Command-line arguments Introduction

Command-line arguments
CS 201 Fundamental Structures
of Computer Science
Introduction
• A command-line argument is the information that
follows the name of the program on the
command line of the operating system
• Command-line arguments are used to pass
information into a program when you run it
– They facilitate the use of your program in batch files
– They give a professional appearance to your program
1
Introduction
• C++ defines two built-in parameters to main()
– They receive the command line arguments
– Their names are argc and argv
• The names of the parameters are arbitrary. However, argc
and argv have been used by convention for several years.
– They are optional
int main( int argc, char *argv[] )
• argc is an integer
– Holds the number of arguments on the command line
– Since the name of the program always corresponds to
the first argument, it is always at least 1
2
int main( int argc, char *argv[] )
• argv is a pointer to an array of character pointers.
– Each character pointer in the argv array corresponds a
string containing a command-line argument
• argv[0] points the name of the program, argv[1] points to the first
argument, argv[2] points to the second argument, …
– Each command-line argument is a string
• If you want to pass numerical information to your program, your
program should convert the corresponding argument into its
numerical equivalent
– Each command-line argument must be separated by
spaces or tabs
• Commas, semicolons, and the like are not valid argument
separators
#include <iostream>
using namespace std;
int main( int argc, char *argv[] ){
cout << "Hello ";
for (int i = 1; i < argc; i++)
cout << argv[i] << " ";
cout << "! " << endl;
return 0;
}
[cgunduz@knuth cgunduz]$ g++ prog1.cpp –o exe_1
[cgunduz@knuth cgunduz]$ ./exe_1 Cigdem Gunduz
Hello Cigdem Gunduz !
[cgunduz@knuth cgunduz]$ ./exe_1 Cigdem Gunduz Demir
Hello Cigdem Gunduz Demir !
3
Passing numeric command-line arguments
• All command-line arguments are passed to the program
as strings
– Your program should convert them into their proper internal
format
• For that, C++ supports standard library functions; most
commonly used ones are:
– atof() : converts a string to a double and returns the result
– atoi() : converts a string to a int and returns the result
– atol() : converts a string to a long int and returns the result
• Each of these functions
– Expects a string containing a numeric value as an argument
– Uses the header <cstdlib>
#include <iostream>
#include <cstdlib>
using namespace std;
int main( int argc, char *argv[] ){
if (argc != 4){
cout << "Usage: \n\t";
cout << "1. Integer (0) or double (1) division\n\t";
cout << "2. Operand 1\n\t3. Operand 2\n\n";
exit(1);
}
if (atoi(argv[1]) == 0){
int a = atoi(argv[2]);
int b = atoi(argv[3]);
cout << a << "\\" << b << " = " << a / b << endl;
}
else{
double a = atof(argv[2]);
double b = atof(argv[3]);
cout << a << "\\" << b << " = " << a / b << endl;
}
return 0;
}
4
[cgunduz@knuth cgunduz]$ g++ prog2.cpp
[cgunduz@knuth cgunduz]$ ./exe_2
Usage:
1. Integer (0) or double (1) division
2. Operand 1
3. Operand 2
[cgunduz@knuth cgunduz]$ ./exe_2 0 5
5\3 = 1
[cgunduz@knuth cgunduz]$ ./exe_2 1 5
5\3 = 1.66667
[cgunduz@knuth cgunduz]$ ./exe_2 0 8.2
8\2 = 4
[cgunduz@knuth cgunduz]$ ./exe_2 1 8.2
8.2\2.9 = 2.82759
-o
exe_2
3
3
2.9
2.9
5