Functions

1
Chapter 3

• Functions

2
Objectives

• Build structured programs that are divided into
  functions.
  
• Describe the flow of execution in a program with
  multiple functions.
  
• Describe what is meant by the phrase "scope of
  variable."

3
Objectives

• Pass data to functions.
• Get values from functions using return.
• Describe and use library functions.
• Use common math functions.
• Use character manipulation functions.

4
How to Build Programs with Functions

• Thus far, you have only written programs with the
  main() function.
  
• Most programs use functions that perform specific
  tasks.
  
• Typically the main function will "call" other
  functions as the program needs their
  functionality.

5
Guidelines Using Functions

• Organization
• Autonomy
• Encapsulation
• Reusability

6
Program Design

• Top-down design begins with the functions at the
  top of the VTOC and works toward the functions at
  the bottom of the VTOC.
  
• Bottom-up design involves beginning with the
  bottom of the VTOC and working your way up.

7
The Syntax of Functions

• You have seen the main function many times so
  far
  
• int main()
• // body of program
• return 0

8
The Syntax of Functions

• When the program reaches the return 0 statement,
  the value zero is returned to the operating
  system, which shows the program ended normally.
  
• The value returned is a standard integer because
  we specified an int data type when the main
  function was declared.
  

9
The Syntax of Functions

• To prevent a value from being returned, the void
  keyword is used in place of a data type.
  
• void main()
• // body of program

10
Calling Functions

• An example of a user created function might look
  like the following
  
• void print_title()
• cout
• cout

11
Calling Functions

• The main function would then "call" the user
  defined function.
  
• int main()
• print_title() // call to print_title
•   // insert the program here
•   return 0

12
Function Prototypes

• To make your own functions work, you must tell
  the compiler at the top of your program that your
  function exists.
  
• You do this by creating a prototype.
• Basically, a prototype defines the function for
  the compiler.

13
Prototype Example

• The top part of a program might look like the
  following
  
• // 1stfunct.cpp
• include
• void print_title()
• // prototype for print_title
• // function

Notice the semicolon.
14
Functions and Program Flow

• When a function is called, the computer executes
  the statements in the function beginning with the
  first statement.
  
• When the end of the function is reached, program
  execution resumes with the statement that follows
  the call to the function.

15
Scope of Variables

• The availability of a variable is known as its
  scope.
  
• A local variable is declared within a function
  and is accessible only within that function.
  
• A global variable is declared before the main
  function.
  
• Global variables are accessible by any function.

16
Data and Functions

• The parentheses after a function's name can be
  used to pass data to a function and in some cases
  to return data from a function.
  
• When a function is called, the data in the
  parentheses (called the argument) is passed into
  the receiving function.
  
• There are two ways to pass data to functions
  passing by value and passing by reference.

17
Passing by Value

• When you pass a variable to a function by value,
  a copy of the value in the variable is given to
  the function for it to use.
  
• If the variable is changed within the function,
  the original copy of the variable in the calling
  function remains the same.

18
Passing by Reference

• Functions that pass variables by reference will
  pass any changes you make to the variables back
  to the calling function.
  
• To pass a variable by reference, simply precede
  the variable name with an ampersand () in the
  function definition.

19
Returning Values Using Return

• Functions other than the main function can return
  their values to the calling function.
  
• The value to be returned is specified using the
  return statement.
  

20
return example

• This is the function that will return the value
• double celsius_to_fahrenheit(double celsius)
• double fahr // local variable
• fahr celsius (9.0/5.0) 32.0
• return(fahr)

21
return statement

• include
•  int main()
• double fahrenheit
• double celsius 22.5
•  fahrenheit celsius_to_fahrenheit(celsius)
•  
• cout F\n"
  
• return 0

22
Using Library Functions

• C compilers include pre-written, ready-to-use
  functions to make programming easier.
  
• The functions that come with your compiler are
  called library functions.
  
• The prototypes for library functions are provided
  to your program using the include compiler
  directive.

23
Example of Library Function

• A common library function is pow() which is used
  to raise a number to a power.
  
• z pow(x, y)
• // z equals x raised to
• // the y power
• In order to use the pow function, you must
  include the math.h header file using the compiler
  directive include .

24
Header Files

• A header file is a text file that provides the
  prototypes of a group of library functions.
  
• The linker uses the information in the header
  file to properly link your program with the
  function you want to use.

25
Popular Math Functions

• abs Returns the absolute value of an integer
• ceil Rounds up to a whole number
• floor Rounds down to a whole number
• Pow10 Calculates 10 to the power of x
• sqrt Calculates the positive square root of x

26
Functions for Working with Characters

• isupper Determines if a character is uppercase
• islower Determines if a character is lowercase
• isalpha Determines if a character is a letter
• isdigit Determines if a character is a digit
• toupper Converts a character to uppercase
• tolower Converts a character to lowercase

27
Summary

• Designing a program that consists of functions
  results in code that is better organized,
  reusable, and easier to debug.
  
• The syntax of functions you create is very
  similar to that of the main function.
  
• You must create a prototype for your functions to
  let the compiler know your function exists.
  
• Prototypes are placed at the top of the program.

28
Summary

• A local variable is created within a function and
  is accessible only from within that function.
  
• A global variable is declared outside of all
  functions and is accessible from any function.
  
• Getting data to and from functions is called
  passing data.
  
• Data can be passed to functions by value or by
  reference.
  
• When possible, you should pass by value.

29
Summary

• Data passed by reference brings back changes made
  to it within a function.
  
• A value can be passed to the calling function
  using return.
  
• A void function does not return a value.
• In a function prototype, you are only required to
  provide the data types of the parameters.
  

30
Summary

• Library functions are functions that come with
  the compiler.
  
• A header file provides the prototypes for library
  functions.
  
• C includes common math functions and functions
  for working with characters.