PIC10A lecture 8

Reading

p103-116

Functions

How it works?

When the program reaches a function call:

the program plugs in the value of the parameters(arguments)
the program execute the codes of the function
value will be returned.

void

A void function performs some actions
but it does not return a value
function call of a void function is similar to a function call that returns a value.
it is terminated with a semicolon and is used as a statement rather than as an expression.

Example

#include<iostream>

using namespace std;

      

void showDate(int month, int day);

      

int main() {

    cout << "Today is ";

    showDate(11,23);

    cout << ".\n";

    return 0;

}

      

      void showDate(int month, int day)
{

    switch(month) {

    case 1:

        cout << "Jan";

        break;

    case 2:

        cout << "Feb";

        break;

    case 3:

        cout << "Mar";

        break;

    case 4:

        cout << "Apr";

        break;

    case 5:

        cout << "May";

        break;

    case 6:

        cout << "Jun";

        break;

    case 7:

        cout << "Jul";

        break;

    case 8:

        cout << "Aug";

        break;

    case 9:

        cout << "Sep";

        break;

    case 10:

        cout << "Oct";

        break;

    case 11:

        cout << "Nov";

        break;

    case 12:

        cout << "Dec";

        break;

    }

      

    cout << " " << day;

}

Output

Today is Nov 23.

Writing your own functions

writing functions mainly requires writing function definitions
functions are like small programs within your program

Example

#include<iostream>
using namespace std;

double finalScore(double midterm, double exam);
char grade(double score);

int main() {
    double midterm, exam; // score for midterm and final exam
    double final; // final result
    cout << "Please enter your midterm and final score: ";
    cin >> midterm >> exam;
	
    final = finalScore(midterm, exam);
	
    cout << "Your grade is " << grade(final) << ".\n";

	return 0;
}


// input: scores for midterm, exam
// return: final score
double finalScore(double midterm, double exam) {
    double result1 = 0.5*midterm + 0.5*exam;
    double result2 = 0.4*midterm + 0.6*exam;
    // the final score is the max of result1 and result2
    if(result1 >= result2) 
	return result1;
    else 
	return result2;
}

// input: score
// output: grade
char grade(double score) {
    if (score >= 90) {
        return 'A';
    } else if (score >= 80) {
        return 'B';
    } else if (score >= 65) {
        return 'C';
    } else if(score >= 50) {
        return 'D';
    } else {
        return 'F';
    }
}

Before writing your functions:

You should determine

the name of the function
what is returned ( void or other type?)
what are the parameters (arguments)
what do you want to do in the function

Example: write a function to calculate tax.

Syntax of functions

function prototype (or called function declaration)
function definition

function header
function body

#include<iostream>

using namespace  std;

      

double myFunction(int parameter1, char parameter2); // function
prototype

      

int main() {                
 
                   
 //
program body

    .........

    return 0;

}

      

double myFunction(int parameter1, char parameter2) { // function header

     // function body

}

Function Prototypes

also called function declaration
indicate the parts of the function without including the definition
similar to declarations for variables
they appear before int main()
require a semicolon;
example : double finalScore(double midterm, double exam); at the begining of the example

Function prototypes consist of

type for value returned (or void)
function name
parameters (type and name)
and a semicolon;

Alternate form for function declarations

The parameter names can be omited
example : double finalScore(double, double);

Function Definition

it includes

function header
function body

Function header

very similar to function prototypes
consists of

same return type (or void) as declared in the function prototype
same function name
same parameter types
parameter names (can be different from the names used in the function prototype)

no semicolon;

Function Body

it follows the function header
like a little program in itself
it consists of variable declarations and executable statements enclosed within a pair of braces
the parameters are declared in the function header. You shouldn't redeclare them.
when the function is called, the argument values are plugged in for the formal parameter, and then the statements in the body are executed.
the value returned by the function is determined when the function executes a return statements

return

Syntax : return expression;
when the program reaches the return statement , the function call stops and the value is returned.

return in void function

Syntax: return
when the program reaches the return statement , the function call stops

Example:

#include<iostream>

#include<cmath>

using namespace std;

      

void squareRoot(double num);

      

int main() {

    squareRoot(3);

    squareRoot(-3.5);

    return 0;

}

      

void squareRoot(double x) {

    if( x < 0) {

        cout << "The square root of
"
<< x << " is meaningless.\n";

        return;

    }

      

    cout << "The square root of " << x
<<
" is " << sqrt(x) <<".\n";

}

Output

The square root of 3 is 1.73205. The square root of -3.5 is meaningless.

Function calling functions

A function body can call other functions
refer to the previous example

Main as a function

main part of the program is actually the definition of a function called main.
when the program starts, the function main is automatically called.

Recursive function

Question: write a function to calculate

square of 1 + square of 2 + square of 3 + .... + square of n

#include<iostream>

using namespace std;

      

int squareSum(int n);

      

int main() {

    cout << squareSum(3) << endl;

      

    return 0;

}

      

int squareSum(int n) {

    if(n <= 0) {

        return 0;

    }

      

    return n*n + squareSum(n-1);

}

The output is 14.