FUNCTIONS I Chapter 5

1
FUNCTIONS - IChapter 5

• Functions help us write more complex programs

2
Agenda

• Introducing Functions
• Tell me a story void Functions
• Parameters and Arguments
• Random numbers
• Do the Math value returning Functions
• Return statement
• Prototypes and file layout
• Its Logicalbool Functions

3
Functions are building blocks
Each box is a mini-program of 5-20 lines of
code
4
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

5
A Simple Function (to begin a story)
Function definition
Function calls
6
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

7
Controlled Jump Illustration
Console Output
Once upon a time, Once upon a time,
Start
8
Function Syntax Rules

• The function call has the same name as the
  function definition
• The function definition must appear before the
  function call in its 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

9
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 finishes
• Part 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 parameters

10
Correct Program Layout
These are the building blocks of code
Main( ) is also a function definition!!
Do not overlap the blocks
11
Incorrect Program Layout
These Function Blocks are Nested Syntax Error!!
12
Parameters and Arguments

• Information 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
• Well modify our program to show this
• opening( ) ? opening(2) // where 2 is an
  argument
• Well also have to modify the function definition
  ?

13
Passing information into a function
m is the parameter
No matter what argument is given, parameter m
gets a copy of it
2 is the argument
Console Output
This morning,
14
Other Argument Possibilities
Console Output
This morning, Once upon a time,Once upon a
time,This morning, This morning,
Each function call demonstrates a different
argument passed to parameter m in opening( )
15
What is a parameter again?
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.
16
The story continues

• In the lab, you will get to write your own
  functions to tell a story
• And youll be able to select a variety of
  components using a random number generator

17
Random Numbers!!

• Sometimes we need the computer to pick a number
  at random
• Very common in video games and computer
  simulations
• Requires a different kind of functionone 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

Notice This function call is in an assignment
statement
18
/- 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
• 2567810 ?
• 2304230910 ?
• -3209809810 ?
• Using 10 gives us a random value between 0 and 9

19
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)

Plug x in here and now the random number selects
the opening
20
Start your creative juices

• Finish Lab4Function1.cpp
• Next Part 2, Value Returning functions

21
Before starting Part 2
22
Part2 Value Returning Functions

• Useful in math and other operations
• These functions compute a result and return it
• For standard math, includeltcmathgt
• We know that 24 2222 16
• In C we could say pow(2.0, 4)
• But nothing will be displayed!!!

23
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) coutltltpow(2.0, 4)
• B) z pow(2.0, 4) coutltltz

16
Notice, 2 arguments
BUT Dont cout a void function coutltltopening(
) ERROR
24
For the Math Whiz
25
Standard C Library

• The Standard C library is a collection of
  pre-defined functions which are accessed through
  header files (such as ltcmathgt, ltiostreamgt
  etc..)
• Notice that these pre-defined functions dont
  show the processing step (function definition)
  we do not need to know how the function works,
  just what it does.

26
Making your own VRFs

• When the Standard Library doesnt 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
• Instead of void before the function name, we put
  the data type of the return value.
• At the end of the function definition, we put a
  return statement to return the result.

27
EXAMPLEDollar Value

• Remember 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

28
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.
• What is the name of the function? ________
• What are the parameters it needs (and their
  type)? __________________________
• What is the return-type of the function (the type
  of the result it computes)?___________________
• Write the function header __________________
• All of the answers can be determined from the
  previous slide!!!

dollarValue
Number of nickels and dimes, both integers
The variable dollar gets the result and its float
float dollarValue( int n, int d )
29
Now we can write the definition
value is a local variable
return-type
Type of return value Consistent with return-type
30
return

• It terminates the execution of the function
• The functions 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

31
Lets 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

32
Function Definition Checklist

• What is the name of the function? ________
• What are the parameters it needs (and their
  type)? __________________________
• What is the return-type of the function (the type
  of the result it computes)?___________________
• Write the function header __________________
• All of the answers can be determined from the
  previous slide!!!

33
Now Write the Function Definition
34
And another!

• Write a function that takes arguments for a
  users 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
• weight150, height70 ? hsize6.2

35
Function Definition Checklist

• What is the name of the function? ________
• What are the parameters it needs (and their
  type)? __________________________
• What is the return-type of the function (the type
  of the result it computes)?___________________
• Write the function header __________________
• All of the answers can be determined from the
  previous slide!!!

36
Now Write the Function Definition
37
Bool Functions for Logic

• A 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 ?

38
A bool function example

• Suppose 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

39
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