C++ Reading from keyboard after giving a .txt to program (used getline() after in)

Question

Ok here's the problem and I'm despaired about it. I get a 9*9 table using

for (int i=0 ; i < 9 ; i++)
    for (int j=0 ; j < 9 ; j++)
         cin >> table [i][j];

I want to make some changes to this table . so I get a command using:

getline (cin , command);

so user would type a command and after parsing it I will make the changes to the table. (it has to be getline() because command is an string and contains spaces)

I have an in.txt containing the table . I give this file to my code using :

g++ main.cpp

./a.out < in.txt .

(these two commands are written in terminal)

after that I want to give my command using keyboard . but the getline() doesn't get anything from keyboard . I tried using

cin.ignore() , cin.clear() , cin.sync () , even cin.get()

but it always passes the getline() .

Any suggestions would be appreciated . thanks in advance.

Don't mix `istream::operator>>` and `getline()`. Better yet, don't use `istream::operator>>` at all. It's quirky and clumsy and incredibly hard to use it correctly. Read lines one by one and parse them using sane functions (such as `std::string::find()` and/or `std::stoi()` if you need to convert to numbers.) — The Paramagnetic Croissant, Mar 02 '15 at 08:21
*"incredibly hard to use it correctly"*? What's hard about `string uIn = ""; cin >> uIn;`? — Jonny Henly, Mar 02 '15 at 08:24
Input doesn't come any longer from keyboard when you do this `./a.out < in.txt` — πάντα ῥεῖ, Mar 02 '15 at 08:24
I'm exactly looking for a way getting inputs from keyboard after reading from file ! — ADAN, Mar 02 '15 at 08:37
@JonnyHenly for example the stuff it will or will not leave in the input buffer after the operator has been executed. that's generally the source of all evil when working with I/O streams. — The Paramagnetic Croissant, Mar 02 '15 at 08:37
@ADAN then use `std::ifstream` to read from a file, and then read from the keyboard. don't expect the shell to read your mind. — The Paramagnetic Croissant, Mar 02 '15 at 08:38

score 1 · Answer 1 · edited May 23 '17 at 11:56

Well this is obvious why any input from standard input won't work. This is not limited to getline. operator>> also won't work, nor anything associated with std.cin or stdin.

This is because ./a.out < in.txt is not "giving a txt to a program". It is redirecting the standard input. By default it is a keyboard input from terminal, but you are switching it to a file on a shell level.

If you wanted to switch from terminal associated input to a file, and then backwards, this would be handy: How to redirect cin and cout to files?. Though, in this your case, you will not have the original buffer associated with the terminal, precisely because you make OS not give it to you. You would need to obtain somehow from the system. This would be more complicated (well it would be os specific and less portable) than below suggestions.

I would suggest you to pass the filename as an argument and do not redirect standard input. Or, if you are gonna write some command prompt anyways, make a load_file command, for loading the file. Then load the file using regular means designed for files e.g. ifstream.

C++ Reading from keyboard after giving a .txt to program (used getline() after in)

1 Answers1