For Loops CSCI N 201 Introduction to Programming

For Loops CSCI N 201 Introduction to Programming Concepts Department of Computer and Information Science IUPUI

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 back. Racer 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 by. Five 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