COMP 110 Augustus Gloop, Augustus Gloop

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COMP 110Augustus Gloop, Augustus Gloop

• Luv Kohli
• September 24, 2008
• MWF 2-250 pm
• Sitterson 014

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Announcements

• Program 2 due Friday, 2pm
• Remember to also print out your code and hand it
  in
• I will be out of town on Friday
• Recitation will still be held by another COMP110
  instructor at the usual time
• No office hours Friday morning
• Ask me questions before 6pm Thursday

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Some notes about printing your code

• Print from jGRASP if you can
• Print with a fixed-width font (such as Consolas
  or Courier New)
• Print double-sided if you can

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Questions?
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Today in COMP 110

• Nested loops
• Some comments on Lab 3

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Nested loops

• Just like we have nested if/else statements, we
  can have nested loops

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Nested loops example friendly greetings
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Nested loops example friendly greetings

• Student 1 shakes Student 4s hand
• Student 1 shakes Student 5s hand
• Student 1 shakes Student 6s hand
• Student 2 shakes Student 4s hand
• Student 2 shakes Student 5s hand
• Student 2 shakes Student 6s hand
• Student 3 shakes Student 4s hand
• Student 3 shakes Student 5s hand
• Student 3 shakes Student 6s hand

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Nested loops example friendly greetings

• Student 1 shakes Student 4s hand
• shakes Student 5s hand
• shakes Student 6s hand
• Student 2 shakes Student 4s hand
• shakes Student 5s hand
• shakes Student 6s hand
• Student 3 shakes Student 4s hand
• shakes Student 5s hand
• shakes Student 6s hand

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Nested loops example friendly greetings

• for (every student in line A)
• Student in line A shakes every Students hand
• in line B

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Nested loops example friendly greetings

• for (every student in line A)
• for (every student in line B)
• (Student in line A) shakes (Student in
  line B)s
• hand

Outer loop
Inner loop
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Nested loops example friendly greetings

• for (int stdLineA 1 stdLineA lt 3 stdLineA)
• for (int stdLineB 4 stdLineB lt 6
  stdLineB)
• System.out.println(Student stdLineA
  
• shakes Student stdLineB
• s hand.)

stdLineA 1 2 3
stdLineB 4 5 6
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Nested loops example friendly greetings

• for (int stdLineA 1 stdLineA lt 3 stdLineA)
• for (int stdLineB 4 stdLineB lt 6
  stdLineB)
• System.out.println(Student stdLineA
  
• shakes Student stdLineB
• s hand.)

Inner loop
Outer loop
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Nested loops

• You can nest different kinds of loops inside
  other loops, or put if/else statements inside
  loops, or put loops inside if/else statements,
  or
• ExamAverager example in jGRASP

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Loop bugs

• Infinite loops already talked about these
• Off-by-one errors

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Off-by-one errors

• Loop repeats one too many or one too few times
• for (count 1 count lt 10 count)
• // loop 9 times 1, 2, 3, ... 9
• for (count 1 count lt 10 count)
• // loop 10 times 1, 2, 3, ... 10
• for (count 0 count lt 10 count)
• // loop 10 times 0, 1, 2, ... 9

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Finding errors

• Trace your variables
• Put output statements in your code to see what
  values are stored in your variables
• System.out.println(variable)
• Check whether the values are correct and what you
  expect
• Remove these extra output statements after the
  program runs correctly
• Read example in the book, p. 188 (4th edition),
  p. 218 (5th edition)
• Use a debugger

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What does this code do?

• int a 0
• int b 0
• for (int i 1 i lt 6 i)
• if (i 2 0)
• b b i
• else
• a a i

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What does this code do?

• int oddSum 0
• int evenSum 0
• for (int i 1 i lt 6 i)
• if (i 2 0)
• evenSum evenSum i
• else
• oddSum oddSum i

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What does this code do?

• int oddSum 0
• int evenSum 0
• for (int i 1 i lt 6 i)
• if (i 2 0)
• evenSum evenSum i
• else
• oddSum oddSum i

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What is wrong with this code?

• int oddSum 0
• int evenSum 0
• for (int i 1 i lt 6 i)
• if (i 2 0)
• evenSum evenSum i
• else
• oddSum oddSum i

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What is wrong with this code?

• int oddSum 0
• int evenSum 0
• for (int i 1 i lt 6 i)
• if (i 2 0)
• evenSum evenSum i
• else
• oddSum oddSum i

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Indentation

• Indentation
• Makes code easier to read
• Helps with finding syntax and logic errors
• Indent code that goes between and
• Be consistent!

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Scope

• Variables declared in outer scopes are visible to
  code inside inner scopes
• public static void main(String args)
• int total 15
• int n 5
• if (n lt 10)
• total total n
• System.out.println(total)

inner
outer
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Scope

• Variables declared in inner scopes are NOT
  visible to outer code
• public static void main(String args)
• int n 5
• if (n lt 10)
• int total 15 n
• System.out.println(total) // ERROR!!!

outer
inner
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else

• if (inputString.equals(BLUE))
• eyeColor Color.BLUE
• else if (inputString.equals(GREEN))
• eyeColor Color.GREEN
• else if (inputString.equals(RED))
• eyeColor Color.RED
• else if (!inputString.equals(BLUE)
• !inputString.equals(GREEN)
• !inputString.equals(RED))
• eyeColor Color.WHITE

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else

• if (inputString.equals(BLUE))
• eyeColor Color.BLUE
• else if (inputString.equals(GREEN))
• eyeColor Color.GREEN
• else if (inputString.equals(RED))
• eyeColor Color.RED
• else
• eyeColor Color.WHITE

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else not needed when empty

• if (inputString.equals(MOUTH))
• mouthStartAngle 0
• else

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Multi-line comments

• / This is a multi-line comment.
• What do you think of it? /
• // You can also have multi-line comments
• // this way.

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Too many comments

• Dont need to comment every line of code,
  especially when it is obvious what your code is
  doing
• // set count to 12
• count 12
• Only comment to enhance understanding and to
  explain why your code does what it does

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Friday

• Program 2 due, 2pm
• Lab 4 help

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