c++ read from file redirection and also keyboard

Question

My program reads a list of integers from user input [ keyboard] and calculates some statistics

The user enters 'x' to terminate the input process.

So for example,

Enter integers separated by space ( enter x to quit) : 1 2 3 4 5 x

But now I want to include the inputs to be read from file redirection also. So if the numbers followed by x is in a data file, the program should take it from there if not then prompt the user

Answer 1

use isatty for your file descriptor (0 for standard input)

example:

#include <unistd.h>
main(){
  if(isatty(0))
    puts("tty");  // print some prompt
  else
    puts("pipe"); // not really needed in your case
}

Answer 2

You can read strings from a file with fstream and iostream.

Then from there you just parse the strings to see if they contain the correct data. If not, prompt the user for input?

Answer 3

If what you mean is that you can either call your program with a file as a command line argument, or not, in which case you want it to read from standard input, you can do something like this:

#include <fstream>
#include <iostream>

void run(std::istream& in) {
   // read all input from 'in' and run your program as normal
}

int main(int argc, char **argv) {
   if(argc == 1) {
      run(std::cin);
   } else if(argc == 2) {
      std::ifstream fin(argv[1]);
      run(fin);
   }

   return 0;
}

This way, if you call

./prog

it'll read from standard input (i.e., the keyboard), and if you call

./prog foo.txt

it'll read from the file foo.txt.

You may want to do a little more work when checking the command line arguments, but this is the basic idea.

Answer 4

one approach that would not require any program alterations is to let the shell do the redirection for you.

On both Windows and Unix shells, < redirects a file to stdin for the program.

So, on unix/linux/mac on a console:

./app < file.txt

or windows, at the command prompt, just:

app < file.txt

will take the contents of file.txt and send it as stdin to the program called 'app'.

Answer 5

On a unix box the following should work:

progream <file

where program is your program and file contain the input that the user would type in.

Answer 6

A method that I use is to put the input routine into a function that uses istream. Call the function using std::cin or from a file.

Example:

#include <iostream>
#include <cstdlib>

bool
Get_Input_Value(std::istream& inp,
                int&  value)
{
  inp >> value;
  return inp.good();
}

int
main(void)
{
  std::cout << "Enter numbers: ";
  std::cout.flush() // Make the prompt appear.
  int the_value = 0;
  bool success = false;
  success = Get_Input_Value(std::cin, the_value);
  if (success)
  {
    std::cout << "\nThe value entered: " << the_value << std::endl;
  }
  else
  {
    std::cerr << "An error occurred with the input stream.\n";
  }
  return EXIT_SUCCESS;
}

The concept here is to use an std::istream variable for your input and assign it to a file or std::cin before any processing.