FUNCTIONS I Chapter 5

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FUNCTIONS - IChapter 5

• Functions help us write more complex programs

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SCENARIO 1

• You need to display some lengthy instructions at
  several places in your program. The instructions
  are identical each time. So you make a function
  to handle the display of instructions and use a
  simple function call, display_insructions( )
  whenever you need to show instructions.

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SCENARIO 2

• You have a complicated formula that you need to
  put in several places in your program but its
  too messy to copy and paste it everywhere you
  need it.

• A function is a way of packaging the formula so
  you can just say
• cents value(nickels, dimes)V
  sphere_volume(R)
• cents value(3, 8)V sphere_volume(4.6)

cents 5 nickels 10 dimes
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SCENARIO 2 Continued

• Perhaps a better exampleyou are doing your
  taxes. When you get to calculating your tax (by
  paper!) you are sent to a table, where you look
  up the tax for your income and bring it back into
  the line on your main worksheet.
• This aspect of jumping to another part of the
  program to perform a side calculation is what
  functions are all about.

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SCENARIO 3

• You are developing a program to run a banks ATM
  machine. Its so complicated you decide to break
  the problem up into smaller units. Each unit will
  handle a specific task, such as a) let a customer
  log in, b) find the balance of a customers
  account, 3) deposit money, 4) withdraw money, 5)
  view the last 5 transactions.
• Each unit is a function, a small "program" that
  accomplishes one task that can tested and
  verified, and then assembled into a larger
  programming structure, the main simulation
  program.

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What are functions ?

• Large programs can be modularized into sub
  programs which are smaller, accomplish a specific
  task and hence are more manageable
• These sub programs are called functions
• They can be compiled and tested separately and
  reused in different programs

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Things to do today Function Basics

• SCENARIO 1 Simplest
• Void Functions ?
• Void Functions with parameters
• Local variables
• Function Prototypes
• SCENARIO 2 A little more complex
• Functions that return a value
• Standard C library functions
• Test Drivers
• SCENARIO 3is project 2 after midterm

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Remember Lab 1?

• include ltiostreamgt
• using namespace std
• int main()
• // This is my first program
• cout ltlt Hello World" ltlt endl
• system("pause")
• return 0

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This does the same thing, using a function

• include ltiostreamgt
• using namespace std
• void message( )
• coutltlt"Hello World"ltltendl
• int main()
• // This is my first function program
• message( )
• system("pause")
• return 0

Function Definition
Function Call
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A little terminology

• A function definition has two parts
• the head and the body
• void message( )
• coutltlt"Hello World"ltltendl

Function name
Function Head
Function Body
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User-defined functions.. Contd..

• The syntax for the head of a function is
• return-type name(parameter-list)
• In the given example the head of the function is
• void msg()
• return-type name(parameter-list)
• (void means nothing no return value)
• Another example
• double dollar_value (int d, int q)
• // return value of dimes and quarters

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The function body

• The body of a function is the block of code that
  follows its head
• It is written between the braces following
  the function header
• It contains the code that performs the functions
  action
• Note that main() is a function whose body is the
  program itself

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Sprechen Sie Deutsches ? Handling multiple
languages

• void message( int choice )
• if (choice 1)
• coutltlt"Hello World"ltltendl
• else coutltlt"hallo Welt"ltltendl
• int main()
• // This is my first function program
• message(2)
• system("pause")
• return 0

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Making your own function, PART 1

• The packaging for a user-defined function is kind
  of complicated (you will get used to it)
• For the formula
• You would say
• float value(int nickels, int dimes)
• float cents 5nickels 10dimes
• return cents

cents 5 nickels 10 dimes
This is a function DEFINITION
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Part 2 Where does the function go?

• OPT 1 definition goes before main( )float
  value(int nk, int dms)
• float val 5nk 10dms
• return val
• int main( )
• int nickels, dimes, cents
• cout ltlt "Enter num nickels and dimes "
• cin gtgt nickels gtgt dimes cents value (
  nickels, dimes) coutltlt"the value is
  "ltltcentsltltendl

Function Definition
Function Call
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Part 2 Where does the function go?

