For Loops

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!

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

The For Loop ● Used when you know how many times something will happen ● Great for counting ● Also often used with lists

Parts of a For Loop ● Sentry Variable ● Initial Value ● Looping Condition ● Increment Statement

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

Initial Value ● Give sentry some meaningful initial value ● Usually 0 or 1

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

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

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

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!

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

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

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

Code Tracing ● Make a chart ● Make each variable a column head ● Make a column for each condition ● Make a column for output

Code Tracing Cont’d ● 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

Code Tracing III ● For conditions, write TRUE or FALSE ● Write any output

Code Tracing Example

Ordinary Slide ● Level 1 – Level 2 ● Level 3

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