COMP 110 Loops, loops, loops, loops, loops, loops

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COMP 110Loops, loops, loops, loops, loops, loops

• Luv Kohli
• September 22, 2008
• MWF 2-250 pm
• Sitterson 014

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Announcements

• Program 2 due date extended to Friday, Sep 26,
  2pm
• Lab 4 due date extended to Wednesday, Oct 1, 2pm
• Print out your programs and hand the in
• Interesting talk today at 4pm, SN011
• William Swartout, Toward the Holodeck
  Integrating Graphics, Artificial Intelligence,
  Entertainment and Learning

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Questions?

• How is Program 2 going?

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Today in COMP 110

• More loop examples
• If time indentation

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Designing a loop

• Initializing statements
• Loop body
• Ending a loop

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Designing a loop body

• Example calculate the sum of numbers entered by
  the user

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Identify the loop body from pseudocode

• Output instructions to the user
• Initialize variables
• Prompt user for input
• Read a number into variable next
• sum sum next
• Prompt user for input
• Read a number into variable next
• sum sum next
• Prompt user for input
• Read a number into variable next
• sum sum next
• ...
• Output the sum

Repeated statementsbecome your loop body
Statements that are only done onceare not part
of your loop body
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Pseudocode with loop body

• Output instructions to the user
• Initialize variables
• Do the following for the appropriate number of
  times
• Prompt user for input
• Read a number into variable next
• sum sum next
• Output the sum

Initializing statements
How do we end the loop?How many iterations?
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Initializing statements

• Variables used in your loop need to be
  initialized (set to a value) before the loop
• next
• Read a number into variable next
• We read a new value for next before using it
  during each iteration of the loop so we do not
  need to initialize it
• sum
• sum sum next
• sum is on the right side of an assignment
  statement. sum MUST have a valid value before the
  loop starts.

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What should sums initial value be?

• Output instructions to the user
• Initialize variables
• Do the following for the appropriate number of
  times
• Prompt user for input
• Read a number into variable next
• sum sum next
• Output the sum

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Initialize sum

• Consider the first iteration.
• After executing the first iteration, the expected
  value of sum should be
• sum next (the first input value)
• The assignment statement is
• sum sum next
• Therefore, initial value of sum is
• sum 0

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What should sums initial value be?

• Output instructions to the user
• sum 0
• Do the following for the appropriate number of
  times
• Prompt user for input
• Read a number into variable next
• sum sum next
• Output the sum

Initializing statements
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Initializing statements

• Always set initial value to 0?
• What if we calculate the product of all the input
  values?
• product product next
• After the first iteration, we expect
• product next (the first input value)
• Initial value of product?
• product 1

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Ending a loop

• Output instructions to the user
• sum 0
• Do the following for the appropriate number of
  times
• Prompt user for input
• Read a number into variable next
• sum sum next
• Output the sum

How do we end the loop?How many iterations?
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Ending a loop

• Count-controlled loops
• If you know the number of loop iterations
• for (count 0 count lt iterations count)
• User-controlled loops
• Ask-before-iterating
• Sentinel value

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Count-controlled loops

• You know how many iterations in advance.
• Ask every student in the class for his/her age
  and calculate the average age
• sum 0
• for (student 1 to student n)
• Ask his/her age
• sum sum age
• average sum / n

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Count-controlled loops

• Use a for loop in most cases
• 37 students in the class 37 iterations
• numStudents 37
• sum 0
• for (count 1 count lt numStudents count)
• Ask student count for his/her age
• sum sum age
• average sum / numStudents

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Count-controlled loops

• Can also use a while loop, but for loop is
  easiest way to implement count-controlled loops
• numStudents 37
• count 1
• sum 0
• while (count lt numStudents)
• Ask student count for his/her age
• sum sum age
• count count 1
• average sum / numStudents

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User-controlled loops

• Also called ask-before-iterating
• Ask the user if it is time to end the loop
• Example add a bunch of numbers
• Lab 4 when the user enters a negative number (a
  sentinel value), its time to end the loop

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User-controlled loops

• Use while and do-while in most cases
• do-while
• If you know that the user wants at least one
  iteration
• Ask the user whether the loop should end at the
  end of the loop body

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User-controlled loops do-while

• do
• // do stuff in your code here
• Prompt user for input
• Read a number into variable next
• sum sum next
• // ask if we should do another iteration
• System.out.print(Continue (yes/no)? )
• answer keyboard.nextLine()
• while (answer.equalsIgnoreCase(yes))

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User-controlled loops while

• Use while if there could be zero iterations
• Example, summing numbers
• When the user inputs a negative value
• End the loop
• The negative value is not used in calculating the
  sum
• If the first input value is negative,no
  iterations

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User-controlled loops while

• sum 0
• while (next gt 0)
• Prompt user for input
• Read a number into variable next
• sum sum next

Is this code correct?
What is the initial value of next?
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Solution

• sum 0
• Prompt user for input
• Read a number into variable next
• while (next gt 0)
• Prompt user for input
• Read a number into variable next
• sum sum next

Is this code correct?
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Solution, another try

• sum 0
• Prompt user for input
• Read a number into variable next
• while (next gt 0)
• sum sum next
• Prompt user for input
• Read a number into variable next

Is this code correct?
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Example problem

• Find the lowest and highest ages in the class
• Use a loop?
• Yes
• What goes in the loop body?

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Loop body

• Ask student 1 for age
• Update min/max ages
• Ask student 2 for age
• Update min/max ages
• ...
• ...
• Ask student 37 for age
• Update min/max ages
• End loop
• Output min and max ages

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Min/max ages

• Which kind of loop?
• Lets use a for loop

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Min/max ages

• int min 2000 // initialize to large value
• int max 0 // initialize to small value
• for (int count 1 count lt 37 count)
• Ask student count for age
• if (age gt max)
• max age
• if (age lt min)
• min age
• Output min and max ages

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Min/max ages
min
max
Ages
2000 20 20 18 18 18 12 12 12
0 20 23 23 23 25 25 94 94
20 23 18 18 25 12 94 36
if (age gt max) max age if (age lt min)
min age
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Wednesday

• Nested loops
• Loop bugs