For Loops. Challenge: Racer ● Simulate a race that says “Now on lap X” for 10 laps. ● Make X...
-
Upload
whitney-kelly -
Category
Documents
-
view
215 -
download
0
Transcript of For Loops. Challenge: Racer ● Simulate a race that says “Now on lap X” for 10 laps. ● Make X...
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 AlgorithmNew 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
CodeMy code
More Code
More Code