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#include <iostream>
#include <string>
// http://en.cppreference.com/w/cpp/language/main_function
// example of expected command line: program_name 10000 5.3 7
// argc == 4 the number of command line arguments; the first is the name of the program
// argv[0] == "program_name", argv[1] == "10000", argv[2] == "5.3", argv[3] == "7"
int main( int argc, char* argv[] )
{
// not very serious because these are constants (with internal linkage),
// but in general, it is a good idea to limit the visibility of names
// to the smallest scope in which they are needed.
// here, we need these values only inside main; they need not be at namespace scope.
const int RULE72 = 72;
const int YDAYS = 365;
if( argc != 4 ) // if the program was invoked with wrong number of command line arguments
{
// inform the user, with a message indicating how the program should be invoked
// std::cerr sends output to stderr, this is where error messages usually go
// see: https://en.wikipedia.org/wiki/Standard_streams#Standard_error_.28stderr.29
std::cerr << "usage: " << argv[0] << " <amount> <interest rate> <years>\n" ;
return 1 ; // return from main (return to the host environment) with status 1
// by convention, if the value returned by main is non-zero,
// it indicates unsuccessful termination of the program
}
try
{
// try to convert the command line arguments (null-terminated strings)
// to the numeric values that we need. eg. the amount argv[1] ( eg. "10000" ) to double
// this attempt at conversion may fail; for example if argv[1] == "hello"
// if the attempted conversion fails, an exception would be thrown
// so we put the conversion code inside a try block
// see: http://www.cplusplus.com/doc/oldtutorial/exceptions/
// (ignore the section 'Exception specifications'. It gives wrong information.)
// http://en.cppreference.com/w/cpp/string/basic_string/stol
// http://en.cppreference.com/w/cpp/string/basic_string/stof
// the links above have more information on these functions
const double amount = std::stod( argv[1] ) ;
const double int_rate = std::stod( argv[2] ) ;
const int years = std::stoi( argv[3] ) ;
// validate: for example, amount is positive etc.
// if validation fails, quit with an error message
std::cout << "amount: " << amount << '\n'
<< "interest rate: " << int_rate << '\n'
<< "number of years: " << years << '\n' ;
// compute stuff, display results
}
catch( const std::exception& )
{
// this block is executed only if an exception was thrown
// when we attempted the conversion of command line arguments to numeric values
std::cerr << "badly formed fields in command line\n" ;
return 1 ;
}
// The main function need not have a return statement:
// if control reaches the end without encountering a return statement,
// the effect is as if we had written return 0;.at the end
}
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