Functions, Part 2 of 42

1
Functions, Part 2 of 42

• Topics
• Functions That Return a Value
• Parameter Passing
• Local Variables
• Miscellaneous hints
• Reading
• Sections 3.1 3.6

2
Functions Can Return Values
/
averageTwo -
calculates and returns the average of two
numbers Inputs num1 - an integer value
num2 - an integer value Outputs
the floating point average of num1 and
num2
/ float
averageTwo (int num1, int num2) float average
/ average of the two numbers / average
(num1 num2) / 2.0 return average
3
Using averageTwo

• include ltstdio.hgt
• float averageTwo (int num1, int num2)
• int main ( )
• float ave
• int value1 5, value2 8
• ave averageTwo (value1, value2)
• printf (The average of d and d is f\n,
  value1, value2, ave)
• return 0
• float averageTwo (int num1, int num2)
• float average
• average (num1 num2) / 2.0
• return average

4
Parameter Passing

• Actual parameters are the parameters that appear
  in the function call.
• average averageTwo (value1, value2)
• Formal parameters are the parameters that appear
  in the function header.
• float averageTwo (int num1, int num2)
• Actual and formal parameters are matched by
  position. Each formal parameter receives the
  value of its corresponding actual parameter.

5
Parameter Passing (cont)

• Corresponding actual and formal parameters do not
  have to have the same name, but they may.
• Corresponding actual and formal parameters must
  be of the same data type, with some exceptions.

6
Local Variables

• Functions only see (have access to) their own
  local variables. This includes main( ) .
• Formal parameters are declarations of local
  variables. The values passed are assigned to
  those variables.
• Other local variables can be declared within the
  function body.
• There is another type of variable called a global
  variable. Do not use them! They are a major
  source of errors!

7
Parameter Passing and Local Variables

• include ltstdio.hgt float averageTwo (int
  num1, int num2)
• float averageTwo (int num1, int num2)
• int main ( ) float average
• float ave average (num1 num2)
  / 2.0
• int value1 5, value2 8 return
  average
• ave averageTwo (value1,
• value2)
• printf (The average of )
• printf (d and d is f\n,
• value1, value2, ave)
• return 0
• value1 value2 ave num1
  num2 average
• 5 8
• int int float
  int int
  float

8
Same Name, Still Different Memory Locations

• include ltstdio.hgt float averageTwo (int
  num1, int num2)
• float averageTwo (int num1, int num2)
• int main ( ) float average
• float average average (num1
  num2) / 2.0
• int num1 5, num2 8 return
  average
• average averageTwo (num1,
• num2)
• printf (The average of )
• printf (d and d is f\n,
• num1, num2, average)
• return 0
• num1 num2 average
  num1 num2 average
• 5 8
• int int float
  int int
  float

9
Changes to Local Variables Do NOTChange Other
Variables with the Same Name

• include ltstdio.hgt
• void addOne (int number) void
  addOne (int num1)
• int main ( ) num1
• printf (In addOne )
• int num1 5 printf (num1 d\n, num1)
• addOne (num1)
• printf (In main )
• printf (num1 d\n, num1) num1
• return 0
• 
  int
• num1
• 5 OUTPUT
• int In addOne num1 6
• In main num1 5

10
Sample Program

• include ltstdio.hgt
• int funcA(int a, int b, int c )
• int main( void )
• int x 3, y 4, z 5
• int result
• result funcA( x, y, z )
• printf( "result d\n", result )
• result funcA( 6 4, y 2, 9 )
• printf( "result d\n", result )
• result funcA( 2.1, 3.5, 4.0 )
• printf( "result d\n", result )
• return 0

11
Sample Program (contd)

• int funcA(int a, int b, int c )
• printf(" a d b d c d\n", a, b, c
  )
• return ( a b c )

12
Sample Program Output

• burt_at_linux3 a.out
• a 3 b 4 c 5
• result 12
• a 10 b 6 c 9
• result 25
• a 2 b 3 c 4
• result 9

13
Sample Program Analysis - 1

• int x 3, y 4, z 5
• result funcA( x, y, z )
• printf( "result d\n", result )
• a 3 b 4 c 5
• result 12
• Looks good!

14
Sample Program Analysis - 2

• result funcA( 6 4, y 2, 9 )
• printf( "result d\n", result )
• a 10 b 6 c 9
• result 25
• 6 4 is 10, OK.
• y is 4 2 is 6, OK.

15
Sample Program Analysis - 3

• result funcA( 2.1, 3.5, 4.0 )
• printf( "result d\n", result )
• a 2 b 3 c 4
• result 9
• Notice that 2.1 became 2, 3.5 became 3 and 4.0
  became 4. This is called truncation. The
  compiler tried to make the data fit the
  prototype! This is called an implied conversion,
  and can be the source for errors that is hard to
  find.
• Do not use implied conversions!

16
Another Example

• include ltstdio.hgt
• include ltstdlib.hgt
• int main( void )
• int negNum -3
• printf( "d\n", abs( negNum ) 2 )
• return 0

17
Another Example Output

• burt_at_linux3 a.out
• 6
• burt_at_linux3
• In this example the expression was the result of
  another function multiplied by a constant.
• Whatever the argument is, it is evaluated to the
  proper data type and then the function is invoked
  that that final version of the argument.

18
system( ) function

• The system( ) function allows you to do any shell
  command from inside a program.
• The function must have an argument that is the
  command you wish to have executed.
• Must include ltstdlib.hgt

19
system( ) Function Example

• include ltstdio.hgt
• include ltstdlib.hgt
• int main ( void )
• / Clear the screen /
• system( "clear" )
• printf( "Hello, World!\n" )
• return 0

20
Good Programming Style

• Functions should do one thing and do it well!
• Functions can only return one thing! In CMSC201
  and CMSC341, you will learn that it can be a data
  structure as will as a simple variable.
• It is better to have more than one function than
  one complicated function.

21
Good Programming Style (contd)

• Function subprograms allow us to remove the the
  main function the code that provides the detailed
  solutions to a subproblem.
• Never write the same code twice.
• Limit your functions to a maximum of 50-100 lines
  of code and comments.
• Many functions have only one line of code.

22
Good Programming Style (contd)

• Forgetting to return a value from a function that
  is suppose to return a value can lead to
  unexpected errors.
• Programs should be written as a collection of
  small functions.
• The function prototype, function header and
  function calls should all agree in the number,
  type, and order of argument s and parameters, and
  the type of the return value.