1 1 5 roles of variables _ SCRATCH

• A variable role is the reason we are using the variable.• Variables always remember data for later use.. • Create a single place to tweak a number used throughout a program – low maint

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?

Quick Summary

Assigned once

 Most recent Assigned unpredictably

 Accumulator

Running total

 Aggregator Running list

• 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: Fixed

Why 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-Recent

Why 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: Accumulator

Why use an accumulator variable?

Variable Role: Accumulator

Pattern: 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: Aggregator

Why use an aggregator variable?

Variable Role: Aggregator

Pattern: 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

pattern:

Variable Role: Stepper

Why use a stepper variable?

• Refer to members of a collection during iteration.

Variable Role: Walker

Why use a walker variable?

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

• 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-far

Why use a best-so-far variable?

Variable Role: Best-so-far

Pattern:

• Initialize

• Check and Set Record

• 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 Flag

Why use a one-way-flag variable?

Variable Role: One-Way Flag

Pattern:

Event-driven example

Variable Role: One-Way Flag

Pattern:

Procedural example