G51PR1

1
G51PR1

• Introduction to Programming I
• 2000-2001
• University of Nottingham
• Unit 3 Conditionals

2
Overview

• Simple if statement
• if else statements
• Further Alternatives
• Possible Conditions
• Nesting if statements
• Ambiguity

• Simple switch statements
• switch Example
• case values
• Program Layout
• CourseWork Exercises

3
Simple if statements

• The if statement, provides decision making
  capabilities.
• It causes selected statements to be executed if a
  certain condition evaluates to true at that
  point in the program.
• Syntax
• if (condition)
• statement
• If the boolean condition is true, the statement
  is executed if it is false, the statement is
  skipped

4
Simple if statements

• int Radius, Result 0
• Radius UserInput.readInt() // user code
• if ( Radius gt 0 )
• Result Radius Radius
• System.out.println("Radius is positive")
• // end if Radius gt 0
• System.out.println(Radius " " Result)

5
Simple if statements

• The condition to be tested appears in parentheses
  after the word "if", and the statement or
  statements whose execution is determined by the
  condition are contained within curly braces. If
  the condition does not hold, everything up to
  the next "" is ignored, and program execution
  continues after the "". In this example, if the
  radius value read in is positive, the program
  then executes in order the statements
  
• Result Radius Radius
• System.out.println("Radius is positive")
• System.out.println(Radius " " Result)
• If the value read in is not positive, the
  statement in the braces following the "if"
  condition will be ignored, and the program will
  execute the single statement
• System.out.println(Radius " " Result)

6
if ... else" statements

• If the condition does not hold, we may wish to
  execute some different statements as alternatives
  to the "if" statements.
• Syntax
• if (condition) statement else
  statement

false
true
statement1
statement2
7
if ... else" statements

• int Money
• int Deposits 0 , Withdrawals 0
• Money UserInput.readInt()
• if ( Money gt 0 )
• System.out.println("Deposit.")
• Deposits Money Deposits
• else
• // if Money lt 0
• System.out.println("Withdrawal.")
• Withdrawals Money - Withdrawls
• // end if else Money gt 0
• System.out.println("Transaction noted.")

8
"If ... else ..." statements

• If the quantity read is positive, the program
  then executes
• System.out.println("Deposit.")
• Deposits Money Deposits
• System.out.println("Transaction noted.")
• If the quantity is zero or negative, the program
  executes
• System.out.println("Withdrawal.")
• Withdrawals Money - Withdrawals
• System.out.println("Transaction noted.")

9
"If ... else ..." statements

• The careful layout of programs becomes more
  important as the programs become more structured.
  Some people prefer to put the braces on separate
  lines as in
• if ( Radius gt 0 )
• ....
• ....
• // end of Radius gt 0
• else
• // if Radius lt 0
• ....
• ....
• // end if else

10
Further alternatives

• We can add further alternatives to an "if"
  statement if we require.
• if ( Radius gt 100 ) // Radius gt 100
• ....
• else if ( Radius gt 10 ) // Radius gt 10 and
  Radius lt 100
• ....
• else if ( Radius gt 1 ) // Radius gt 1 and
  Radius lt 10
• ....
• else // Radius lt 1
• ....
• // end if Radius various values
• The tests in the if statements are executed
  exactly in the order in which they are
  encountered. The first test here is "Radius gt
  100" if this is false, the second test if
  executed, testing whether "Radius gt 10", so that
  this is effectively the test "Radius lt 100
  Radius gt 10", since we know that the first test
  is false.

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Possible conditions (logical expressions)

• We looked at the format of conditions in unit 2,
  when we were discussing expressions in general.
  
• // Test equality, use double equals
• if ( code 3 ) ....
• // Two tests ANDed together
• if ( Radius gt 0 Radius lt 10 ) ....
  
• // Two tests ORed together
• if ( code 0 code 1 ) ....

12
Possible Conditions

• Note that the AND ("") and OR ("") forms, are
  lazy left-to-right operators. This means that the
  expressions to be AND-ed together are evaluated
  from left to right, one at a time, and as soon as
  one is found which is FALSE, evaluation finishes
  and a FALSE result is returned. The remaining
  expressions are not evaluated at all. Only the
  minimum amount of work is done to determine the
  result. It is then perhaps safe to write
• if ( Div gt 0.001 Total/Div lt
  100 ) ...
• since we can never get division be zero.
• Similarly with OR, the expressions are evaluated
  left-to-right, and as soon as one is found which
  is TRUE, evaluation ceases and a TRUE result is
  returned.

