The switch Statement

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The switch Statement
Selection Revisited
2
Problem

• Using OCD, design and implement a program that
  allows the user to perform an arbitrary
  temperature conversion (Celsius-to-Kelvin,
  Kelvin-to-Celsius, Fahrenheit-to-Celsius,
  Celsius-to-Fahrenheit, Fahrenheit-to-Kelvin, or
  Kelvin-to-Fahrenheit).

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Preliminary Analysis

• One way to solve the problem is to provide a menu
  of choices, from which the user selects the
  operation they want to perform

Please enter a - to convert Fahrenheit to
Celsius b - to convert Celsius to Fahrenheit
c - to convert Fahrenheit to Kelvin d - to
convert Kelvin to Fahrenheit e - to convert
Celsius to Kelvin f - to convert Kelvin to
Celsius q - to quit --gt
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Behavior main()

• Our program should display its purpose and a menu
  of choices. It should then read the users
  choice. It should then prompt for and read the
  temperature to be converted. It should then
  compute the converted temperature by applying to
  the input temperature the particular conversion
  specified by the users choice. It should then
  display the converted temperature.

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Objects main()

• Description Type Kind Name

purpose string constant --
menu string constant MENU
choice char variable choice
input temp. double variable tempIn
output temp. double variable tempOut
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Operations main()

• Description Predefined? Library?
  Name

display strings yes iostream ltlt
read a double yes iostream gtgt
convert a temp. no -- --
display double yes iostream ltlt
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Algorithm main()

• 0. Display via cout the purpose of the program.
• 1. Loop
• a. Display MENU via cout.
• b. Read choice from cin.
• c. If choice is q, terminate repetition.
• d. Prompt for and read tempIn from cin.
• e. Compute tempOut by converting tempIn using
  choice.
• f. Display tempOut.
• End loop.

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Organization

• Well need
• a main function
• six conversion functions
• a function to apply the right conversion
• The conversion functions seem likely to be
  reuseable someday, so well create a library
  named Heat in which to store them.

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Behavior FahrenheitToCelsius()

• Our function should receive from its caller the
  temperature to be converted. It should return
  the result of subtracting 32.0 from that
  temperature, and then dividing the difference by
  1.8.

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Objects FahrenheitToCelsius()

• Description Type Kind Name

the original double variable originalTemp
temperature
the converted double variable --
temperature
We can use this as a pattern for each of the
temperature conversion functions -- they differ
only in the particular formula they use.
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Coding Conversion Prototypes

• / Heat.h
• ...
• /
• double FahrenheitToCelsius(double originalTemp)
• double CelsiusToFahrenheit(double originalTemp)
• double FahrenheitToKelvin(double originalTemp)
• double KelvinToFahrenheit(double originalTemp)
• double CelsiusToKelvin(double originalTemp)
• double KelvinToCelsius(double originalTemp)

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Operations FahrenheitToCelsius()

• Description Predefined? Library?
  Name

receive a double yes built-in
subtract doubles yes built-in -
divide doubles yes built-in /
return a double yes built-in return
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Algorithm FahrenheitToCelsius()

• 0. Receive originalTemp.
• 1. Return (originalTemp - 32.0) / 1.8.

We can use this algorithm as a pattern for each
of the temperature conversion functions -- each
differs only in the particular formula used.
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Coding Conversion Definitions

• / Heat.cpp
• ...
• /
• double FahrenheitToCelsius(double originalTemp)
• return (originalTemp - 32.0) / 1.8
• double CelsiusToFahrenheit(double originalTemp)
• return originalTemp 1.8 32.0
• double FahrenheitToKelvin(double originalTemp)
• return (originalTemp - 32.0) / 1.8 273.0

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Organization (ii)

• Once our library is completed, we can design and
  implement a function to perform the particular
  conversion specified by the users menu choice.
• We will call this function Convert().
• Since it is less likely to be reuseable than our
  conversion functions, we will store its prototype
  and definition in the same file as our main
  function.

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Behavior Convert()

• Our function should receive from its caller the
  temperature to be converted and the users menu
  choice. Based on the menu choice, our function
  should return the result of applying the
  appropriate temperature conversion function to
  the received temperature.

