Week 3 - Part2: Command Line Arguments and Compilation of Multiple Files

Command Line Arguments

Examples: Why Command Line Arguments

apt-get install vlc
git commit -a -m "kill me please!"
cp [file] [target path]

Application	Argument 1	Argument 2	Argument 3	Argument 4
`apt-get`	`install`	`vlc`	`null`	`null`
`git`	`commit`	`-a`	`-m`	`"kill me please!"`
`cp`	`[file]`	`[target path]`	`null`	`null`

Command Line Arguments in C and C++

#include <iostream>
// argc is counter for the arguments, including the application-name.
// argv is array of strings representing the arguments.
int main( int argc, char **argv )
{
    std::cout << "Arguments count:" << argc << std::endl;

    for( int i = 0 ; i < argc ; ++i )
    {
        std::cout << "Argument:" << argv[ i ] << std::endl;
    }
}

Compile the above example and run the application with any arguments.

Example: Simple calculater from command line arguments

What if we have a simple calculator that we pass our operarands directly from the terminal.

$ ./myCalculator 13 + 7
20

$ ./myCalculator 50 - 39
11

argv[0]	argv[1]	argv[2]	argv[3]
”./myCalculator”	“13”	”+”	“7”
”./myCalculator”	“50”	”-“	“39”

Note that “13”, “7”, “50”, and “39” are strings! Not integers How to convert from string representation to integer representation?

int x = "40"; // Compiler Error (Type Mismatch)

int y = std::atoi("40"); // Now this works, and y = 40.

double z = std::atof("13.9"); // z = 13.9

std::atoi converts a string representation of numbers into the equivalent integer.
std::atof converts a string representation of numbers into the equivalent double.
You need to #include <cstdlib> to use this function.

In our application we can use the function implemented by Emmanuel and Mostafa from here.

#include <iostream>
#include <cstdlib>

// Our logic
double calculation( double a , double b , char operation );

int main( int argc , char *argv[] )
{
    double a = std::atof( argv[1] );
    double b = std::atof( argv[3] );

    char *op_string = argv[2];
    char op = op_string[0];

    std::cout << calculation( a , b , op ) << std::endl;
    return 0;
}

double calculation( double a , double b , char operation )
{
    // Copy-paste the implementation of Emmanuel here.
}

After implementing (or copying) the calculation function, compile this example.

FAQ #3. Why not using `std::cin` inside functions of particular logic

After working with different kind of input (i.e input from the command line arguments instead of std::cin), you can now realize why it is pointless to use std::cin and std::cout in our interesting and usefull functions. See FAQ #3: Why not using std::cin inside functions of particular logic

Making Your Own Header Libraries

You may realized that we used calculation multiple times in different program. And you also may realized that I used the word “Copy The Implementation”! Which is very frustrating; copying a particular implementation in multiple programs!

What about making a library containing your very useful function:

So you use that library whenever you need, without copying sources. This will make you adopt DRY principle (i.e Don’t repeat your self).
So if you have figured out a mistake in your function, you just fix it from one place; No need to jump across sources to fix the buggy function.

multiple