How to declare and use command-line arguments?

Declaring and Using Command-Line Arguments in C

Command-line arguments are a way to pass data to a program when it is executed. They are commonly used to customize the behavior of a program or to provide input data. In the C programming language, command-line arguments are accessed through the main() function, which has the following signature:

int main(int argc, char *argv[])

Here, argc (argument count) is an integer that represents the number of command-line arguments, including the program name itself. argv (argument vector) is an array of strings, where each string represents a command-line argument.

Accessing Command-Line Arguments

To access the command-line arguments, you can use a simple loop to iterate through the argv array:

#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;
}

In this example, the program will print the number of command-line arguments, followed by each argument on a new line.

When you run the program with the following command:

./program arg1 arg2 arg3

The output will be:

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

Note that the first argument (argv[0]) is always the name of the program, and the remaining arguments are the actual command-line arguments provided by the user.

Using Command-Line Arguments

Command-line arguments can be used to customize the behavior of a program. For example, you can use them to specify input files, output files, or various options that control the program's execution. Here's an example of a program that takes two command-line arguments and performs a simple arithmetic operation:

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

int main(int argc, char *argv[]) {
    if (argc != 4) {
        printf("Usage: %s <number1> <operator> <number2>\n", argv[0]);
        return 1;
    }

    int num1 = atoi(argv[1]);
    char op = argv[2][0];
    int num2 = atoi(argv[3]);

    int result;
    switch (op) {
        case '+':
            result = num1 + num2;
            break;
        case '-':
            result = num1 - num2;
            break;
        case '*':
            result = num1 * num2;
            break;
        case '/':
            result = num1 / num2;
            break;
        default:
            printf("Invalid operator: %c\n", op);
            return 1;
    }

    printf("%d %c %d = %d\n", num1, op, num2, result);
    return 0;
}

In this example, the program expects three command-line arguments: the first number, the operator, and the second number. It then performs the specified arithmetic operation and prints the result.

When you run the program with the following command:

./program 10 + 5

The output will be:

10 + 5 = 15

Handling Errors and Validating Input

It's important to handle errors and validate the input when working with command-line arguments. In the previous example, we checked if the correct number of arguments was provided and handled invalid operators. This helps to ensure that the program behaves as expected and provides meaningful error messages to the user.

Here's a Mermaid diagram that illustrates the flow of the program:

By understanding how to declare and use command-line arguments in C, you can create more flexible and powerful programs that can be customized and used in a variety of scenarios.