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Chapter 3
Functions
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Overview

• 3.2 Using C functions
• Passing arguments
• Header files libraries
• 3.4-5 Writing C functions
• Prototype
• Definition
• Parameters

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Functions

• Every C program must have a main function
• execution starts by looking for a main
• All other functions are called directly from
  main, or indirectly through a chain of function
  called from main
• Function Calls
• One function calls another by using the name of
  the called function next to ( ) enclosing an
  argument list
• ex. strlen(FirstName)
• A function call temporarily transfers control
  from the calling function to the called function

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Function call syntax

• FunctionName(Argument List)
• The argument list is a way for functions to
  communicate with each other by passing
  information
• The argument list can contain 0 or more actual
  arguments, separated by commas

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What does the function do?

• The function uses it actual arguments (inputs)
• to return a calculation
• To produce a side-effect
• The function's return value is substituted for
  the function call in the expression
• If the function does things other than a
  calculation (such as print a message to the
  screen), it is said to produce side-effects

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Example of calling a function
include ltiostreamgt include ltcmathgt using
namespace std int main() float x, root
cout ltlt "Enter a number " cin gtgt x root
sqrt(x) cout ltlt endl ltlt "The square root of "
ltlt x ltlt " is " ltlt root ltlt "." ltlt endl
return 0
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Using library functions

• We've already seen how to use library-provided
  objects like cin and cout to manage I/O
• Libraries also contain the implementation of
  functions
• A library has 2 parts
• Interface (stored in a header file) tells what
  items are in the library and how to use them
• Implementation (stored in another file) contains
  the definitions of the items in the library
• include ltiostreamgt
• Refers to the header file for the iostream
  library needed for use of cout and endl

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Some Mathematical Library Functions
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Some Mathematical Library Functions
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User Defined Functions

• In addition to the functions in libraries,
  programmers can use functions that they write on
  their own
• Using functions for common code segments can
  improve your program in several ways
• more readable
• is easier to update
• modular code

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Two parts of a function definition
Heading

• int Cube ( int n )
• return n n n
• The heading declares the functions name,
  specifying its return type and the name and type
  of each of its parameters
• The body is a compound statement (block) which
  defines the behavior of the function

Body
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What is in a heading?
int Cube ( int n )
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What is in a prototype?

• A prototype looks like the heading, followed by a
  semicolon
• parameter list must contain the type of each
  parameter, but names are optional
• Prototypes are used to declare functions before
  they are defined
• float volume(float, float)
• //C does require parameter names in prototypes
• float volume(float height, float radius)
• //it is good style to include them

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• include ltiostreamgt
• int GCD (int n1, int n2) // prototype
• using namespace std
• int main()
• int x, y
• cout ltlt "enter 2 positive integers "
• cin gtgt x gtgt y
• cout ltlt "The GCD of " ltlt x ltlt " and " ltlt y
• ltlt " is " ltlt GCD( x, y) ltlt endl
• cout ltlt "The GCD of " ltlt x ltlt " and 100 "
• ltlt " is " ltlt GCD( x, 100) ltlt endl
• cout ltlt "The GCD of " ltlt y ltlt " and " ltlt xx - 1
• ltlt " is " ltlt GCD( y, xx -1) ltlt endl
• return 0

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A program with several functions
function prototypes main function
Square function
Cube function
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A complete program prototypes

• include ltiostreamgt
• int Square (int) // prototypes
• int Cube (int)
• using namespace std
• int main()
• cout ltlt "The square of 27 is "
• ltlt Square (27) ltlt endl
• cout ltlt "The cube of 27 is "
• ltlt Cube (27) ltlt endl
• return 0

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A complete program definitions

• int Square (int n)
• return n n
• int Cube (int n)
• return n n n

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void function has no return value

• void DisplayMessage(int n)
• cout ltlt "I have liked math for "
• ltlt n ltlt " years" ltlt endl
• return

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Classified by location

• Arguments Parameters

Always appear in a function call, using
variables whose scope is within the calling
statement.
Always appear in the function heading, or
function prototype. Their scope is only within
the function definition.
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Equivalent terms

• The term "argument", as used in this slideshow,
  is equivalent to
• Actual argument ? used in our text
• Actual parameter (confusing, but used by others)
• The term "parameter", as used in this slideshow,
  is equivalent to
• Formal parameter ? used in our text

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Scope

• A variable or constant can be declared outside of
  any function
• Such variables or constants are known as globals
• Globals can be accessed from anywhere in a
  program
• Variables declared in a function are known as
  local variables
• Local variables can only be referenced from
  within the function in which they are declared
• You may use global constants, but no CS201
  program should use global variables

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Drivers and Stubs

• When building a large project, it is sometimes
  helpful to test each function in isolation
• This allows the programmer to test a single
  function before the entire program is written
• The function to be tested must be called
• A simple main function, called a stub, can call
  the function with appropriate arguments
• The function to be tested might call other
  functions which have not yet been implemented
• Use functions which match the signature of these,
  but which do no real work, are called stubs

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Strings

• C allows programmers to create new data types,
  called classes
• Many classes have been implemented in the
  standard libraries
• The ltstringgt library implements the string class,
  which is used to store a sequence of characters

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Declaring a string

• If you include the string library in your
  program, you can declare string variables
• include ltstringgt string first_name,
  last_name
• You can assign string literals to a string
  variable
• last_name "Doe" first_name "John"

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String operations

• Mathematical operations, such as multiplication
  and division, have no meaning for strings
• But there are operations that do make sense
• String concatenation full_name first_name
  last_name
• Nth position
• char first_initial first_name0

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String member functions

• There are several functions designed for strings
• Called member functions, they are invoked
  differently int size first_name.length()
• int location first_name.find("o")
• There is also a function that allows a programmer
  to read an entire line of input into a string
  cin gtgt my_string //reads one word getline(cin,
  my_string) //reads whole line