JavaScript V

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JavaScript V

• Conditional Execution

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Outline

• Conditional statement
• If-then
• If-then-else
• Nested If
• Boolean expressions

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Program execution

• Unconditional execution
• interpreter executes each line in sequence
• Conditional execution
• evaluate a test
• execute code only if test is true
• we can think of this as one step
• not always executed
• Need not be a single step
• might be a chunk of code controlled by the
  condition
• call the "body"

program step 1 program step 2 program step
3 program step 4 program step 5
condition conditional step program step
2 program step 3 program step 4
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Examples

• if due date on book is before today's date
• charge overdue fee
• if day of the week is not sunday or holiday
• parking meter must be used
• if even day of the month and date between Nov 1
  and April 1
• car will be ticketed and towed

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JavaScript

• if statement
• if (condition) statement or
• if (condition) statements
• Example
• if (dateToday gt dueDate)
• bookOverdue true
• or
• if (dateToday gt dueDate)
• bookOverdue true
• calculateFine (dateToday, dueDate)
• ...

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If-then-else

• Conditional execution
• evaluate a test
• execute one piece of code if test is true
• execute another piece if test is false
• we can think of this as one step
• one part or the other will be executed

If (condition) conditional true step(s) else
condition false step(s) program step 2 program
step 3 program step 4
program step 1
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Examples

• if the title is a new release
• then charge 3.00 and lending period is 2 days
• else charge 2.00 and lending period is 5 days
• if the answer is correct
• then add 1 to score
• else print help message

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JavaScript

• if statement
• if (condition)
• then part
• else
• else part
• Example
• if (newRelease(title))
• charge 3.00
• lendingPeriod 2
• else
• charge 2.00
• lendingPeriod 5

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Stylistic issues

• Where to put brackets
• I prefer after condition
• if (condition)
• ... body ...
• else
• ... body ...
• Some prefer on separate lines
• if (condition)
• ... body ...
• else
• ... body ...
• In some cases, you might put whole statement on
  one line
• if (condition) ... body ...

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Example

• ifdemo.html
• ifdemo2.html (with else)
• chill.html

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Nested If

• There may be more than two possibilities to be
  handled
• Put one if statement inside another
• if (temp lt 50)
• if (windSpeed lt 3)
• windChill temperature
• else
• windChill ...calculation here...
• else
• ... wind chill undefined ...
• Example chill2.html

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Exercise

• if (x gt y)
• if (x10 lt 100)
• document.write(ONE)
• else
• document.write(TWO)
• else
• document.write(THREE)

x 0 y 5 output ? ? x 0 y
-5 output ? ? x 9 y 9 output ? ? x
22 y 21 output ? ?
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How to handle

• Lots of possibilities
• if affiliation is Republican
• Republican primary ballot
• non-partisan referendum ballot
• if affiliation is Democrat
• Democrat primary ballot
• non-partisan referendum ballot
• if affiliation is Green
• Green Party primary ballot
• non-partisan referendum ballot
• otherwise
• non-partisan referendum ballot
• Nesting gets ugly

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Cascading ifs

• Easier to read than nested style
• It is clear that multiple alternatives are
  involved
• if (condition1) then
• action1
• else if (condition2) then
• action2
• else if (condition3) then
• action3
• etc
• Some languages (perl) actually have a special
  elsif construct

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Examples

• dice.html
• Reed style
• twodice.html
• note image list access
• with counter
• In-class Exercise with double counter?
• Exercise (on your own)
• Rewrite wind chill code as cascading ifs

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Equality operators

• Equal
• Not equal !
• Inequality gt, gt, lt, lt
• These are operators just like , -, etc.
• What data type do they return?
• i lt 5

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Decision Making Equality and Relational
Operators
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If Statement Gotcha 1Type Sensitivity

• The result of an inequality depend on data type
• 100 gt 18
• "100" lt "18"

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If Statement Gotcha 2Equality vs assignment

• Equality test (d 0)
• Boolean value
• Assignment (d 0)
• the assignment statement also has a value
• whatever the new value is
• bad language design!
• Unexpected behavior
• if (done false)
• .. do something ..
• never executed
• Worse yet
• "", 0 are automatically converted to false
• if (p 5)
• .. do something ..
• always executed
• Check that if statements only use the equality
  operator

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Boolean Operators

• and
• or
• !
• not

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Boolean expressions

• Expressions returning Boolean values
• can be used as a condition in an if statement
• if wind speed above 3 and temperature below 50
• calculate wind chill
• if (windSpeed gt 3 temperature lt 50)
• calculate (windSpeed, temperature)

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Other Examples

• die1 RollDie()
• die2 RollDie()
• doubles?
• total is 7?
• roll is a 1 and 1?
• roll contains a 3?
• total is 7 or 11?