13
Nesting "if" statements

• "if" statements can be nested if that is what the
  program logic requires.
• if ( Radius gt 0 )
• if ( Result gt 0 ) // Radius gt 0 and Result gt
  0
• ....
• else // Radius gt 0 and Result lt 0 here
• ....
• // end if else Result gt 0
• else // not Radius gt 0
• if ( Result gt 0 ) // Radius lt 0 and Result
  gt 0
• ....
• else // Radius lt 0 and Result lt 0
• ....
• // end if else Result gt 0
• // end if else Radius gt 0

14
Ambiguity

• We have specified above that you must always use
  curly braces after an "if" condition and after
  the "else". The Java official definition states
  that the curly braces are essential only if there
  is more than one statement to be executed as a
  result of the condition. Many commercial users of
  Java insist, as we do, that the curly braces
  should always be there.
• If you don't use curly braces, the following can
  be ambiguous (to the reader)
• if ( Radius gt 0 )
• if ( Result gt 0 )
• xxx
• else
• yyy

15
Ambiguity

• The above executes as
• if ( Radius gt 0 )
• if ( Result gt 0 )
• xxx
• else
• yyy

• But can be easily mistaken for
• if ( Radius gt 0 )
• if ( Result gt 0 )
• xxx
• else
• yyy

16
Ambiguity

• These two have quite different effects. In the
  case when, for example, "Radius gt 0" and not
  "Result gt 0", the first will execute "yyy", the
  second will have no effect. If we have "Radius lt
  0" (the first condition is FALSE) and "Result gt
  0", the first example will have no effect, while
  the second would execute "yyy".
• The two are thus quite different in their effect.
  It is a good principle always to use curly
  braces, and to put a comment after any closing
  curly brace which is not close to its opening
  partner. An example might be
• if ( Radius gt 0 )
• ...
• ...
• // end if Radius gt 0
• Our rule (enforced by the CourseMaster marking
  system) will be that a closing curly brace must
  have a comment if it is more than 10 lines after
  its opening curly brace. This ensures that you
  see a comment on the computer terminal screen if
  the complete "if" statement is unlikely to fit
  onto one screenful of information.

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switch statements

• The "if" statement essentially gives a choice
  between two alternatives. We may sometimes need a
  choice between a larger number of possibilities.
  The "switch" construct allows for any number of
  different actions to be taken dependent of the
  value of an integer calculation.
• The syntax of the switch statement is
• switch (expression)
• case value1
• statement
• case value2
• statement
• case

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switch example

• int inputValue
• inputValue UserInput.readInt()
• switch ( inputValue )
• case 0 // if "inputValue" is 0
• do_this()
• break
• case 4 // if "inputValue" is 4
• do_that()
• break
• case 7 // if "inputValue" is 7
• do_the_other()
• break
• default // if "inputValue" is anything else
• something_else()
• // end switch( inputValue )
• The expression you switch on must be one of int,
  byte, char, short or long.

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case Values

• The value after the word "case" must be a
  constant, you could not put case PartType, where
  PartType is a variable. You must put either an
  explicit constant (i.e. the numeric value 7), or
  a declared final, for example static final
  int insPerFoot 12
• Two "case" labels can be adjacent. In this case,
  the two specified values will cause the same code
  to be executed.
• Don't forget the "break" statements where you
  need them. You will normally want control to
  leave the "switch" statement at the end of each
  separate "case".
• The "default" entry is optional, but often is
  included.

