Iteration and Simple Menus

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Iteration and Simple Menus

• Deterministic / Non-deterministic loops and
  simple menus

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Where are we?

• The 6 Coding Concepts
• Input getting things into the program
• Output passing things to the outside world
• Assignment pass a value from one part of the
  program to another
• Sequence - do each thing in the order specified
• Decision-making -check something then do the
  appropriate action depending on that condition
• Repetition - do the set of commands several times

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Deterministic loops (for loop)?

• Use if you know how many times the loop should
  execute (deterministic loop)?
• int num Console.readInt("How many lines?")
• for (int i 0 i lt num i)?
• System.out.println("I must stay awake in
  class")

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Breakdown of the for Loop

• for (int i 0 i lt num i)?
• What does this mean?
• The for loop consists of 3 items
• The initialiser (int i 0)?
• The terminating condition (i lt num)?
• The incrementor (i)?
• These items are separated by a
• Like the if statement, you can omit the that
  follow it if there is only statement to be
  repeated

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Nested for loops

• You have seen how if-else statements can be
  nested inside each other
• if( condition )?
• if( condition )?
• // do something
• else
• // do something different
• else

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Nested for loops

• Well you can do the same with for loops
• for( int i 0 i lt 10 i )?
• System.out.println(Outer loop)
• for( int j 0 j lt 5 j )?
• System.out.println(Inner loop)

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Loop examples

• for( int i 0 i lt 10 i ) will repeat 10
  times by incrementing i
• for( int i 0 i lt 10 i ) will repeat 11
  times
• for( int i 0 i lt 10 i2 ) will repeat 5
  times
• for( int i 10 i gt 0 i-- ) will repeat 10
  times by decrementing i
• for() will repeat infinitely!

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Alternative syntax For-Each

• There is another way of specifying the Java for
  loop when using arrays and collections.
• int myArray
• for(int i myArray)?
• sum i
• instead of
• for(int i i lt myArray.length i)?
• sum myArrayi

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Non-deterministic loops (while)?

• use if you want the loop to execute until a
  condition is false (non-deterministic loop)?
• must allow tested variable to change within the
  loop, otherwise youll have an infinite loop!
• int num Console.readInt("How many lines?")
• while (num gt 0)?
• System.out.println("I love Java!!!")
• num--

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Non-deterministic loops (do-while)?

• There is a similar while loop called the do-while
  loop
• int num Console.readInt("How many lines?")
• do
• System.out.println("I love Java!!!")
• num--
• while (num gt 0)

Note the semi-colon!
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Non-deterministic loops (do-while)?

• So whats the difference between while and
  do-while?
• The contents of a while loop is not executed if
  the test condition fails at the start.
• The contents of a do-while loop is guaranteed to
  execute at least once.
• The test condition is only performed after the
  first iteration through the loop.

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Exiting a loop

• The normal way to exit a loop is for the
  condition that is tested to become false.
• This is true of all three types of loops in Java
  for, while, and do-while.
• However, there might be times when you want a
  loop to end immediately, even if the condition
  being tested is still true.
• You can do this with a break statement, as shown
  in the following code

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The break statement

• int index 0
• while (index lt 1000)?
• index index 5
• if (index 400) break
• System.out.println("The index is " index)
• The break statement is especially handy if you
  need to search a list and then exit the search
  loop code if you found what you are looking for

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Menus

• Can use a combination of a while loop and a
  switch statement to implement a menu
• NB may be better to have 0 Exit - why?
• What would you like to do?
• 1 Register student
• 2 Register student on program
• 3 Exit
• ?

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Simple menu display

• public int showMenu()?
• int choice
• System.out.println("\nWhat would you like to
  do?")
• System.out.println("\t1\tRegister student")
• System.out.println("\t2\tChange student's
  program")
• System.out.println("\t3\tExit")
• choice Console.readInt("? ")
• return choice

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Simple menu control

• int choice showMenu()
• while (choice ! 3)?
• switch (choice)?
• case 1
• registerStudent()
• break
• case 2
• changeProgram()
• break
• default
• System.out.print(Invalid, please try
  again")
• choice showMenu()

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Implementing amenu-based interface

• Would have to write appropriate methods
• showMenu()?
• registerStudent()?
• changeProgram()?
• Look at StudentInterface.java listing

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Summary

• We have looked at
• for loops
• Nested for loops
• while loops
• do-while loops
• break
• We have put them together into a menu-driven
  interface for the Student class (see tutorial).

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Further work

• Practical
• Have a look at the Student class tutorial sheet
• Have a look at the Control Flow tutorial sheet
• Add more functionality
• print student details
• increment level
• Read http//www.faqs.org/docs/javap/c3/s1.html to
  go over all the main blocks of Java code youve
  seen so far.