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FUNCTIONS - IChapter 5

Functions help us write more complex programs

Agenda

Introducing Functions

Tell me a story– void FunctionsParameters and Arguments

Random numbers

Do the Math– value returning FunctionsReturn statement

Prototypes and file layout

It’s Logical—bool Functions

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Functions are building blocks…

Each box is a

“mini-program” of 5-20 lines of

code

We use functions…

When a task (sequence of code) gets routine and needs to be repeated several times in a program.

When we want to hide details of a complicated calculation from the main flow of the program

As a tool to break down complicated problems into easily solvable pieces

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A Simple Function (to begin a story)

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Function calls

Function definition

A function call is a “controlled jump”

Program execution always begins in main( )

A function call tells computer to jump into another part of the file (where the definition is)

When finished, the computer returns to the line following the function call and continues on thru the program

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Controlled Jump Illustration

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Start

Once upon a time,

Once upon a time,

Console Output

Function Syntax Rules

The function call has the same name as the function definition

The function definition must appear before the function call in it’s own separate block of code

Curly braces mark the beginning and end of a function definition

A function definition cannot be put inside another function definition

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What the heck is “void”?

The first line of a function definition is called the function head or header:

void opening( )

The syntax for the function header is: return-type function-name ( parameters )

The word void indicates the function does not return any data when it finishesPart 2 deals with non-void or value-returning functions

Notice int main( )? This allows main( ) to return a 0 when it completes successfully.

Also, right now, we are not using any parameters9

Correct Program Layout

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These are the

“building blocks” of

code

Do not overlap the

blocks

Main( ) is also a

function definition!!

Incorrect Program Layout

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These

Function

Blocks are

Nested—

Syntax Error!!

Parameters and ArgumentsInformation can be given to a function during the function call

This allows a variety of function behaviors

Makes functions more useful

Information is passed from an argument in the function call

to a parameter in the definition

We’ll modify our program to show this:• opening( ) opening(2); // where 2 is an argument

• We’ll also have to modify the function definition 12

Passing information into a function

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2 is the argument

m is the parameter

No matter what argument is

given, parameter m

get’s a copy of it

This morning,Console Output

Other Argument Possibilities

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This morning,

Once upon a time,Once upon a time,This morning,

This morning,

Console Output

Each function call demonstrates a different argument passed to

parameter m in opening( )

What is a parameter again?

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The parameter m receives whatever argument is given in

the function call.

When the argument is a variable, the value of the variable is given

to m

Here m in main( ) is different from m in opening( ). Since they

live in different functions, they are like separate variables.

The story continues…

In the lab, you will get to write your own functions to tell a story

And you’ll be able to select a variety of components using a random number generator

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Random Numbers!!Sometimes we need the computer to pick a number “at random”

Very common in video games and computer simulationsRequires a different kind of function…one that returns a value so we can use it later.

int x; x = rand( );

x now holds a “pseudo” random number between +/-2 billion, for example: 230923, 102912009, 1942390439, -2039329, etc

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Notice: This function call is in an assignment statement

+/- 2 billion?The output of rand( ) is not very useful

Too broad a range

To make useable, we use modulus (%) to chop it down:

Example:• 25678%10 = ?

• 23042309%10 = ?

• -32098098%10 = ?

Using %10 gives us a random value between 0 and 9

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To Randomize…Problem:

rand( ) by itself always gives the same random number.

Solution: Start your program with a call to srand(time(NULL));

This starts the math formula in rand( ) off with a unique #,• The number of seconds past Jan 1, 1950 (or so)

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Plug x in here and now the random number selects the opening

Start your creative juices…

Finish Lab4Function1.cpp

Next: Part 2, Value Returning functions

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Before starting Part 2…

a) Function Call _______

b) Function Definition_______

c) Parameter_______

d) Argument_______

e) Local Variable_______

f) Return Type_______

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Part2 Value Returning Functions

Useful in math and other operationsThese functions compute a result and return it

For standard math, #include<cmath>

We know that 24= 2*2*2*2 = 16

In C++ we could say pow(2.0, 4);But nothing will be displayed!!!

Show your Result

Unlike for void functions, when a math function returns, the original function call is replaced with the result.

pow(2.0, 4);

To show the result you must either cout or store it in a variable:

A) cout<<pow(2.0, 4) ;

B) z = pow(2.0, 4); cout<<z ;

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Notice, 2 arguments

BUT Don’t cout a void function: cout<<opening( ); ERROR

For the Math Whiz…

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Standard C++ Library

The Standard C++ library is a collection of pre-defined functions which are accessed through header files (such as <cmath>, <iostream> etc..)

Notice that these pre-defined functions don’t show the processing step (function definition): we do not need to know how the function works, just what it does.

Making your own VRFsWhen the Standard Library doesn’t have what you

need, you can make your own. A value returning function is very similar to a void function with only two small changes:

1. Instead of void before the function name, we put the data type of the return value.

2. At the end of the function definition, we put a return statement to return the result.

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EXAMPLE—Dollar ValueRemember Lab 2?

We can use a function call to calculate dollars: dollars = dollarValue( nickels, dimes);

We are now going to write the definition of the function. But first…

Use the Function Definition Checklist

When you are given an example function call, there are a few steps to follow before writing the function definition to make sure your code works.

1. What is the name of the function? ________

2. What are the parameters it needs (and their type)? __________________________

3. What is the return-type of the function (the type of the result it computes)?___________________

4. Write the function header __________________

All of the answers can be determined from the previous slide!!!

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float dollarValue( int n, int d )

dollarValue

Number of nickels and dimes, both integers

The variable dollar gets the result and it’s float

Now we can write the definition

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Type of return value

Consistent with return-type

value is a “local variable”return-type

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return…

It terminates the execution of the function

The function’s return-type specifies the data type of the values that it would return to the calling program

Its syntax is :

return expression ;

where the data-type of the expression

value = function's return-type

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Let’s try another!Define a function named calcSize; the function is passed a character argument representing a size code and the function returns the size in inches (an integer) according to the following chart and sample function calls:

Function Definition Checklist

1. What is the name of the function? ________

2. What are the parameters it needs (and their type)? __________________________

3. What is the return-type of the function (the type of the result it computes)?___________________

4. Write the function header __________________

All of the answers can be determined from the previous slide!!!

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Now Write the Function Definition

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And another!Write a function that takes arguments for a user’s height and weight, and then computes their hat size according the the following formula:

Hat size = 2.9 times the weight in pounds divided by height in inches

Here is an example function call:hsize = hat(weight, height);

Here is some example test data:weight=150, height=70 hsize=6.2

Function Definition Checklist

1. What is the name of the function? ________

2. What are the parameters it needs (and their type)? __________________________

3. What is the return-type of the function (the type of the result it computes)?___________________

4. Write the function header __________________

All of the answers can be determined from the previous slide!!!

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Now Write the Function Definition

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Bool Functions for LogicA bool function is a special type of VRF

bool data type has only two values: true, false

This means a function that returns a bool value can be used in an if-statement

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A bool function exampleSuppose you want to check if n is between 0 and 100. You could say

Or you could put the condition in a bool function:

And use that function instead of the boolean logic

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Time to explore on your own…

Finish Lab4Function2.cpp

Practice will help you master this topic

Recommend you solve one more function from the Assignment 4 handout