1. Roles of Variables with Examples in Scratch
© 2014 Project Lead The Way, Inc.
Unpublished work © 2013 Project Lead The Way, Inc.

2. What are variable roles?
Presentation Name
Course Name
Unit # – Lesson #.# – Lesson Name
What are variable roles?
A variable role is the reason we are using the variable.
Variables always remember data for later use. But why are we trying to remember something?

3. What are variable roles?
Presentation Name
Course Name
Unit # – Lesson #.# – Lesson Name
What are variable roles?
Certain reasons for using a variable come up over and over.
Eight roles cover 90% of the variable use by first-year programmers.
Fixed
Most recent
Stepper
Walker
Accumulator
Aggregator
Best-so-far
One-way flag

4. Quick Summary Fixed Assigned once Most recent Assigned Accumulator
Presentation Name
Course Name
Unit # – Lesson #.# – Lesson Name
Quick Summary
Fixed
Assigned once
Most recent
Assigned
unpredictably
Accumulator
Running total
Aggregator
Running list
Stepper
Predetermined
sequence of values
Walker
Elements of iterator
Best-so-far
Record holder
One-way flag
Won’t reset flag
until after iteration

5. What are variable roles?
Presentation Name
Course Name
Unit # – Lesson #.# – Lesson Name
What are variable roles?
Roles say why we are using the variable.
Roles are not syntax.
Examples of syntax include whether the variable is used in a conditional, in an expression, or in an assignment block.

6. Why use a fixed variable?
Presentation Name
Course Name
Unit # – Lesson #.# – Lesson Name
Variable Role: Fixed
Why use a fixed variable?
Create a single place to tweak a number used throughout a program – low maintenance
Make code easier to read – no wondering “why subtract 20 here?”
Make it easy to add features: user decides on the constant
A major source of bugs is when one “thing” appears in multiple places. It is common for that thing to be changed in some but not all of those places during revisions.
Adding user input could be text input or by Graphical User Interface. Either way, using the user’s value is much easier if all the places that need that value already use a variable. Then, instead of being a fixed role,… (see next slide)

7. Variable Role: Fixed Pattern: Assigned at the head
Presentation Name
Course Name
Unit # – Lesson #.# – Lesson Name
Variable Role: Fixed
Pattern:
Assigned at the head
of a program or at the
head of a code block
Used in any way later but never assigned again
Convention suggests all caps
A major source of bugs is when one “thing” appears in multiple places. It is common for that thing to be changed in some but not all of those places during revisions.
Adding user input could be text input or by Graphical User Interface. Either way, using the user’s value is much easier if all the places that need that value already use a variable. Then, instead of being a fixed role,… (see next slide)

8. Variable Role: Most-Recent Why use a most recent variable?
Presentation Name
Course Name
Unit # – Lesson #.# – Lesson Name
Variable Role: Most-Recent
Why use a most recent variable?
Retrieve or calculate once, use multiple times
Remember state of a process
Remember user input until needed
Embed explanation
Debug by printing
Pattern: Appears on left of assignment and then in a variety of syntax

9. Variable Role: Most-Recent
Presentation Name
Course Name
Unit # – Lesson #.# – Lesson Name
Variable Role: Most-Recent
Initialized
Value is used
Unpredictably
assigned

10. Variable Role: Accumulator Why use an accumulator variable?
Presentation Name
Course Name
Unit # – Lesson #.# – Lesson Name
Variable Role: Accumulator
Why use an accumulator variable?
To keep a running total or cumulative value – could be multiplication, addition, net, . . .
Common pattern:
Assigned to initial value before loop,
Assigned with inside of loop
Result used after loop

11. Variable Role: Accumulator Pattern: Initialize-Accumulate-Report
Presentation Name
Course Name
Unit # – Lesson #.# – Lesson Name
Variable Role: Accumulator
Pattern: Initialize-Accumulate-Report

12. Variable Role: Aggregator Why use an aggregator variable?
Presentation Name
Course Name
Unit # – Lesson #.# – Lesson Name
Variable Role: Aggregator
Why use an aggregator variable?
To collect items and remember them all separately
Common pattern:
Initialize to empty collection before a loop,
Append element to aggregate during iteration
The aggregate is used during or after the loop

13. Variable Role: Aggregator Pattern: Initialize-Aggregate-Report
Presentation Name
Course Name
Unit # – Lesson #.# – Lesson Name
Variable Role: Aggregator
Pattern: Initialize-Aggregate-Report

14. Variable Role: Stepper Why use a stepper variable?
Presentation Name
Course Name
Unit # – Lesson #.# – Lesson Name
Variable Role: Stepper
Why use a stepper variable?
Iterate a specific number of times
Know that 5th or 7th or nth iteration is being executed
Represent integers– e.g., for factorials
Common
pattern:
“Counter” is a fine word if the stepper starts at 1 and increments by 1, but “stepper” is more general and groups a larger number of reasons that programmers use a variable

15. Why use a walker variable?
Presentation Name
Course Name
Unit # – Lesson #.# – Lesson Name
Variable Role: Walker
Why use a walker variable?
Refer to members of a collection during iteration.

16. Variable Role: Walker
Scratch requires “indexing” with a stepper like C++. Python and Java are easier! 01 02
for fruit in fridge:
do_something_with(fruit)
Python, Java
Scratch,
C++, C

17. Variable Role: Best-so-far Why use a best-so-far variable?
Presentation Name
Course Name
Unit # – Lesson #.# – Lesson Name
Variable Role: Best-so-far
Why use a best-so-far variable?
To remember the record holder while iterating across many opportunities to set the record
Frequent pattern:
Initialize to worst-possible value before loop,
During iteration, compare something to best-so-far and maybe assign a new record
After loop, best-so-far used as the true record-best from all iterations

18. Variable Role: Best-so-far Pattern: Initialize Check and Set Record
Report
Presentation Name
Course Name
Unit # – Lesson #.# – Lesson Name

19. Variable Role: One-way Flag Why use a one-way-flag variable?
Presentation Name
Course Name
Unit # – Lesson #.# – Lesson Name
Variable Role: One-way Flag
Why use a one-way-flag variable?
To remember whether any of several opportunities meet a single condition
Common pattern:
“Clear” the flag (initialize) to say the opportunity has not yet been met
Check for condition with each iteration and “raise” flag if true
Flag is not cleared during iteration
After loop, check if flag was raised during the iterations

20. Variable Role: One-Way Flag Pattern: Initialize to clear flag
Presentation Name
Course Name
Unit # – Lesson #.# – Lesson Name
Variable Role: One-Way Flag
Pattern:
Initialize to clear flag
Check and Raise Flag
Report
Event-driven example

21. Variable Role: One-Way Flag Pattern: Initialize to clear flag
Presentation Name
Course Name
Unit # – Lesson #.# – Lesson Name
Variable Role: One-Way Flag
Pattern:
Initialize to clear flag
Check and Raise Flag
Report
Procedural example