Ways to Accept Program’s Arguments in C/C++

While I was solving some problem sets on Hackerrank, one curiosity pops up of what might be happening for the program to accept external sample input test cases before executing to see its expected result.

The point is Hackerrank doesn’t use a obvious way to accept arguments, but instead rely on scanf or std::cin. I did further research and testing. This writeup is a result. Here I present you various ways to accept program’s arguments in C/C++.

1. Main’s arguments

We always see the following boilerplate code

int main(int argc, char* argv[])
{
    ...
}

that is the main way to accept program’s arguments from external. So users can execute the program and supply parameters at the same time.

./myProgram.out param1 param2

and in terms of code, argc tells us how many parameters sending in included the first one which is the name of the program (./myProgram.out itself). argc is 3 in this case.

We can access actual value for each one via argv i.e. argv[0] is ./myProgram.out, argv[1] = param1 and so on.

2. Standard Input

Standard input in C is scanf(), and std::cin for C++.

This is the most interesting way to accept program’s arguments. We can also take advantage of OS support for piping external input into the program, pipe output from one program to be an input of another. This makes it very convenient and flexible especially for competitive programming and platform such as Hackerrank to be able to operate. Thus allows it to accept submitted source code from users before compiling and executing with various several input test-cases.

Let’s imagine if input requirement of a program is two numbers separated by space like below

1 5

and output requirement is a result of addition of those two numbers like below

6

With above two requirements of input and output, we then call scanf() or std::cin for number of times matching with input requirement. So the complete program would be as follows

C program

#include <stdio.h>

int main(int argc, char* argv[])
{
    int a, b;
    scanf("%d %d", &a, &b);
    printf("%d", a + b);
    return 0;
}

C++ program

#include <iostream>

int main(int argc, char* argv[])
{
    int a, b;
    std::cin >> a >> b;
    std::cout << (a + b);
    return 0;
}

As mentioned about taking advantage of OS support for piping, we can do the following

echo "1 5" | ./myProgram.out

Above will send in 1 5 as input string into myProgram.out, and either scanf() or std::cin will be able to read from it properly.

Not only that, we can pipe input from file as follows

./myProgram.out < input.txt

wheres content of input.txt contains either oneline of input string

1 5

or multiple lines like

1
5

It will work the same!

So imagine further that we can pipe output from myProgram.out to be an input of another in chain like following

input.txt > ./myProgram.out | ./myProgram2.out | ./myProgram3.out

That’s amazing.

3. Change stream buffer of std::cin (C++ only)

As a bonus, we can change stream buffer std::basic_ios::rdbuf_ref which is the upper class¹ that cin inherits from.

Just set std::cin’s rdbuf to whatever stream you want to read from i.e. file stream.

...

// content we want to read from via std::cin
std::ifstream fileIn("res/sample_input.txt");

// save pointer to stream buffer of std::cout
auto coutSBuf = std::cin.rdbuf();

// substitute internal std::cout buffer with buffer of fileIn
std::cin.rdbuf(fileIn.rdbuf());

// read content from std::cin into a string
std::string readString;
std::cin >> readString;
std::cout << readString << std::endl;

...

(Extra) 4. Change stream buffer of stdout and stdin (C only)

It’s possible to do similar in C only as well. It can be done via freopen() and optinal with the help of dup() to set back stream buffer. Anyway it’s by no mean reliable and portable, thus I’m not showing any source code example here. You can take a look for more information here, and here.

In short, better use fscanf() or fprintf() everywhere as we can control which stream buffer to input or output, thus we can set back any stream buffer under control at will, at anytime. See this solution for more information on example code.

First published on Sep, 8, 2019

Sep, 9, 2019

• Added (Extra) 4.

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1. See an overview of class inheritance and relationship of I/O related, and stream buffer classes in C++ here
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Written by Wasin Thonkaew
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