• OPT 2 definition goes after main( ),
  withfloat value(int nk, int dms)int main( )
• int nickels, dimes, cents
• cout ltlt "Enter num nickels and dimes "
• cin gtgt nickels gtgt dimes cents value (
  nickels, dimes) coutltlt"the value is
  "ltltcentsltltendlfloat value(int nk, int dms)
• float val 5nk 10dms
• return val

Function Prototype(signature)
Function Call
Function Definition
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Standard C Library

• The Standard C library is a collection of
  pre-defined functions which are accessed through
  header files
• Notice about these pre-defined functions that the
  processing step is hidden we do not need to
  know what the function does to produce the output

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Example The sqrt() function

• This function returns the square root of a given
  positive number
• // sqroot.cpp
• include ltiostreamgt
• include ltcmathgt
• using namespace std
• int main()
• int x
• x 2
• cout ltlt sqrt(9) ltlt endl
• cout ltlt sqrt (10x 5) ltlt endl
• cout ltlt sqrt(x) ltlt endl
• return 0

New library
3 5 1.41421
Calls sqrt()
Method 1, function call in a cout statement
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Invoking a function, method 2

• A function can be invoked or called in a cout
  statement as shown in the previous example or by
  assigning its value to a variable
• Example
• float x25, y
• ysqrt(x)
• coutltlt x ltlt "ltlt y ltlt endl

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Method 2, function call in an assignment statement
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Passing by value Whats that ?

• Consider the function sqrt(x)
• The expression x is called the argument or actual
  parameter of the function call and we say that it
  is passed by value to the function
• So when x is 3, the actual value 3 is passed to
  the sqrt() function by the call sqrt(x)

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This can be illustrated as
main()
sqrt()
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float x
3
1.73205
float y
1.73205
Shaded box, Processing hidden
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Check this out!!

• Nesting of function calls
• int main()
• float y
• y sqrt(1 2sqrt(3 4sqrt(5)))
• coutltlt yltlt endl

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abs(k) works with integers
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Things to do today

• Functions Basics including
• Standard C library functions
• User - defined functions
• Test Drivers
• Function Declarations and Definitions
• Local variables and Functions
• void Functions

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User-defined functions

• A user defined function has two parts
• the head and the body
• Here is a simple example of a user defined
• function
• float cube(float x)
• return xxx //returns cube of x

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User-defined functions.. Contd..

• The syntax for the head of a function is
• return-type name(parameter-list)
• In the given example the head of the function is
• float cube(float x)
• return-type name(parameter-list)
• Another example
• double dollar_value (int d, int q)
• // return value of dimes and quarters

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The function body

• The body of a function is the block of code that
  follows its head
• It is written between the braces following
  the function header
• It contains the code that performs the functions
  action
• Note that main() is a function whose body is the
  program itself

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In the example

• The function body is
• return xxx //returns cube of x
• This includes the return statement that specifies
  the value that the function sends back to the
  place where it was called

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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

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Things to do today

• Functions Basics including
• Standard C library functions
• User - defined functions
• Test Drivers
• Function Declarations and Definitions
• Local variables and Functions
• void Functions

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Does my function work right ?

• That is the purpose of the test driver
• It is an ad-hoc program(minus all the usual
  niceties such as user prompts, output labels and
  documentation) written to test a function that we
  have created
• Remember the cube() function ? Lets write a test
  driver for that

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• float cube(float x)
• // returns cube of x
• return xxx
• int main()
• // test driver for cube() function
• coutltlt"cube(1) "ltltcube(1)ltltendl
• coutltlt"cube(-5) "ltltcube(-5)ltltendl
• coutltlt"cube(4) "ltltcube(4)ltltendl

1 -125 64
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Things to do today

• Functions Basics including
• Standard C library functions
• User - defined functions
• Test Drivers
• Function Declarations and Definitions
• Local variables and Functions
• void Functions

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Two ways for defining functions

• The complete definition of the function is listed
  above the main program like
• float cube(float x)
• // returns cube of x
• return xxx
• int main()
• float n5
• coutltlt cube(n) ltlt endl

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Another way is

• List only the functions header above the main
  program (declaration) like
• float cube(float) (or)
• float cube(float x)
• List the functions head and body below the main
  program (definition)
• declaration is also called a function prototype
• A function declaration is like a variable
  declaration

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The previous program can also be written as

• float cube(float x) //declaration
• int main()
• float n5
• coutltlt cube(n) ltlt endl
• float cube(float x) //definition
• // returns cube of x
• return xxx

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Actual Vs. Formal Parameters

