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: 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?
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?
Iterate a specific number of times Know that 5 th or 7 th or n th iteration is being executed Represent integers– e.g., for factorials Common 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 easier! for fruit in fridge: do_something_with(fruit) Variable Role: Walker Python, Java Scratch, C++, C
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 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 Flag Why use a one-way-flag variable?
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