Control Structures

1
Control Structures
Corresponds with Chapters 3 and 4
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Control Statements

• Decisions (selections)
• if statements
• switch statements
• Iterations (repetitions)
• for loops (enumerated)
• while loops (top-tested)
• do while loops (bottom-tested)

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if Statement

• if (booleanExpression)
• single statement

Remember to indent the subordinate statement
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if Statement
if (booleanExpression) statement1 statement
2 statementn
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if flowchart
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Example

• if (i gt 0 i lt 10)
• System.out.println("i is an
• integer between 0 and 10")

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The if...else Statement

• if (booleanExpression)
• statement(s)-for-the-true-case
• else
• statement(s)-for-the-false-case

Note blocks with braces are necessary when there
are multiple subordinate statements. They are
optional when there is a single subordinate
statement.
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if-- else flowchart
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if...else Example
The if branch statement must be in a block
because there are two of them.

• if (radius gt 0)
• area radiusradiusPI
• System.out.println("The area for the "
• "circle of radius " radius " is"
• area)
• else
• System.out.println("Negative input")

There is only one else-branch statement. It does
not need to be in a block, but you can if you
want.
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Nested if Statements
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FlowChart for if else if -- else
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Nesting an if - else inside the if portion

• if (boolean expression1)
• if (boolean expression2)
• statement1
• else
• statement2

The nested if-else. NOTE an else is always
associated with the most recent if. This else is
the result of boolean expression2, not boolean
expression1.
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FlowChart for if with nested if -- else
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Outer if-else with a nested if (no inner else)

• if (boolean expression1)
• if (boolean expression2)
• statement1
• else
• statement2

The nested if. NOTE here, because of the
braces, the else is associated with the if for
boolean expression 1.
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FlowChart for if -elsewith nested if
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Shortcut if Statements

• if (x gt 0)
• y 1
• else
• y -1
• is equivalent to
• y (x gt 0) ? 1 -1

if
else
Returned if true
Returned if false
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switch Statements

• switch (switch-expression)
• case value1
• statement(s) 1
• break
• case value2
• statement(s) 2
• break
• case valueN interestRate 9.0
• statement(s) N
• break
• default
• statement(s) for default

No break needed for last condition
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switch Statements

• switch (status)
• case 0 compute taxes for single filers
• break
• case 1 compute taxes for married file
  jointly
• break
• case 2 compute taxes for married file
  separately
• break
• case 3 compute taxes for head of household
• break
• default System.out.println("Errors invalid
  status")
• System.exit(0)

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switch Statement Flow Chart
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Equivalent if statement

• if (status 0)
• compute tax for single filers
• else if (status 1)
• compute tax for married filing jointly
• else if (status 2)
• compute tax for married filing separately
• else if (status 3)
• compute tax for head of household
• else
• System.out.println(
• ("Errors invalid status")
• System.exit(0)

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Exercise

• if (a1)
• x 6
• else if (a 2)
• x 10
• else if (a 3)
• x 16
• else if (a 4)
• x 34

Convert this if statement into a switch statement
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while Loops

• while (continue-condition)
• // loop-body

Top-tested loop Test before performing statements
of loop Loop body may never execute
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Listing 4.4 p109 User Decides when to quit
Initial data input before the loop
Subsequent data inputs toward the end of the
loop body
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Do-while Loops

• do
• //loop body
• while (continue-condition)

Bottom-tested loop Perform statements of loop
before testing Always enter the loop body at
least once
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Listing 4.5 a Do-While Loop
Test is at the bottom
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for Loops
1) assignment statement

• for (control variable initialization
• continue-test adjustment-statement)
• //loop body

2) boolean expression
3) assignment statement

• Set initial value for loop counter (only once,
  before looping starts)
• Test loop counter (also called control variable)
  to see if it is less than the max loop allowed
  (before executing loop body)
• Increment loop counter (after executing loop
  body) (usually an increment operator).

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for Loop Flow Chart
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for Loop Example

• int i
• for (i 0 ilt100 i)
• System.out.println(Hi. i is i)

Here, the loop counter variable (called i ) is
available for use outside the loop.
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Alternative SyntaxDeclare Loop Counter in
Initialization

• for (int i 0 ilt100 i)
• System.out.println(Hi. i is i)

Here, the loop counter variable (called i ) is
available for use only within the loop.
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The Nested for Loop
for (int i0 iltouterLoopLimiti) outer loop
statement(s) for (int j0jltinnerLoopLimitj)
inner loop statement(s) outer loop
statement(s)
Body of outer loop will execute outerLoopLimit
times
Body of inner loop will execute innerLoopLimit
times for each time the outer loop body executes.
? Total innerLoopLimit outerLoopLimit times!
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The Nested for Loop -- Flowchart
Statements preceding loop
No
Statements following loop
Yes
No
Yes
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Nested Loop Example(my example)
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Example Program Run
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A couple points about the example
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Reminders about packages from the example
The class NestedForLoop is in the package called
controlStructurePackage. Therefore, there is a
file called NestedForLoop.class in a folder
called controlStructurePackage.
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Reminders about packages from the example
In order to use a class from outside your own
package, you must import the package that the
class is contained in. Here, we import the
JOptionPane class from the javax.swing package.
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Another Example -- listing 4.6 p115
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Listing 4.6
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Listing 4.6
An if statement nested inside a for loop, which
is itself nested in another for loop.
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Listing 4.6
At the end, the string is displayed in the output
screen
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Listing 4.6 program analysis
Question 1 how many times will outer loop body
execute?
Question 2 how many times will inner loop body
execute?
Question 3 how many times will IF branch execute?
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The break Keyword
break can be used to terminate the loop based on
decisions inside the loop body.
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Listing 4.11 Using the break keyword
Question how many times will this loop body
execute? What will be the value in sum?
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The continue Keyword
continue can be used to branch to the top of the
loop (skipping statements in loop body) based on
decisions inside the loop body.
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Listing 4.12Using the continue keyword
Question 1 how many times will this loop body
execute?
Question 2 how many times sum number execute?
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Typical Nested Structure Decision within a
Loop(menu-driven as first statement in loop)
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A Java Implementation
This implementation nests a switch inside a while
loop.
Exercise Do the same thing by nesting an if
statement inside a do-while loop.
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Other Useful Examples

• Listing 4.8 finding the greatest common divisor
  (while loop with nested if statement)
• Listing 4.9 finding a sales amount (do-while
  loop with nested if - else if - else statement)
• Listing 4.10 printing a pyramid (outer for loop
  with three nested for loops inside it)
• Listing 4.14 finding prime numbers (outer while
  loop with nested for loop with nested if
  statement)