Input/Output in C (scanf, printf)

📖 Introduction

Input/Output (I/O) operations allow C programs to interact with users and external devices. The most common I/O functions are printf() for output and scanf() for input, both declared in <stdio.h>.

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🎯 Standard I/O Streams

C provides three standard streams that are automatically opened when a program starts:

┌─────────────────────────────────────────────────────────────┐
│                     Standard Streams                         │
├─────────────────────────────────────────────────────────────┤
│                                                              │
│   Keyboard ──► stdin  ──┐                                   │
│                         │                                   │
│                         ▼                                   │
│                   ┌──────────┐                              │
│                   │ C Program │                              │
│                   └──────────┘                              │
│                    │        │                               │
│                    ▼        ▼                               │
│                 stdout   stderr                             │
│                    │        │                               │
│                    ▼        ▼                               │
│                  Screen  Screen                             │
│                                                              │
└─────────────────────────────────────────────────────────────┘

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📤 Output with printf()

Basic Syntax:

int printf(const char *format, ...);

printf() returns the number of characters printed (negative on error).

Simple Output:

#include <stdio.h>

int main() {
    printf("Hello, World!\n");  // Print string with newline
    printf("Hello ");
    printf("World\n");          // Continues on same line
    return 0;
}

Format Specifiers:

int age = 25;
float price = 19.99;
char grade = 'A';
char name[] = "John";

printf("Age: %d\n", age);       // Integer
printf("Price: %f\n", price);   // Float
printf("Grade: %c\n", grade);   // Character
printf("Name: %s\n", name);     // String

Complete Format Specifier Table:

Format Modifiers:

Format: %[flags][width][.precision][length]specifier

Width:

printf("[%10d]\n", 42);     // [        42]  Right-aligned, width 10
printf("[%-10d]\n", 42);    // [42        ]  Left-aligned, width 10
printf("[%*d]\n", 10, 42);  // [        42]  Width from variable

Precision:

printf("%.2f\n", 3.14159);      // 3.14       (2 decimal places)
printf("%.5s\n", "Hello World"); // Hello      (5 characters)
printf("%10.2f\n", 3.14159);    //       3.14 (width 10, 2 decimals)

Flags:

Examples:

int num = 42;
float f = 3.14159;

// Width and alignment
printf("[%10d]\n", num);      // [        42]
printf("[%-10d]\n", num);     // [42        ]
printf("[%010d]\n", num);     // [0000000042]

// Precision
printf("[%.2f]\n", f);        // [3.14]
printf("[%10.2f]\n", f);      // [      3.14]
printf("[%-10.2f]\n", f);     // [3.14      ]

// Signs
printf("[%+d]\n", num);       // [+42]
printf("[%+d]\n", -num);      // [-42]
printf("[% d]\n", num);       // [ 42]

// Alternate forms
printf("[%#x]\n", 255);       // [0xff]
printf("[%#o]\n", 64);        // [0100]
printf("[%#.0f]\n", 3.0);     // [3.]

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📥 Input with scanf()

Basic Syntax:

int scanf(const char *format, ...);

scanf() returns the number of successfully read items (0 or negative on failure).

⚠️ Important: Use Address-of Operator (&)

int age;
scanf("%d", &age);    // CORRECT: Pass address
// scanf("%d", age);  // WRONG: Undefined behavior!

Simple Input:

#include <stdio.h>

int main() {
    int age;
    float height;
    char grade;

    printf("Enter age: ");
    scanf("%d", &age);

    printf("Enter height: ");
    scanf("%f", &height);

    printf("Enter grade: ");
    scanf(" %c", &grade);  // Note: space before %c

    printf("Age: %d, Height: %.2f, Grade: %c\n", age, height, grade);

    return 0;
}

scanf Format Specifiers:

Reading Multiple Values:

int a, b, c;

// Space-separated input
printf("Enter three integers: ");
scanf("%d %d %d", &a, &b, &c);

// Or any whitespace works
scanf("%d%d%d", &a, &b, &c);  // Spaces optional in format

// Reading different types
int age;
float height;
char initial;
scanf("%d %f %c", &age, &height, &initial);

The Newline Problem:

int num;
char ch;

printf("Enter number: ");
scanf("%d", &num);

printf("Enter character: ");
scanf("%c", &ch);  // PROBLEM: Reads leftover '\n'!

// Solutions:

// Solution 1: Add space before %c
scanf(" %c", &ch);  // Space consumes whitespace

// Solution 2: Use getchar() to consume newline
scanf("%d", &num);
getchar();  // Consume '\n'
scanf("%c", &ch);

// Solution 3: Use %*c to skip the newline
scanf("%d%*c", &num);  // %*c reads and discards a character
scanf("%c", &ch);

Reading Strings:

char name[50];

// Method 1: %s (stops at whitespace)
printf("Enter name: ");
scanf("%s", name);  // Note: No & for arrays!
// Input: "John Doe" → Only "John" is stored

// Method 2: %[^\n] (reads until newline)
scanf(" %[^\n]", name);  // Reads whole line including spaces
// Input: "John Doe" → "John Doe" is stored

// Method 3: Limit input length (SAFE)
scanf("%49s", name);     // Max 49 chars + null terminator
scanf("%49[^\n]", name); // Max 49 chars with spaces

