Basic input

It is important in any modern program be able to receive information at runtime instead of know all needed values at compile time.

Yes, there are advantages about knowing all inputs at compile time, such as speed optimizations, but true reusable programs can process somewhat arbitrary input.

In this exercise we sample the 3 basic ways to capture external information: by arguments, environment and interactively.

Arguments

#include <iostream>

int main(int argc, char **argv)
{
  using namespace std;

  if (argc < 2)
  {
    cout << "usage: " << endl
         << argv[0] << " <your name>" << endl;
    return 1;
  }
  cout << "Hello, " << argv[1] << endl;
  return 0;
}

Environment

#include <iostream>
#include <cstdlib>

using namespace std;

int main(int argc, char **argv)
{
  if (char *name = getenv("USER"))
  {
    cout << "Hello, " << name << endl;
  }
  else
  {
    cerr << "Variable $USER not set!" << endl;
    return 1;
  }
  return 0;
}

Interactive

#include <iostream>
#include <string>

int main(int argc, char **argv)
{
  std::cout << "Please provide your name: " << std::endl;
  std::string name;
  std::cin >> name;
  std::cout << "Hello, " << name << std::endl;
  return 0;
}

How to build

clang++ input-arguments.cc -o input-arguments
clang++ input-environment.cc -o input-environment
clang++ input-interactive.cc -o input-interactive

How to run

./input-arguments Leonardo
USER=Leonardo ./input-environment
./input-interactive # or "echo 'Leonardo' | ./input-interactive"

Noteworthy

By adding the using namespace std declaration, we can save a few characters of code. Just cout instead of std::cout. But beware, by doing that, we lost a bit of clarity and makes the namespace a little dirty.

C++ has a very neat string class, so you don't need to worry about memory allocation when reading user input in a interactive way.