Basic Input and Output in C

1#include <stdio.h>
2
3int main() {
4    // Simple text output
5    printf("Hello, World!\n");
6    
7    // Displaying variable values
8    int age = 25;
9    float height = 1.75;
10    char grade = 'A';
11    
12    printf("Age: %d years\n", age);
13    printf("Height: %.2f meters\n", height);
14    printf("Grade: %c\n", grade);
15    
16    // Multiple values in one statement
17    printf("Student Information:\nAge: %d\nHeight: %.2f\nGrade: %c\n", 
18           age, height, grade);
19    
20    return 0;
21}
22
23// Output:
24// Hello, World!
25// Age: 25 years
26// Height: 1.75 meters
27// Grade: A
28// Student Information:
29// Age: 25
30// Height: 1.75
31// Grade: A

1#include <stdio.h>
2
3int main() {
4    int num = 42;
5    float pi = 3.14159;
6    
7    // Width specification
8    printf("[%5d]\n", num);     // Output: [   42] (right-aligned in 5 spaces)
9    printf("[%-5d]\n", num);    // Output: [42   ] (left-aligned in 5 spaces)
10    
11    // Precision for floating-point
12    printf("%.2f\n", pi);       // Output: 3.14 (2 decimal places)
13    printf("%.4f\n", pi);       // Output: 3.1416 (4 decimal places)
14    
15    // Both width and precision
16    printf("[%8.3f]\n", pi);    // Output: [   3.142] (8 spaces, 3 decimals)
17    
18    // Zero padding
19    printf("[%05d]\n", num);    // Output: [00042] (padded with zeros)
20    
21    // Showing positive sign
22    printf("[%+d]\n", num);     // Output: [+42] (explicit positive sign)
23    
24    return 0;
25}
26
27// Output:
28// [   42]
29// [42   ]
30// 3.14
31// 3.1416
32// [   3.142]
33// [00042]
34// [+42]

1#include <stdio.h>
2
3int main() {
4    // putchar() - Outputs a single character
5    putchar('A');
6    putchar('\n');
7    
8    // puts() - Outputs a string followed by a newline
9    puts("Hello, World!");  // Note: puts() automatically adds a newline
10    
11    // fprintf() - Prints to a specific stream (like a file)
12    fprintf(stdout, "This goes to standard output\n");
13    
14    return 0;
15}
16
17// Output:
18// A
19// Hello, World!
20// This goes to standard output

1#include <stdio.h>
2
3int main() {
4    int age;
5    float height;
6    char initial;
7    
8    // Reading an integer
9    printf("Enter your age: ");
10    scanf("%d", &age);  // Note the & operator (address-of)
11    
12    // Reading a floating-point number
13    printf("Enter your height in meters: ");
14    scanf("%f", &height);
15    
16    // Reading a character
17    printf("Enter your first initial: ");
18    scanf(" %c", &initial);  // Note the space before %c to skip whitespace
19    
20    // Displaying the input
21    printf("\nYou entered:\n");
22    printf("Age: %d years\n", age);
23    printf("Height: %.2f meters\n", height);
24    printf("Initial: %c\n", initial);
25    
26    return 0;
27}
28
29// Example Interaction:
30// Enter your age: 25
31// Enter your height in meters: 1.75
32// Enter your first initial: J
33// 
34// You entered:
35// Age: 25 years
36// Height: 1.75 meters
37// Initial: J

1#include <stdio.h>
2
3int main() {
4    char name[50];  // Array to store the string (max 49 characters + null terminator)
5    
6    printf("Enter your full name: ");
7    scanf("%s", name);  // No & needed for arrays/strings
8    
9    printf("Hello, %s!\n", name);
10    
11    return 0;
12}
13
14// Example Interaction:
15// Enter your full name: John Doe
16// Hello, John!

1#include <stdio.h>
2#include <string.h>  // For string functions like strlen()
3
4int main() {
5    char full_name[50];
6    
7    printf("Enter your full name: ");
8    
9    // Clear input buffer
10    fflush(stdin);  // Note: This is not standard C, but works on many systems
11    
12    // Read a full line of text
13    fgets(full_name, sizeof(full_name), stdin);
14    
15    // Remove the newline character that fgets keeps
16    full_name[strcspn(full_name, "\n")] = 0;
17    
18    printf("Hello, %s!\n", full_name);
19    
20    return 0;
21}
22
23// Example Interaction:
24// Enter your full name: John Doe
25// Hello, John Doe!