Return Value Checking:

int num;
printf("Enter a number: ");

if (scanf("%d", &num) != 1) {
    printf("Invalid input!\n");
    return 1;
}

printf("You entered: %d\n", num);

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🔡 Character I/O: getchar() and putchar()

getchar() - Read Single Character:

#include <stdio.h>

int main() {
    int ch;  // Use int, not char (for EOF detection)

    printf("Enter a character: ");
    ch = getchar();

    printf("You entered: ");
    putchar(ch);
    putchar('\n');

    return 0;
}

putchar() - Write Single Character:

putchar('A');       // Print 'A'
putchar('\n');      // Print newline
putchar(65);        // Print 'A' (ASCII value)

Reading a Line Character by Character:

int ch;
printf("Enter text (press Enter to stop): ");
while ((ch = getchar()) != '\n') {
    putchar(ch);
}
putchar('\n');

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📝 String I/O: gets() and puts()

⚠️ Warning: Never Use gets()

gets() is unsafe and removed from C11. It doesn't check buffer size:

// DANGEROUS - Never use!
char buffer[10];
gets(buffer);  // Can overflow buffer!

Use fgets() Instead:

char buffer[100];
printf("Enter a line: ");
fgets(buffer, sizeof(buffer), stdin);  // Safe!

// Note: fgets keeps the newline character
// Remove it if needed:
buffer[strcspn(buffer, "\n")] = '\0';

puts() - Print String with Newline:

puts("Hello, World!");  // Automatically adds newline
// Equivalent to: printf("Hello, World!\n");

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🔄 Buffer Flushing

fflush() - Flush Output Buffer:

printf("Loading");
for (int i = 0; i < 5; i++) {
    printf(".");
    fflush(stdout);  // Force output immediately
    sleep(1);
}
printf("\n");

Clearing Input Buffer:

// Method 1: Read and discard until newline
int c;
while ((c = getchar()) != '\n' && c != EOF);

// Method 2: Using scanf
scanf("%*[^\n]");  // Discard until newline
scanf("%*c");      // Discard newline itself

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📊 Formatted I/O Examples

Creating Tables:

printf("%-10s %10s %10s\n", "Name", "Age", "Score");
printf("%-10s %10s %10s\n", "----", "---", "-----");
printf("%-10s %10d %10.2f\n", "Alice", 25, 95.50);
printf("%-10s %10d %10.2f\n", "Bob", 30, 88.75);
printf("%-10s %10d %10.2f\n", "Charlie", 22, 92.00);

Output:

Name              Age      Score
----              ---      -----
Alice              25      95.50
Bob                30      88.75
Charlie            22      92.00

Progress Bar:

for (int i = 0; i <= 100; i += 10) {
    printf("\r[%-20s] %3d%%", "####################" + (20 - i/5), i);
    fflush(stdout);
    usleep(100000);  // Sleep 100ms
}
printf("\n");

Hexdump:

unsigned char data[] = {0x48, 0x65, 0x6C, 0x6C, 0x6F};
for (int i = 0; i < sizeof(data); i++) {
    printf("%02X ", data[i]);
}
printf("\n");
// Output: 48 65 6C 6C 6F

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⚠️ Common Mistakes

1. Missing & in scanf:

int x;
scanf("%d", x);    // WRONG: crashes or corrupts memory
scanf("%d", &x);   // CORRECT

2. Wrong Format Specifier:

float f = 3.14;
scanf("%d", &f);   // WRONG: expects int
scanf("%f", &f);   // CORRECT

double d;
scanf("%f", &d);   // WRONG for double
scanf("%lf", &d);  // CORRECT for double

3. Buffer Overflow with %s:

char name[10];
scanf("%s", name);      // DANGEROUS: could overflow
scanf("%9s", name);     // SAFE: max 9 chars + null

4. Not Checking Return Value:

// Bad: No error checking
scanf("%d", &num);

// Good: Check for valid input
if (scanf("%d", &num) != 1) {
    printf("Invalid input\n");
}

5. Mixing scanf and getchar:

scanf("%d", &num);
char ch = getchar();  // Gets '\n' from previous input!

// Fix:
scanf("%d", &num);
getchar();            // Consume the newline first
char ch = getchar();  // Now gets actual character

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✅ Best Practices

1. •Always check scanf() return value for input validation
2. •Use field width with %s to prevent buffer overflow: %49s
3. •Prefer fgets() over scanf() for strings - safer and more flexible
4. •Use appropriate format specifiers - %lf for double in scanf
5. •Add space before %c when following other inputs: " %c"
6. •Flush stdout when printing without newline and expecting input
7. •Initialize buffers before reading into them
8. •Handle EOF in character input loops

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🔑 Key Takeaways

1. •printf() displays formatted output; scanf() reads formatted input
2. •Format specifiers must match the variable type
3. •scanf requires address (using &) except for arrays
4. •Whitespace in scanf skips whitespace in input
5. •Check return values to handle input errors
6. •Buffer overflow is a major security concern - always limit input size
7. •The newline problem requires special handling with character input

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⏭️ Next Topic

Continue to Operators to learn about arithmetic, relational, logical, and bitwise operators.