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Command Line Arguments

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Command line arguments are a way to provide input to a program at the time of its execution. This allows users to pass data to the program without hardcoding it into the source code. In C, command line arguments are handled using the main function's parameters: argc and argv.

Key Concepts

  1. argc (Argument Count):

    • An integer representing the number of command line arguments passed to the program, including the program's name.

  2. argv (Argument Vector):

    • An array of character pointers (strings) listing all the arguments. argv[0] is the name of the program, and argv[1] to argv[argc-1] are the actual command line arguments.

Syntax

int main(int argc, char *argv[]) {
    // Your code here
}
  • argc: Number of arguments.
  • argv: Array of arguments.

Practical Example

Let's create a simple program that prints out all the command line arguments passed to it.

Code Example

#include <stdio.h>

int main(int argc, char *argv[]) {
    printf("Number of arguments: %d\n", argc);
    for (int i = 0; i < argc; i++) {
        printf("Argument %d: %s\n", i, argv[i]);
    }
    return 0;
}

Explanation

  1. Include the Standard I/O Library:

    • #include <stdio.h> is necessary for using the printf function.

  2. Main Function:

    • The main function takes two parameters: argc and argv.

  3. Print Number of Arguments:

    • printf("Number of arguments: %d\n", argc); prints the total number of arguments.

  4. Loop Through Arguments:

    • A for loop iterates through each argument and prints it using printf.

Running the Program

To compile and run the program, use the following commands in your terminal:

gcc -o command_line_args command_line_args.c
./command_line_args arg1 arg2 arg3

Expected Output

Number of arguments: 4
Argument 0: ./command_line_args
Argument 1: arg1
Argument 2: arg2
Argument 3: arg3

Practical Exercises

Exercise 1: Sum of Integers

Write a program that takes a list of integers as command line arguments and prints their sum.

Solution

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

int main(int argc, char *argv[]) {
    int sum = 0;
    for (int i = 1; i < argc; i++) {
        sum += atoi(argv[i]);
    }
    printf("Sum: %d\n", sum);
    return 0;
}

Explanation

  1. Include Libraries:

    • #include <stdio.h> for printf.
    • #include <stdlib.h> for atoi (ASCII to Integer conversion).

  2. Initialize Sum:

    • int sum = 0; initializes the sum variable.

  3. Loop Through Arguments:

    • The loop starts from 1 to skip the program name.
    • sum += atoi(argv[i]); converts each argument to an integer and adds it to sum.

  4. Print Sum:

    • printf("Sum: %d\n", sum); prints the total sum.

Exercise 2: Concatenate Strings

Write a program that concatenates all command line arguments into a single string and prints it.

Solution

#include <stdio.h>
#include <string.h>

int main(int argc, char *argv[]) {
    char result[1000] = ""; // Ensure the array is large enough
    for (int i = 1; i < argc; i++) {
        strcat(result, argv[i]);
        if (i < argc - 1) {
            strcat(result, " ");
        }
    }
    printf("Concatenated String: %s\n", result);
    return 0;
}

Explanation

  1. Include Libraries:

    • #include <stdio.h> for printf.
    • #include <string.h> for strcat.

  2. Initialize Result String:

    • char result[1000] = ""; initializes an empty string with a large enough buffer.

  3. Loop Through Arguments:

    • strcat(result, argv[i]); concatenates each argument to result.
    • if (i < argc - 1) { strcat(result, " "); } adds a space between arguments.

  4. Print Result:

    • printf("Concatenated String: %s\n", result); prints the concatenated string.

Common Mistakes

  1. Buffer Overflow:

    • Ensure the buffer for concatenated strings is large enough to hold all arguments.

  2. Incorrect Indexing:

    • Remember that argv[0] is the program name, and actual arguments start from argv[1].

  3. Type Conversion:

    • Use atoi or similar functions to convert string arguments to integers or other types.

Conclusion

Command line arguments provide a flexible way to pass input to your programs. Understanding how to use argc and argv allows you to create more dynamic and user-friendly applications. Practice with the provided exercises to solidify your understanding and prepare for more advanced topics.

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