Switch Statements

1
Switch Statements

• Selecting Among More than Two Choices

2
Why Multiple Transfers

• Each "if" statement transfers control to either
  statement-1 or statement-2, depending on the
  boolean expression, since the boolean expression
  can only be true or false.
• if (boolean-expression)
• statement-1
• else
• statement-2

3
When you have more than 2 options, you can ...

• use additional "if" statements, each of which has
  two branches
• these can be placed in sequence, or
• these can be "nested," i.e. an if within an if
• use a "switch" statement

4
To Use "if" Statements

• if (boolean-expression-1)
• statement-1
• else if (boolean-expression-2)
• statement-2
• else if (boolean-expression-3)
• statement-3
• else
• statement-4

5
Rewriting in Conventional Style (which uses
indents to show relationships)

• if (boolean-expression-1)
• statement-1
• else
• if (boolean-expression-2)
• statement-2
• else
• if (boolean-expression-3)
• statement-3
• else
• statement-4

6
Example Use "if else if" Statements

• if (studentGradeLevel 1)
• froshStudentList.add(studentId)
• else if (studentGradeLevel 2)
• sophStudentList.add(studentId)
• else if (studentGradeLevel 3)
• juniorStudentList.add(studentId)
• else
• seniorStudentList.add(studentId)

7
Example Use "if" with Conventional Formatting

• if (studentGradeLevel 1)
• froshStudentList.add(studentId)
• else
• if (studentGradeLevel 2)
• sophStudentList.add(studentId)
• else
• if (studentGradeLevel 3)
• juniorStudentList.add(studentId)
• else
• seniorStudentList.add(studentId)

8
Implications of Use of Deeply Nested ifs ...

• Deeply indented if's are tough to write and tough
  to read (and get them correct)
• If you have a lot of choices, then you will end
  up with many indents, maybe too many
• When you use the "if else if" notation it can
  be fairly simple, but it requires different
  formatting than for an "if" statement, and it is
  still interpreted as if it is indented in the
  traditional way

9
Structure of the "Switch" Stmt

• switch (controlVar)
• case 'a'
• statement-1
• break
• case 'b'
• statement-2
• break
• default
• statement-3
• break

10
Points to Note

• Control Variable can be an integer expression,
  generally an int or char, but can not be a String
• The name of the control variable is placed in
  parentheses
• Following the word switch and control variable,
  you have all the cases set up in a single block
  (i.e., set of curly braces)
• All the reserved words (switch, case, default)
  start with a lower case letter

11
Points to Note (more)

• Do not use a semi-colon after (control var)
• Each case ends with a colon character (note not
  a semi-colon)
• Each statement can be a simple statement or a
  compound statement (i.e., block)
• Normally each case (including the last one) ends
  with a break statement
• Each case label has to be unique
• Good idea to always use default
• There can be only one "default" label

12
How the "switch" Stmt Works

• The "switch" statement identifies which case
  matches the control variable, and starts
  executing there
• Statements get executed in sequential order until
  the end of the switch (i.e., until the closing
  curly brace)
• If a "break" statement is executed, then control
  passes to the statement after the closing curly
  brace of the switch statement

13
What Happens?

• charges 0.
• switch (actType)
• case 'B'
• charges charges 2.00
• case 'D'
• charges charges 5.00
• default
• charges charges 10.00
• Question if actType is B, what value will
  charges have?

14
Documenting "FALLTHROUGH"

• char actType
• charges 0.
• switch (actType)
• case 'B'
• charges charges 2.00
• //FALLTHROUGH
• case 'D'
• charges charges 5.00
• //FALLTHROUGH
• default
• charges charges 10.00
• //FALLTHROUGH

15
What Happens

• char actType
• charges 0.
• switch (actType)
• case 'B'
• case 'R'
• charges charges 2.00
• break
• case 'D'
• case 'G'
• case 'T'
• charges charges 5.00
• break
• default
• charges charges 10.00
• break

16
Written with Better Style

• char actType
• charges 0.
• switch (actType)
• case 'B' case 'R'
• charges charges 2.00
• break
• case 'D' case 'G' case 'T'
• charges charges 5.00
• break
• default
• charges charges 10.00
• break

17
What Happens if There is No Match

• actType 'Z'
• charges 0.
• switch (actType)
• case 'B'
• charges charges 2.00
• break
• case 'D'
• charges charges 5.00
• break

18
What Happens Here

• actType "B"
• charges 0.
• switch (actType)
• case "B"
• charges charges 2.00
• break
• case "D"
• charges charges 5.00
• break

19
Summary of the Switch Statement

• Is easier to use for multi-way selection than
  nested if's
• Switch can only rely on one variable or
  expression (unlike if's which can involve several
  unrelated conditions)
• Can only switch on int and char variables (unlike
  if's)
• Looks for first matching value in a case
  statement, and then starts working sequentially
  from there
• Be careful of drop throughs
• You can specify a default action to be performed
  if none of the other case values match