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Character switch example

• char commandChar
• commandChar UserInput.readChar()
• switch ( commandChar )
• // Perhaps 'e' for "edit"
• case 'e'
• edit()
• break
• // Perhaps 'l' for list/print
• case 'l'
• case 'p'
• print()
• break
• // Some other character
• default
• System.out.println("Don't understand "
  commandChar)
• // end switch( commandChar )

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Character switch example

• Note again the careful indentation, and the
  comment after the closing curly brace.
• It is also possible to omit the break statements
  on the rare occasions when you wish to cause code
  to follow through as in this example, although
  this is regarded by many as bad practice.
• In this case, 'r' causes "run" 'c' causes
  "compile" then "run" 'e' causes "edit" then
  "compile" then "run"
• case 'e' edit()
• case 'c' compile()
• case 'r' run()
• break

22
Program layout

• As we learn new constructs, programs become more
  complex, and the neat layout of programs becomes
  more important. Our automatic marking system will
  check your layout.The suggested form of layout
  for "if" statements and "switch" statements is as
  shown in the above examples. Generally, we indent
  each set of statements from "" to "" about 3-4
  spaces more than the outer code.
• if ( Radius gt 0 )
• Result Radius Radius
• System.out.print(Positive Radius")

if ( Radius gt 0 ) // end if Radius gt 0
Result Radius Radius
System.out.print("Positive Radius") // end if
Radius gt 0
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Program layout

• if ( Money gt 0 )
• System.out.println("Deposit.)"
• Deposits Money
• else // if Money lt 0
• System.out.println("Withdrawal.)"
• Withdrawals - Money
• // end if else Money gt 0
• if ( Money gt 0 )
• System.out.println("Deposit.)"
• Deposits Money
• else if ( Money gt 100 )
• System.out.println("Money greater than 100.
  ")
• else // if Money lt 0
• System.out.println("Withdrawal.")
• Withdrawals - Money
• // end if else Money gt 0

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Program layout

• if ( x gt 10 )
• System.out.println("x positive")
• if ( y gt 0 )
• System.out.println("y positive too")
• else
• System.out.println("x is but y isn't")
• // end if x positive

25
Program layout

• switch ( inputValue )
• case 0 // if "inputValue" is 0
• do_this()
• break
• case 3
• case 4 // if "inputValue" is 3 or 4
• do_that()
• break
• case 7 // if "inputValue" is 7
• do_the_other()
• break
• default // if "inputValue" is anything else
• yet_else()
• // end switch ( inputValue )

26
Program layout

• CourseMaster marking has the same rules for ""
  and "" and for "(" and ")". If the matching
  symbols will fit on the same line, all is well.
  You are permitted
• Cash Number ( CostPerItem Overhead )
• if ( x gt 0 ) System.out.println("Message ")
  
• If the matching pair will not fit on one line,
  then the closing one must start a line further
  down, lining up with the start of the line
  containing the opening one
• Cash Number (
• CostPerItem Overhead // perhaps a longer
  expression
• )
• if ( x gt 0 )
• System.out.println("A long message -- A long
  message -- A long message ")
• // and a comment here

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Coursework exercise marking

• The exercises in this unit are marked partly on
  dynamic correctness, partly on typographic
  layout, and sometimes you will see a heading
  Features. For dynamic correctness and typographic
  layout, earlier comments apply (see the end of
  unit 2 notes1 and the Typographic Layout
  paper2 and the program layout section above).
• The features mark is problem specific, and
  concerns the items for which a teacher would look
  by eye if marking your programs by hand. For
  example, if the problem involves converting
  centimetres to inches, the explicit conversion
  factor 2.54 should appear as a single global
  constant, it should NOT appear more than once, it
  should NOT appear in the program. If you are
  converting minutes to hours, the constant 60
  should appear only once, as a global constant,
  and it is bad practice to use the constant 59.
• Always use gt 60 rather than gt 59 when the
  value which represents the factor which concerns
  us (the number of minutes in an hour) is 60, not
  59.
• So, if you loose marks on features you must stop
  and think of all the good programming practices
  you have been taught, and decide which principle
  you have broken.

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References

• 1. Eric Foxley, Programming in C - Basics, LTR
  Report, Computer Science Dept, Nottingham
  University, 1996.
• 2. Cleveland Gibbon, Writing Typographically
  Sound Programs with Ceilidh, LTR Report, Computer
  Science Dept, Nottingham University, 1995.

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Summary

• Simple switch statements
• switch Example
• case values
• Program Layout
• CourseWork Exercises

• Simple if statements
• if else statements
• Further Alternatives
• Possible Conditions
• Nesting if statements
• Ambiguity