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Objects Convert()

• Description Type Kind Name

the temp. double variable origTemp being
converted
a menu choice char variable choice
the converted double variable --
temperature
Our prototype is thus double Convert(double
origTemp, char choice)
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Operations Convert()

• Description Defined? Library? Name

return a double yes built-in return
convert F-to-C yes Heat ...
convert C-to-F yes Heat ...
convert F-to-K yes Heat ...
convert K-to-F yes Heat ...
convert C-to-K yes Heat ...
convert K-to-C yes Heat ...
perform selection yes built-in switch based
on a char
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Algorithm Convert()

• 0. Receive origTemp, choice.
• 1. If choice is a return FahrenheitToCelsius(or
  igTemp)
• Else if choice is b return
  CelsiusToFahrenheit(origTemp)
• Else if choice is c return
  FahrenheitToKelvin(origTemp)
• Else if choice is d return
  KelvinToFahrenheit(origTemp)
• Else if choice is e return
  CelsiusToKelvin(origTemp)
• Else if choice is f return
  KelvinToCelsius(origTemp)
• Else display error message and return 0.0.

We could implement this algorithm with a
multi-branch if, but lets learn something new.
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Coding Convert()

• / main.cpp
• ...
• /
• // rest of main.cpp
• double Convert(double origTemp, char choice)
• switch (choice)
• case a return FahrenheitToCelsius(origTemp
  )
• case b return CelsiusToFahrenheit(origTemp
  )
• case c return FahrenheitToKelvin(origTemp)
  
• case d return KelvinToFahrenheit(origTemp)
  
• case e return CelsiusToKelvin(origTemp)
• case f return KelvinToCelsius(origTemp)
• default cerr ltlt \nUnsupported menu
  choice
• ltlt choice ltlt in Convert()
  ltlt endl
• return 0.0

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Discussion
The switch statement provides an alternative
means of doing multi-branch selection.

• double Convert(double origTemp, char choice)
• switch (choice)
• case a return FahrenheitToCelsius(origTemp
  )
• case b return CelsiusToFahrenheit(origTemp
  )
• case c return FahrenheitToKelvin(origTemp)
  
• case d return KelvinToFahrenheit(origTemp)
  
• case e return CelsiusToKelvin(origTemp)
• case f return KelvinToCelsius(origTemp)
• default cerr ltlt \nUnsupported menu
  choice
• ltlt choice ltlt in Convert()
  ltlt endl
• return 0.0

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Organization (iii)

• Once our library and conversion function have
  been designed and completed, we can proceed to
  implement the algorithm for our main function.
• The primary control structure is a forever loop
  that continues repetition until the user enters
  menu choice q...

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Coding main()

• include ltiostreamgt // cin, cout, ltlt,
  gtgt, ...
• using namespace std
• include Heat.h //
  FahrenheitToCelsius(), ...
• double Convert(double originalTemp, char
  menuChoice)
• int main()
• const MENU \nPlease enter\n
• a - to convert Fahrenheit to
  Celsius\n
• b - to convert Celsius to
  Fahrenheit\n
• c - to convert Fahrenheit to
  Kelvin\n
• d - to convert Kelvin to
  Fahrenheit\n
• e - to convert Celsius to
  Kelvin\n
• f - to convert Kelvin to
  Celsius\n
• q - to quit\n
• --gt

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Coding main()

• char choice
• double tempIn, tempOut
• for ()
• cout ltlt MENU
• cin gtgt choice
• if (choice q choice Q) break
• cout ltlt \nEnter the temperature to be
  converted
• cin gtgt tempIn
• tempOut Convert(tempIn, choice)
• cout ltlt \nThe converted temperature is
• ltlt tempOut ltlt endl

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Testing
This program converts temperatures. Please
enter a - to convert Fahrenheit to Celsius
b - to convert Celsius to Fahrenheit c - to
convert Fahrenheit to Kelvin d - to convert
Kelvin to Fahrenheit e - to convert Celsius to
Kelvin f - to convert Kelvin to Celsius q -
to quit\n --gt a Enter the temperature to be
converted 212 The converted temperature is 100
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Testing
Please enter a - to convert Fahrenheit to
Celsius b - to convert Celsius to Fahrenheit
c - to convert Fahrenheit to Kelvin d - to
convert Kelvin to Fahrenheit e - to convert
Celsius to Kelvin f - to convert Kelvin to
Celsius q - to quit\n --gt b Enter the
temperature to be converted 0 The converted
temperature is 32
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Testing
Please enter a - to convert Fahrenheit to
Celsius b - to convert Celsius to Fahrenheit
c - to convert Fahrenheit to Kelvin d - to
convert Kelvin to Fahrenheit e - to convert
Celsius to Kelvin f - to convert Kelvin to
Celsius q - to quit\n --gt c Enter the
temperature to be converted 32 The converted
temperature is 273 ...
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Summary

• To perform multibranch selection, C provides an
  alternative statement called the switch
  statement.
• Well learn more of the details of the if and
  switch statements next time, including when you
  should use which statement.