1 1 5 roles of variables _ SCRATCH

Roles of Variables with Examples
in Scratch

Computer Science and Software Engineering

© 2014 Project Lead The Way, Inc.


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?


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
 Stepper
 Accumulator
 Best-so-far

 Most recent
 Walker
 Aggregator
 One-way flag


Quick Summary

 Fixed
Assigned once

 Most recent
Assigned
unpredictably

 Accumulator
Running total

 Stepper
Predetermined

 Aggregator
Running list

 Walker
Elements of iterator

sequence of values

 Best-so-far

Record holder

 One-way flag
Won’t reset flag
until after iteration


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.


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


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


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


Variable Role: Most-Recent

Initialized

Value is used

Unpredictably
assigned


Variable Role: Accumulator
Why use an accumulator variable?

•

To keep a running total or cumulative value – could be multiplication, addition,
net, . . .

•

Common pattern:

1.
2.
3.

Assigned to initial value before loop,
Assigned with
Result used after loop

inside of loop


Variable Role: Accumulator
Pattern: Initialize-Accumulate-Report


Variable Role: Aggregator
Why use an aggregator variable?

•
•

To collect items and remember them all separately
Common pattern:

1.
2.
3.

Initialize to empty collection before a loop,
Append element to aggregate during iteration
The aggregate is used during or after the loop


Variable Role: Aggregator
Pattern: Initialize-Aggregate-Report


Variable Role: Stepper
Why use a stepper variable?

•
•
•
•

Iterate a specific number of times
Know that 5

th

th
th
or 7 or n iteration is being executed

Represent integers– e.g., for factorials
Common
pattern:

Variable Role: Walker
Why use a walker variable?

•

Refer to members of a collection during iteration.


Variable Role: Walker
Scratch requires “indexing” with a stepper like C++. Python and Java are

02

for fruit in fridge:
do_something_with(fruit)

Py
tho
n,

01

Ja
va

easier!

Sc
rat

ch
,
C+
+,
C

•


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:

1.
2.

Initialize to worst-possible value before loop,
During iteration, compare something to best-so-far and maybe assign a
new record

3.

After loop, best-so-far used as the true record-best from all iterations


Variable Role: Best-so-far
Pattern:

•
•
•

Initialize
Check and Set Record
Report


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:

1.
2.
3.
4.

“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


Variable Role: One-Way Flag
Pattern:

•
•
•

Initialize to clear flag
Check and Raise Flag
Report
Event-driven example


Variable Role: One-Way Flag
Pattern:

•
•
•

Initialize to clear flag
Check and Raise Flag
Report

Procedural

example