Question

I have to read input according to these rules:

“The input consists of several lines of text. Some lines may be empty. The input can be fed from a file, using a line such as prog.exe < input.txt in which case the end of input is indicated appropriatelly by the operating system. If you enter input using a keyboard, there is normally a way to signal the end of input with some control key, depending on the operating system (e.g., Ctrl+d in Unix/Linux-style systems, and Ctrl+z in Microsoft systems).”

Previously I have been doing it this way

while(getline(cin, data)) {
     if(data == "0") break;
     / * do stuff */
}

So I can read as many lines as I want and preform calculations, and then when I’m done just type a 0 and end my program. I tried entering a list of things in a .txt file one per line, and then calling program.exe < myfile.txt but nothing happened.

What’s this < file.txt doing?

How can I properly handle content inside it when calling my program like that?

And how can I make it calculate things when you hit ctrl+z?

Answer 1

Paraphrasing the text of your exercise:

Using command1 < file1 executes command1, with file1 as the source of input (as opposed to the keyboard).

This is known as redirecting standard input.

std::cin will get its input from file1 instead of from the keyboard.

The end of the input file is analogous to CTRL+Z (on Microsoft systems, CTRL+D on most others). Once std::getline() reaches the end of the file (or you read a line with just “0”), you’ll exit your while-loop and then you can do your calculation on the data you’ve collected (and presumably stored in some container).