I/O in C

Chapter: Input and Output in C

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Input and output (I/O) operations are essential in any program, as they allow interaction with the user or external devices. In C, I/O operations are handled using functions from the standard input/output library (<stdio.h>). This chapter covers various methods for reading inputs and displaying outputs, ranging from simple console I/O to working with files.

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1. Introduction to I/O in C

C provides three standard streams for input and output:

• Standard Input (stdin): Used for reading input, typically from the keyboard.

• Standard Output (stdout): Used for displaying output, typically to the screen.

• Standard Error (stderr): Used for error messages or diagnostics.

The most commonly used I/O functions in C are:

• printf and scanf for formatted I/O.

• putchar and getchar for character-based I/O.

• puts and gets for string-based I/O.

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2. Console Input and Output

2.1 Printing Output with printf

The printf function is used for formatted output.

Syntax:

printf("format string", arguments);

• The format string contains text and format specifiers (e.g., %d, %s) to display variables.

• The arguments correspond to the format specifiers.

Example:

#include <stdio.h>

int main() {
    int age = 25;
    float height = 5.9;

    printf("Age: %d\n", age);
    printf("Height: %.1f\n", height);

    return 0;
}

Output:

Age: 25
Height: 5.9

Common Format Specifiers:

Specifier	Description	Example
%d	Integer	123
%f	Floating-point number	123.45
%c	Character	'A'
%s	String	"Hello"
%x	Hexadecimal	7b (for 123)
%o	Octal	173 (for 123)
%%	Prints % character	%

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2.2 Reading Input with scanf

The scanf function is used to read formatted input.

Syntax:

scanf("format string", &variable);

• The format string specifies the data type of the input.

• The & (address-of operator) is used to pass the memory address of the variable where the input will be stored.

Example:

#include <stdio.h>

int main() {
    int num;
    printf("Enter a number: ");
    scanf("%d", &num);
    printf("You entered: %d\n", num);

    return 0;
}

Output:

Enter a number: 42
You entered: 42

Notes on scanf:

1. Multiple inputs can be read at once:

   scanf("%d %f", &int_var, &float_var);

2. scanf stops reading strings at spaces. Use fgets for multi-word input.

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2.3 Character Input and Output

For single-character I/O, use getchar and putchar.

1. getchar: Reads a single character from the input.

   Example:

   char ch = getchar();

2. putchar: Prints a single character to the output.

   Example:

   putchar('A');

Example Program:

#include <stdio.h>

int main() {
    char ch;

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

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

    return 0;
}

Output:

Enter a character: G
You entered: G

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2.4 String Input and Output

1. gets: Reads an entire line, including spaces. Avoid gets, as it can cause buffer overflows.

2. fgets (Preferred): Reads a line of text safely, including spaces.

   Example:

   char str[50];
   fgets(str, sizeof(str), stdin);

3. puts: Prints a string followed by a newline.

   Example:

   puts("Hello, World!");

Example Program:

#include <stdio.h>

int main() {
    char name[50];

    printf("Enter your name: ");
    fgets(name, sizeof(name), stdin);

    printf("Hello, ");
    puts(name);

    return 0;
}

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3. File Input and Output

File I/O allows reading from and writing to files. The <stdio.h> library provides functions like fopen, fprintf, fscanf, fclose, etc.

3.1 Opening and Closing Files

Use fopen to open a file:

FILE *file_pointer = fopen("filename", "mode");

Modes include:

• "r": Read mode.

• "w": Write mode (overwrites the file).

• "a": Append mode.

• "r+": Read and write.

Use fclose to close the file:

fclose(file_pointer);

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3.2 Writing to a File

Use fprintf or fputs for writing data to a file.

Example:

#include <stdio.h>

int main() {
    FILE *fp = fopen("output.txt", "w");
    if (fp == NULL) {
        printf("Error opening file.\n");
        return 1;
    }

    fprintf(fp, "Hello, File!\n");
    fclose(fp);

    return 0;
}

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3.3 Reading from a File

Use fscanf or fgets to read data from a file.

Example:

#include <stdio.h>

int main() {
    FILE *fp = fopen("output.txt", "r");
    if (fp == NULL) {
        printf("Error opening file.\n");
        return 1;
    }

    char buffer[100];
    while (fgets(buffer, sizeof(buffer), fp) != NULL) {
        printf("%s", buffer);
    }

    fclose(fp);
    return 0;
}

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4. Formatted and Unformatted I/O

4.1 Formatted I/O

Formatted I/O functions, like printf and scanf, allow the use of format specifiers for structured data input and output.

4.2 Unformatted I/O

Unformatted I/O functions, like getchar, putchar, gets, and puts, deal directly with raw data.

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5. Best Practices for I/O in C

1. Validate Inputs: Always check for valid inputs to prevent errors or undefined behavior.

2. Handle File Errors: Check file pointers for NULL after fopen.

3. Use Safe Functions: Prefer fgets over gets, as it avoids buffer overflows.

4. Close Files: Always close files after reading or writing.

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6. Sample Questions and Solutions

Question 1: Write a program to read and print a sentence using fgets and puts.

Solution:

#include <stdio.h>

int main() {
    char sentence[100];

    printf("Enter a sentence: ");
    fgets(sentence, sizeof(sentence), stdin);

    printf("You entered: ");
    puts(sentence);

    return 0;
}

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Question 2: Write a program to copy content from one file to another.

Solution:

#include <stdio.h>

int main() {
    FILE *source, *destination;
    char ch;

    source = fopen("source.txt", "r");
    if (source == NULL) {
        printf("Error opening source file.\n");
        return 1;
    }

    destination = fopen("destination.txt", "w");
    if (destination == NULL) {
        printf("Error opening destination file.\n");
        fclose(source);
        return 1;
    }

    while ((ch = fgetc(source)) != EOF) {
        fputc(ch, destination);
    }

    fclose(source);
    fclose(destination);

    printf("File copied successfully.\n");
    return 0;
}

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Summary

Input and output operations are fundamental for creating interactive and file-based C programs. In this chapter, we covered console I/O using printf, scanf, getchar, and putchar, as well as file I/O with fopen, fscanf, and fprintf. By understanding these concepts, you can create robust programs that interact with users and external files. Practice the sample questions to solidify your skills.

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