Programming and Graphics

5.4 Command line arguments 141

we will see on the screen:

1os

2is

3 running

We can use a double pointer to simplify the arguments of the main func-
tion by stating:

int main(int argc, char ** argv)

Building commands

The ability to receive information from the operating system allows us to
build command-line applications. For example, suppose that we want to build
a command that generates a file with a specified name, prints a zero in the file,
and then closes the file. The command is implemented in the following code
contained in the filenfz.ccand compiled into an executable namednfz:

#include <fstream>

using namespace std;

int main(int argc, char **argv)

{

ofstream file1;

file1.open(argv[1]);

file1 << "0";

file1.close();

return 0;

}

Running the code by typing in the command line:

nfz sage

generates a file namedsagecontaining a zero.

What if we forget to type the name of the file or accidentally type multiple
file names? In the first caseargc=1, and in the second caseargc>2. To issue
a warning, we include the iostream system header file and insert the following
block at the top of code:

if(argc != 2 )

{

cout<< "Please use: "<< argv[0] <<" <filename>" << endl;

return 1;

};