For Loops

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For Loops
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Challenge Racer

• Simulate a race that says Now on lap X for 10
  laps.
• Make X vary, so it says 1, then 2, then 3
• Use only one output statement!

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Loops

• A loop is used for repetitive behaviour
• A set of commands is placed in a code block
• A condition determines when the block stops
  repeating

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The For Loop

• Used when you know how many times something will
  happen
• Great for counting
• Also often used with lists

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Parts of a For Loop

• Sentry Variable
• Initial Value
• Looping Condition
• Increment Statement

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Sentry Variable

• A special variable designed to control the loop
• In for loops, usually an integer
• Use a meaningful name when practical
• Traditionally use i if no other name makes sense

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Initial Value

• Give sentry some meaningful initial value
• Usually 0 or 1

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Looping Condition

• A condition
• Always involves sentry variable
• Indicates when loop should continue
• If condition is true, loop will repeat
• When condition becomes false, loop will end

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Increment Statement

• A line of code
• Always involves sentry variable
• Changes value of sentry
• Usually i, i--, I something
• Must make it possible for condition to become
  false eventually

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Racer Algorithm

• New Program Racer by me
• New integer variable lap starts at 1
• For loop with lap going from 1 to 10 by 1
• Output Now on lap lap
• End For loop
• End racer

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Challenge Backwards Racer

• Simulate a race that says Now on lap X for 10
  laps.
• This time go backwards!
• Make X vary, so it says 10, then 9, then 8
• Use only one output statement!

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Counting Backwards

• Use same for loop elements
• Use larger value for initial sentry value
• Decrement variable each time through
• Check for smaller result

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Backwards Racer Algorithm

• New Program backRacer by me
• New integer variable lap starts at 10
• For loop with lap going from 10 to 1 by -1
• Output Now on lap lap
• End For loop
• End racer

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Counting by 5 Algorithm

• New Program byFive by me
• New integer variable lap starts at 0
• For loop with lap going from 0 to 50 by 5
• Output Now on lap lap
• End For loop
• End racer

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Code Tracing

• Make a chart
• Make each variable a column head
• Make a column for each condition
• Make a column for output

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Code Tracing Contd

• Walk through code one line at a time
• Each time a variable changes, change it on the
  chart
• Each time you get to a condition statement,
  evaluate the condition

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Code Tracing III

• For conditions, write TRUE or FALSE
• Write any output

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Code Tracing Example
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Ordinary Slide

• Level 1
• Level 2
• Level 3

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Code

• My code
• More Code
• More Code