Void Functions

1
Void Functions

• Tutorial 5

2
Objectives

• Create and invoke a function that does not return
  a value
• Pass information, by reference, to a function
• Pass a String variable, by value and by
  reference, to a function

3
Concept Lesson
4
More About Functions

• Programmers use functions for two reasons
• to avoid duplicating code in different parts of a
  program
• to allow large and complex programs to be broken
  into small and manageable tasks

5
More About Functions
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Creating Void Functions

• Void function - a function that does not return a
  value after completing its assigned task
• You might want to use a void function in a
  program simply to display information
• Example title and column headings at the top of
  each page in a report

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Creating Void Functions
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Creating Void Functions

• Differences Between the Syntax of a
  Value-Returning Function and that of a Void
  Function
• The function header in a void function begins
  with the keyword void, rather than with a data
  type
• The keyword void indicates that the function does
  not return a value
• The function body in a void function does not
  contain a return expression statement

9
Passing Variables

• Most programming languages allow you to pass to
  the receiving function either
• the variables value (pass by value)
• or its address (pass by reference)

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Passing Variables by Value
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Passing Variables by Reference

• Passing a variables address (passing by
  reference) gives the receiving function access to
  the variable being passed
• You pass a variable by reference when you want
  the receiving function to change the contents of
  the variable
• To pass a variable by reference to a C
  function
• Include an ampersand (), called the address-of
  operator, before the name of the corresponding
  formal parameter in the function header

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Passing Variables by Reference
13
Passing Variables by Value and by Reference
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Passing Variables by Value and by Reference
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Passing Variables by Value and by Reference

• salary and raiseRate passed by value because the
  receiving function needs to know the values
  stored in the variables, but does not need to
  change the values
• raise and newSalary passed by reference, because
  it is the receiving functions responsibility to
  calculate the raise and new salary amounts and
  then store the results in these memory locations

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Passing Variables by Value and by Reference
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Passing Variables by Value and by Reference
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Passing Variables by Value and by Reference
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Passing a String Variable to a Function

• String data type is not a fundamental data type
  in C.
• It is a data type that is added to the C
  language through the use of a class
• To pass a String variable by value to a C
  function
• use the syntax String variablename in the
  function header, and the syntax String in the
  function prototype
• To pass a String variable by reference to a C
  function
• use the syntax String variablename in the
  function header, and the syntax String in the
  function prototype

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Passing a String Variable by value and by
reference
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Application Lesson
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Analyzing, Planning, and Desk-checking
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Analyzing, Planning, and Desk-checking

• Data for first desk-check
• Customer name Joe Smith
• Current reading (gallons) 9000
• Previous reading (gallons) 8000
• Rate per gallon .00175
• Data for second desk-check
• Customer name Suman Patel
• Current reading (gallons) 14000
• Previous reading (gallons) 7500
• Rate per gallon .00175

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Completed Desk-check Tables
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Coding the main() Function
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Coding the getInput() Function

• The main() function will pass to the getInput()
  function the memory addresses of its name,
  current, and previous variables
• The getInput() function will use the following
  formal parameters to receive the information
• cust
• cur
• prev

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Code for the getInput() Function
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Coding the calculate() Function

• The main() function will pass to the calculate()
  function
• the values of three memory locations (current,
  previous, and RATE)
• the addresses of two memory locations (gallons
  and charge)

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Code for the calculate() Function
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Coding the displayBill() Function
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Completing the Water Bill Program

• To open the partially completed C program
• Start Microsoft Visual Studio .NET. If necessary,
  close the Start Page window
• Click File on the menu bar, and then click Open
  Solution. The Open Solution dialog box opens
• Locate and then open the CppNet\Tut05\T5App
  Solution folder
• Click T5App Solution (T5App Solution.sln) in the
  list of filenames, and then click the Open button
• If the T5App.cpp source file is not displayed,
  right-click T5App.cpp in the Solution Explorer
  window, and then click Open

32
Testing the Program

• To test the program
• Save and then build the solution. Verify that the
  program generated no warnings
• Execute the program. When prompted for the
  customers name, type Joe Brown and press Enter
• When prompted for the current reading, type 9000
  and press Enter. When prompted for the previous
  reading, type 8000 and press Enter

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Command Prompt Window Showing the Results of the
First Test
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Testing the Program (Cont.)

• Press Enter to close the Command Prompt window,
  then close the Output window
• Execute the program again. Enter Suman Patel as
  the customer name, 14000 as the current reading,
  and 7500 as the previous reading. The Command
  Prompt window shows that the gallons used and the
  water charge are 6,500 and 11.38, respectively,
  which agree with the results shown earlier in the
  desk-check tables.
• Press Enter to close the Command Prompt window,
  then close the Output window
• Click File on the menu bar, and then click Close
  Solution to close the current solution
• Click File on the menu bar, and then click Exit
  to exit Visual Studio .NET

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Summary

• Create and invoke void functions
• Pass information by reference to a function
• Allows function to change variable that is passed
  in
• Pass a String variable by value and by reference
  to a function