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Function Prototype
A function may take input parameters and produce a return value
- need to know its name, type of result , type of each parameter
y = sqrt(9.0);
Function prototype
- describes how the function is called
- tells everything you need to know to make a function call
double sqrt ( double number);
result function parameter formal type name type parameter
- formal parameter is a place holder to stand for the actual parameter
- function prototype terminates with semi-colon
Function Definition
Pre-defined or programmer-defined
- Predefined functions in libraries: a partial list on p. 105
- Programmer-defined functions: need to specify the function prototype and function definition
Terminology
- A function is defined using a function header and function body
— A function is defined once
Function header
- written exactly the same as the function prototype, except ";"
- declares the type of return value, name of the function, type of each parameter, and name of each formal parameter
Function body
- follows the function header and completes the function definition
- consists of declarations and executable statements -- similar to the body of the main part of the program
Simple Value-Returning Function
// Illustrate the function definition and its // use in the main program
#include <iostream.h>
int two_times(int number); // Returns the number multiplied by two
int main() { int x = 10, y = 0;
y = two_times(x); cout << "y is: " << y << endl; y = two_times(y) * 100; cout << "Now y is: " << y << endl; return 0; } int two_times( int number) { return (2 * number); }
Some more on Pre-defined Functions
A variety of "libraries"
- iostream.h - for streams of character I/O
- math.h - for some mathematical functions
— sqrt(double), pow(double, double), fabs(double)
- stdlib.h - "standard library"
— abs(int), labs(long)
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Type-changing ("casts")
int value; double result; value = 5; result = value/2; // ouch - stores 2.0 - not //what we intended // try result = double (value) / 2; // stores 2.
Parameter Passing
#include <iostream.h> double total( int number, double price); // Returns the total cost including tax
int main() { double cost, bill; int units = 10;
cost = 50.00; bill = total(units, cost);// the CALL cout << "Total cost is: " << bill << endl; return 0; } double total( int number, double price) { double subtotal; subtotal = number * price; return (subtotal * 1.05); }
Arguments are used for all instances of the formal parameters
that occur in the function body
- units -> number cost -> price
Placing Calls
int prompt_and_read();
// prompt for an integer and return it
int main() { int first_number,second_number ; first_number = prompt_and_read(); // CALL second_number = prompt_and_read(); // CALL
cout << "The numbers read in are:\n" << first_number << "\n" << second_number << "\n"; return 0; }
int prompt_and_read()
{
int value_user_typed; cout << "Input an integer number: " ; cin >> value_user_typed ; return value_user_typed ; }
Scope
When writing a function, how do you know you haven’t inadvertantly overwritten a variable, say cost, that was also used by the calling program?
#include <iostream.h> double total( int number, double price); // Returns the total cost including tax
int main() { double cost, bill; int units = 10;
cost = 50.00; bill = total(units, cost); cout << "Unit cost is: " << cost << endl; cout << "Total cost is: " << bill << endl; return 0; } double total( int number, double price) { double cost; cost = number * price return (cost * 1.05); }
Local Variables
The concept of variable scope addresses this issue
- variables declared in the function are only visible to that function: local variables
- variables declared in the calling program are only visible to the calling program
- unless the programmer wants them to be visible to the function
Global Variables
#include <iostream.h>
int main () { int x ; char y ; ........................... ............................. return 0 ; }
const double z = 3.14;
int sum () { double x ; double y ; ................. }
int root() { int x; ............................. }
