JavaScript VI

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JavaScript VI

• Loops Repetition Statements

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Iteration

• Instructions on a shampoo bottle
• put on hair
• lather
• rinse
• repeat
• We call this "iteration"
• executing some action repeatedly
• usually not forever, but according to some
  algorithm

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Examples

• Roll the dice until you make your roll doubles
• Calculate a grade for each student in the class
• Scan each word in a document, looking for one
  that is misspelled
• Compute the monthly interest on the loan for each
  of the next 12 months

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JavaScript constructs

• while loop
• Used to repeatedly perform a sequence of
  statements as long as some condition holds
• for loop
• Used to repeatedly perform a sequence of
  statements for a specified number of times

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while

• Syntax
• while (condition)
• ... body ...

Meaning Upon reaching the while, check the
condition. Execute the body of the loop
repeatedly as long as the condition remains
true. If and when the condition becomes false,
then exit the loop and continue with the rest of
the program
Note that the body of the loop must change the
condition. Why?
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Comparison of if and while

• Appearance is similar
• if (condition)
• ... body ...
• while (condition)
• ... body ...
• Meaning is similar
• true condition means body is executed
• Difference is in repetition
• body in if statement is executed at most once
• body in while loop is repeatedly executed until
  condition is false

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Example

• Get a positive number N as value from user,
  compute and print the sum from 1 to N
• N prompt(Enter a number, 0)
• N parseInt(N)
• sum 0
• i 1
• while (i lt N)
• sum sum i
• i i 1
• document.write(Sum of numbers from 1 to
• N is sum .)
• Exercise modify this to compute the sum of all
  numbers between two user-specified number M and
  N, with M lt N.

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While Loop Example

• example roll two dice repeatedly until doubles
  are obtained

• note even though while loops and if statements
  look similar, they are very different control
  statements
• an if statement may execute its code 1 time or
  not at all
• a while loop may execute its code an arbitrary
  number of times (including not at all)

sample output
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While Loop Page
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Counter-Driven Loops

• The Sum of 1 to N program was an example of a
  counter-driven loop
• Often we want to repeat an action some number of
  times
• roll the dice 1000 times
• print out the first 10 lines of a file
• we need
• a loop that executes some number of times

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Counter while loop

• General form to execute body N times
• var counter 0
• while (count lt N)
• body
• counter counter 1
• Note
• counter is only used to keep track of the
  repetitions
• what happens if we don't increment the counter?
• why isn't the test count lt N or count N?

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Counter-Driven Loops
examples
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Counter-Driven Loops Page
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Common Errors
i 1 while (i lt N) sum sum i

• Body of the code doesn't change the condition

i 0 while (i lt N) sum sum i i i
1

• Wrong initialization

i 1 while (i lt N) i i 1 sum sum
i

• Modification step is out of order

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More Common Errors
i 1 while (i lt N) sum sum i
Wrong condition
// print odd numbers lt 10 x 1 while (x ! 10)
document.writeln(x) x x 2
How about this?
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An Alternate Formulation

• Count down instead of up
• var counter N
• while (count gt 0)
• body
• counter counter - 1
• Points
• is this the right test?

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Example

• count.html
• Purpose Count down to 0
• Some new features
• Continuous addition of text to text area

document.CountForm.Output.value
document.CountForm.Output.value count "\n"
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Countdown Page
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Loops Without Counters

• Loop conditions can be based on any Boolean
  expression
• Not just involving counters
• Such as comparison of two variables or quantities
• Example roll.html
• Purpose keep rolling until doubles
• Again using continuous addition of text to text
  area

document.DiceForm.Output.value
document.DiceForm.Output.value "You
rolled " roll1 " " roll2 "\n"
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Other Examples

• stats.html

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For loops

• Simplifies the counter-driven pattern
• To execute body N times
• var counter 0
• while (count lt N)
• body
• counter counter 1
• For-loop version
• for (counter 0 counter lt N counter counter
  1)
• body

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For syntax

• for (variable initial value exit condition
  increment step)
• fairly flexible
• but almost always used for simple counting
• for (i 0 i lt N i)
• body
• Note
• repeats the body N times
• i is a conventional name for a loop counter
• i is the same as i i 1

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Special case

• the for loop is a special case of the while loop
• you can always rewrite a for loop as a while loop
• for (variable initial value condition
  increment step)
• body
• Rewritten as
• variable initial value
• while (condition)
• body
• increment step

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Example

• For version of Sum-1-to-N program
• N prompt(Enter a number, 0)
• N parseInt(N)
• sum 0
• for (i 1 i lt N i)
• sum sum i
• document.write(Sum of numbers from 1 to
• N is sum .)