine - flag for left alignment and numeric width for consistent column sizes.
Q4: What is the difference between printf() and puts() for printing strings?
puts(str) is equivalent to printf("%s\n", str) — it outputs the string and adds a newline. puts() cannot format variables; it only prints raw strings. It's slightly faster because there's no format parsing. Use puts() for simple string output and printf() when formatting is needed.
Summary
- Combine
printf()formatting withscanf()/fgets()for interactive programs - Use width specifiers and alignment flags for professional table output
- Always validate user input — check
scanf()return values and use range checks fgets()+sscanf()is the safest pattern for robust input handling- Menu-driven programs use loops with switch statements and proper buffer clearing
- Format specifiers like
%-10s,%8.2f,%05dgive precise output control - Clear the input buffer between different types of reads to avoid issues
Exam Focus
Revise definitions, diagrams, examples, and short-answer points for C Input Output Programs and Examples.
Interview Use
Prepare one clear explanation, one practical example, and one common mistake for this C Programming topic.
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