1#include <stdio.h>
2
3int main() {
4    char c;
5    char buffer[100];
6    
7    // getchar() - Reads a single character
8    printf("Enter a character: ");
9    c = getchar();
10    printf("You entered: %c\n\n", c);
11    
12    // Clear the input buffer (consume the newline left by getchar)
13    while ((getchar()) != '\n');
14    
15    // gets() - Reads a line of text (DEPRECATED - unsafe!)
16    // printf("Enter a line of text: ");
17    // gets(buffer);  // UNSAFE - can cause buffer overflow!
18    
19    // Better alternative: fgets()
20    printf("Enter a line of text: ");
21    fgets(buffer, sizeof(buffer), stdin);
22    printf("You entered: %s", buffer);
23    
24    return 0;
25}
26
27// Example Interaction:
28// Enter a character: A
29// You entered: A
30//
31// Enter a line of text: Hello, C programming!
32// You entered: Hello, C programming!

1#include <stdio.h>
2
3int main() {
4    // User information
5    char name[50];
6    int age;
7    float height;
8    char gender;
9    
10    // Gather information
11    printf("===== User Registration =====\n");
12    
13    printf("Enter your name: ");
14    fgets(name, sizeof(name), stdin);
15    // Remove newline character
16    for (int i = 0; name[i] != '\0'; i++) {
17        if (name[i] == '\n') {
18            name[i] = '\0';
19            break;
20        }
21    }
22    
23    printf("Enter your age: ");
24    scanf("%d", &age);
25    
26    printf("Enter your height in meters: ");
27    scanf("%f", &height);
28    
29    // Clear input buffer before reading a character
30    while ((getchar()) != '\n');
31    
32    printf("Enter your gender (M/F): ");
33    scanf("%c", &gender);
34    
35    // Display information in a formatted way
36    printf("\n===== User Information =====\n");
37    printf("| %-20s | %s\n", "Name", name);
38    printf("| %-20s | %d\n", "Age", age);
39    printf("| %-20s | %.2f meters\n", "Height", height);
40    printf("| %-20s | %c\n", "Gender", gender);
41    printf("============================\n");
42    
43    return 0;
44}
45
46// Example Interaction:
47// ===== User Registration =====
48// Enter your name: John Smith
49// Enter your age: 30
50// Enter your height in meters: 1.85
51// Enter your gender (M/F): M
52// 
53// ===== User Information =====
54// | Name                 | John Smith
55// | Age                  | 30
56// | Height               | 1.85 meters
57// | Gender               | M
58// ============================

1#include <stdio.h>
2
3int main() {
4    FILE *file_ptr;
5    char name[50];
6    int age;
7    
8    // Writing to a file
9    file_ptr = fopen("user_data.txt", "w");
10    
11    if (file_ptr == NULL) {
12        printf("Error opening file for writing!\n");
13        return 1;
14    }
15    
16    printf("Enter your name: ");
17    fgets(name, sizeof(name), stdin);
18    // Remove newline
19    for (int i = 0; name[i] != '\0'; i++) {
20        if (name[i] == '\n') {
21            name[i] = '\0';
22            break;
23        }
24    }
25    
26    printf("Enter your age: ");
27    scanf("%d", &age);
28    
29    fprintf(file_ptr, "Name: %s\nAge: %d\n", name, age);
30    fclose(file_ptr);
31    
32    printf("\nData has been written to user_data.txt\n");
33    
34    // Reading from the file
35    char buffer[100];
36    file_ptr = fopen("user_data.txt", "r");
37    
38    if (file_ptr == NULL) {
39        printf("Error opening file for reading!\n");
40        return 1;
41    }
42    
43    printf("\nFile contents:\n");
44    while (fgets(buffer, sizeof(buffer), file_ptr) != NULL) {
45        printf("%s", buffer);
46    }
47    
48    fclose(file_ptr);
49    
50    return 0;
51}
52
53// Example Interaction:
54// Enter your name: John Smith
55// Enter your age: 30
56// 
57// Data has been written to user_data.txt
58// 
59// File contents:
60// Name: John Smith
61// Age: 30