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Parameter variables

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• Parameter variables
  • The specification of a parameter variable in a function definition is actually a variable declaration, but without using the extern keyword

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  • Example:

    double cube( double x ) { return( x * x * x ); }

    The parameter specification

    double x

    is actually a declaration.

    It's somewhat like the construct:

    extern double x

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• Evidence that parameter specification is a declaration
  • We can tell that a parameter specification is in fact a declaration by using an array as parameter

  • Recall that in a declaration of an array variable, the SIZE (dimension) can be omitted

    Example:

    Array definition: int a[10]; // SIZE is required // in array definition Array declaration: extern int a[]; or: extern int a[10];

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  • Consider the following function that sum the elements in an array of 10 elements:

    int sum( int a[10] ) { int s = 0; for ( i = 0; i < 10; i++ ) s = s + a[i]; } int main() { int x[10]; int s; fill x with value; s = sum(x); }

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

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  • The program will work without the SIZE information in the array:

    int sum( int a[] ) // No size info !!! { int s = 0; for ( i = 0; i < 10; i++ ) s = s + a[i]; } int main() { int x[10]; int s; fill x with value; s = sum(x); }

    This is only allowed when the array variable (a) is declared !!!

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• The parameters of the main function

  • Now consider the parameters of the C++'s main function:

    int main ( int argc, char * argv[] ) { ... }

  • The parameter specification declares the following types of parameters:

    argc:      int Read the specification as: extern int argc argv:      one-dimensional array of char * Read the specification as: extern char * argv[]

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  • The array of char * is used to point to a sequence of strings:

    In the figure above, we see that:

    argv[0] points to the string "Hello" argv[1] points to the string "World"

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• How to use the parameters of the main function
  • It is quite easy to use argv[] because Java's main() function, also uses an String array as parameter.

  • The only difference is that Java's array has an internal length information, while the length of argv[] in C++ is given in the first parameter of main()

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  • Here is an example program that prints all the strings in the array argv[]:

    int main(int argc, char *argv[]) { int i; for ( i = 0; i < argc; i++ ) cout << "argv[" << i << "] = " << argv[i] << endl; }

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  • Example Program: (Demo above code)                                                
    • Prog file: click here

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• Converting strings to int and double
  • It is common to provide integers or double as arguments to a program

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  • In Java, you convert the string command line arguments using:

    int Integer.parseInt( String s ); double Double.parseDouble( String s );

    Notice that these are class (static) methods.

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  • In C++, you convert the string command line arguments using:

    int atoi( char *s ); // ASCII to Integer double atof( char * s ); // ASCII to float

    Example:

    int main(int argc, char *argv[]) { int i; double sum = 0; for ( i = 1; i < argc; i++ ) sum = sum + atof( argv[i] ); cout << "sum = " << sum << endl; }

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  • Example Program: (Demo above code)                                                
    • Prog file: click here