For faster input, I read that you can do file-redirection and include a file with the cin inputs already set.
In theory it should be used like following:
App.exe inputfile outputfile
As far as I understood from C++ Primer book, The following C++ code[1] should be reading cin input from the text file and shouldn't need to any other special indication like[2]
[2]
include <fstream>
ofstream myfile;
myfile.open ();
[1] The following C++ code...
#include <iostream>
int main()
{
int val;
std::cin >> val; //this value should be read automatically for inputfile
std::cout << val;
return 0;
}
Am I missing something?
4 Answers
To use your code [1] you have to call your program like this:
App.exe < inputfile > outputfile
You can also use:
App.exe < inputfile >> outputfile
In this case the output wouldn't be rewritten with every run of the command, but output will be appended to already existing file.
More information about redirecting input and output in Windows you can find here.
Note that the <, > and >> symbols are to be entered verbatim — they are not just for presentation purposes in this explanation. So, for example:
App.exe < file1 >> file2
In addition to original redirection >/ >> and <
You can redirect std::cin and std::cout too.
Like following:
int main()
{
// Save original std::cin, std::cout
std::streambuf *coutbuf = std::cout.rdbuf();
std::streambuf *cinbuf = std::cin.rdbuf();
std::ofstream out("outfile.txt");
std::ifstream in("infile.txt");
//Read from infile.txt using std::cin
std::cin.rdbuf(in.rdbuf());
//Write to outfile.txt through std::cout
std::cout.rdbuf(out.rdbuf());
std::string test;
std::cin >> test; //from infile.txt
std::cout << test << " "; //to outfile.txt
//Restore back.
std::cin.rdbuf(cinbuf);
std::cout.rdbuf(coutbuf);
}
[I am just explaining the command line argument used in Question]
You can provide file name as command line input to the executible, but then you need to open them in your code.
Like
You have supplied two command line arguments namely inputfile & outputfile
[ App.exe inputfile outputfile ]
Now in your code
#include<iostream>
#include<fstream>
#include<string>
int main(int argc, char * argv[])
{
//argv[0] := A.exe
//argv[1] := inputFile
//argv[2] := outputFile
std::ifstream vInFile(argv[1],std::ios::in);
// notice I have given first command line argument as file name
std::ofstream vOutFile(argv[2],std::ios::out | std::ios::app);
// notice I have given second command line argument as file name
if (vInFile.is_open())
{
std::string line;
getline (vInFile,line); //Fixing it as per the comment made by MSalters
while ( vInFile.good() )
{
vOutFile << line << std::endl;
getline (vInFile,line);
}
vInFile.close();
vOutFile.close();
}
else std::cout << "Unable to open file";
return 0;
}
It is important that you understand the concept of redirection. Redirection reroutes standard input, standard output, and standard error.
The common redirection commands are:
>redirects standard output of a command to a file, overwriting previous content.$ command > file>>redirects standard output of a command to a file, appending new content to old content.$ command >> file<redirects standard input to a command.$ command < file|redirects standard output of a command to another command.$ command | another_command2>redirects standard error to a file.$ command 2> file$ command > out_file 2> error_file2>&1redirects stderr to the same file that stdout was redirected to.$ command > file 2>&1
You can combine redirection:
# redirect input, output and error redirection
$ command < in_file > out_file 2> error_file
# redirect input and output
$ command < in_file > out_file
# redirect input and error
$ command < in_file 2> error_file
# redirect output and error
$ command > out_file 2> error_file
Even though, it is not part of your question, you can also use other commands are powerful when combined with redirection commands:
