Computer Science and Software Engineering © 2014 Project Lead The Way, Inc.

Roles of Variables with Examples in Scratch

• 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.

What are variable roles?

Fixed Most recent Stepper Walker Accumulator Aggregator Best-so-far One-way flag

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

• 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.

What are variable roles?

• 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: FixedWhy use a fixed variable?

• 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: Fixed

• 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-RecentWhy use a most recent variable?

Variable Role: Most-Recent

Initialized

Unpredictably assigned

Value is used

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

• Common pattern:

1. Assigned to initial value before loop,

2. Assigned with inside of loop

3. Result used after loop

Variable Role: AccumulatorWhy use an accumulator variable?

Variable Role: AccumulatorPattern: Initialize-Accumulate-Report

• To collect items and remember them all separately

• Common pattern:

1. Initialize to empty collection before a loop,

2. Append element to aggregate during iteration

3. The aggregate is used during or after the loop

Variable Role: AggregatorWhy use an aggregator variable?

Variable Role: AggregatorPattern: Initialize-Aggregate-Report

• 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:

Variable Role: StepperWhy use a stepper variable?

• Refer to members of a collection during iteration.

Variable Role: WalkerWhy use a walker variable?

• Scratch requires “indexing” with a stepper like C++. Python and Java are easier!0102

for fruit in fridge: do_something_with(fruit)

Variable Role: Walker

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• To remember the record holder while iterating across many opportunities to set the record

• Frequent pattern:

1. Initialize to worst-possible value before loop,

2. 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-farWhy use a best-so-far variable?

Variable Role: Best-so-farPattern:

• Initialize• Check and Set Record• Report

• To remember whether any of several opportunities meet a single condition

• Common pattern:

1. “Clear” the flag (initialize) to say the opportunity has not yet been met

2. Check for condition with each iteration and “raise” flag if true

3. Flag is not cleared during iteration

4. After loop, check if flag was raised during the iterations

Variable Role: One-way FlagWhy use a one-way-flag variable?

Variable Role: One-Way FlagPattern:

• Initialize to clear flag• Check and Raise Flag • Report

Event-driven example

Variable Role: One-Way FlagPattern:

• Initialize to clear flag• Check and Raise Flag • Report

Procedural example