• Actual parameters (We use ARGUMENTS)
• Parameters of the function in the function
  call
• Passed by value
• Formal parameters
• Variables listed in the functions
  parameter-list in the function definition
• Local to that function
• The Arguments are copied to the Formal Parameters
  during function call

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Actual Vs. Formal Parameters

• float cube(float) // function declaration
• int main()
• float n1
• while (n gt 0)
• cingtgt n
• coutltlt"cube("ltlt n ltlt") "ltlt cube(n) ltltendl
• // function definition
• float cube(float x)
• // returns cube of x
• return xxx

Actual Parameter n
Formal parameter x
The value of n is copied into x for each function
call
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Things to do today

• Functions Basics including
• Standard C library functions
• User - defined functions
• Test Drivers
• Function Declarations and Definitions
• Local variables and Functions
• void Functions

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Local variables and functions

• A local variable is declared inside a block and
  is accessible only from within that block
• Similarly a variable declared within a function
  is local to that function.. i.e it exists only
  when the function is executing
• A functions formal parameters (used at that
  point where the function is actually defined) are
  also regarded as being local to the function

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The variable larger is local to maxonly can
be used inside max

• int max (int a, int b)
• int larger
• if (a gt b)
• larger a
• else
• larger b
• return larger
• int main()//tests the max() function
• coutltltmax(5, 8) ltltmax(5,8)ltltendl
• coutltltmax(7, 3) ltltmax(7,3)ltltendl
• coutltltmax(7, 7) ltltmax(7,7)ltltendl

Parameters a b are like local variables
tooonly work inside max
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In the last example

• The function has three local variables
• a,b and larger
• The parameters a b are local because it is
  declared in the functions formal parameter list
• The parameter larger is local because it is
  declared within the body of the function

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The max function could also be written without a
local variable

• int max(int a, int b)
• int larger
• if (a gt b)
• larger a
• else
• larger b
• return larger

int max(int a, int b) if (a gt b)
return a else return b
Hint for Prob 7 min function
Both work the same way
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Whats wrong with this ?

• float cube(float x)
• // returns cube of x
• return xxx
• int main()
• // test driver for cube() function
• x cube(1)
• coutltlt"cube(1) "ltlt x ltltendl
• coutltlt"cube(-5) "ltltcube(-5)ltltendl
• coutltlt"cube(4) "ltltcube(4)ltltendl

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This would work !

• float cube(float x)
• // returns cube of x
• return xxx
• int main()
• // test driver for cube() function
• float x cube(1)
• coutltlt"cube(1) "ltlt x ltltendl
• coutltlt"cube(-5) "ltltcube(-5)ltltendl
• coutltlt"cube(4) "ltltcube(4)ltltendl

But for now, try not to reuse variable names in
functions
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Data Types Must Match!
int main() // test driver for grade() float
x grade(95) coutltltgrade(95) "ltlt x

• char grade (double avg)
• if (avg gt 92)
• return 'H'
• else if (avg gt 65)
• return 'P'
• else
• return 'F'

Whats wrong here?
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Data Types Must Match!
int main() // test driver for grade() char
G grade(95) coutltltgrade(95) "ltlt G

• char grade (double avg)
• if (avg gt 92)
• return 'H'
• else if (avg gt 65)
• return 'P'
• else
• return 'F'

Hint for Problem 7 grade fn
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Things to do today

• Functions Basics including
• Standard C library functions
• User - defined functions
• Test Drivers
• Function Declarations and Definitions
• Local variables and Functions
• void Functions

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void Functions

• If a function does not return any value its
  return-type is void
• The void type specifies the empty set
• Think of an example for this type of function
  that does not return anything

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This function does not return anything

• void printhello(float x)
• for (int i0 iltx i)
• coutltlt "Hi There !!!!" ltlt endl
• int main()
• printhello(2)

Also Notice a void function call is just on a
line by itself Dont use in cout or assignment
statements (like float functions)
No return statement
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And now for.

• Global Variables--
• A variable declared above/outside all functions
• Can be accessed by everyone
• Generally not a good idea for beginners
• Global Constants
• A variable declared above/outside all functions
• Can be accessed by everyone
• Works great for things like PI that you use
  often

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Summarizing what we did today..

• Standard C library functions
• User - defined functions
• Test Drivers
• Function Declarations and Definitions
• Local variables and Functions
• void functions