ECE 242 Spring 2003 Data Structures in Java

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ECE 242 Spring 2003Data Structures in Java

• http//rio.ecs.umass.edu/ece242
• Functions and Parameters
• Prof. Lixin Gao

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Todays Topics

• Function or Method
• Parameters

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The Problem

• Suppose we have a computation or operation we
  need to perform over and over again within a
  program
• Or, suppose we have a computation that many
  programs need to perform
• How can we write those operations just once, but
  allow them to be easily used whenever needed
  within a program or from many programs?

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Functions

• A function is a named code sequence that can be
  executed by giving its name
• The function may take some input parameters on
  which it computes
• The function may compute and return a value

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Common Functions

• We have already seen and used several functions
• class HelloWorld
• public static void main(String args)
• lt--- function definition for main()
• System.out.println("Hello, World!")
• lt--- function calls to println()

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Defining a function

• A function is defined by giving its name and
  listing the code it is to execute

Function name
double cube(double n) return (nnn)
Function body
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Function Type and Value

• A function can return a value
• The function is declared to have the type of its
  return value

function type
parameter type
double cube(double n) return (nnn)
function parameter
return statement
return value
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Calling a function

• To use the function, it is called or invoked from
  within a program or another function
  (CallCubeFunction.java)

class CallCubeFunction static double
cube(double n) return (nnn)
public static void main(String args)
double length 3.0 double
itsCube itsCube cube(length)
System.out.println("Its cube is " itsCube)

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void

• The type void is used when a function returns no
  result or takes no parameters
• Note A function that returns no result does not
  require a return statement.

void prompt(void) System.out.println(En
ter next input)
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Function Control Flow
void prompt(void) System.out.println(En
ter the next integer)
main() prompt() prompt()
prompt
public static void main(String args)
prompt() prompt()
return (0)
prompt
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Function Parameters

• The calling program may need to pass the function
  several values on which to operate
• Such values are function input parameters
• The function specifies its inputs as formal
  parameters in the function declaration

dummy, formal input parameter
double cube(double n) return( nnn )
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Parameter Parsing

• The calling program provides an actual parameter
  on the call.
• Within the function, the value of the actual
  parameter is substituted for the formal parameter.

Parameter parsing
double length 3.0 double itsCube itsCube
mybox.cube(length)
double cube(double n) return (nnn)

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Control and Data Flow

• When a function is called, control transfer to
  function body, the function executes, then
  control returns to the point of call.

public static void main(String args) x
6.0 y cube(x/2.0) z 3.4cube(4.0)
double cube (double n) return (nnn)
3.0
27.0
4.0
64.0
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Multiple parameters

• A function can take multiple parameters
• Parameters must match in number, order and type

int m, n double gpt, gpa gpt 3.0 3.3
3.9 gpa avgFunc(gpt, 3)
double avgFunc(double total, int count)
return (total/(double)count)
actual parameters
formal parameters
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GetGrade Example

• Defining Function, example
• char GetGrade( int score )
• char grade
• if( score gt 85 ) gradeA
• else if( score gt 75 ) grade B
• else if( score gt 65 ) grade C
• else if( score gt 60 ) grade D
• else grade E
• return grade

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GetGrade Example

• Calling Function, example
• int myscore 88
• char mygrade
• mygrade GetGrade( myscore )
• Complete Example QueryMyGrade.java

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Organizing Functions in a Class

• class className
• int variable1 // instance variable
• static int function1()
• // end of function1
• static int function2(float f)
• // end of function2
• public static void main (String args)
• // end of class

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QueryMyGrade.java (1)

• class QueryMyGrade
• static char GetGrade( int score )
• char grade
• if( score gt 85 ) grade'A'
• else if( score gt 75 ) grade 'B'
• else if( score gt 65 ) grade 'C'
• else if( score gt 60 ) grade 'D'
• else grade 'E'
• return grade

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QueryMyGrade.java (2)

• public static void main (String args)
• EasyIn easy new EasyIn()
• int thescore
• char thegrade
• System.out.print("Input your score")
• thescore easy.readInt()
• thegrade GetGrade( thescore )
• System.out.println("Your grade is "
  thegrade)

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GetGrade Parameters

• Function parameter
• mygrade GetGrade( myscore )
• myscore is the parameter
• Return Value
• mygrade GetGrade( myscore )
• mygrade is the return value