Software Engineering Table-driven methods General control issues

Table-driven methods A table-driven method is a schema to look up information in a table instead of using (possibly complicated) logical statements (if and case, for example).

Table-driven methods General considerations:  How to index the dimensions of a table, to avoid making it unnecessarily sparse and to make it easy to access.  What to store inside the table – data or actions.  How to declare a table in your language, so that it can accommodate its contents.

Table-driven methods Table organisation:  Direct access tables: dimensions represent meaningful values of variables. if (month = 1) { days = daysPerMonth(month - 1); days = 31; } else { daysPerMonth = if (month = 2) { days = 28; } else (...)

Table-driven methods Table organisation:  Indexed access tables: values point to indices that point to the table. value index value index

General control issues Boolean expressions: When appropriate, compare boolean values to true and false implicitly. Instead of: while (done == false)... while ((a>b) == true)... Give preference to: while (! done)... while ( a > b )...

General control issues Boolean expressions:  Simplify complicated expressions: Break complicated tests into partial tests with new boolean variables. Move complicated expressions into boolean functions. Use decision tables to replace complicated conditions.

General control issues Boolean expressions:  Form boolean expressions positively: “I ain't not no undummy.” Homer Simpson In if statements, convert negatives to positives and flip-flop the code in the if and else clauses. Use DeMorgan's laws to simplify boolean tests, e.g. prefer (A || B) to (!(!A && !B)).

General control issues Boolean expressions:  Use parentheses to clarify boolean expressions. Do not rely on the language's evaluation order. Use redundant parentheses if appropriate to make the expression more readable.  Write numeric expressions in number-line order, e.g. MIN <= i && i <= MAX, i < MIN || MAX < i.

General control issues Compound statements (blocks):  A compound statement is a collection of statements that are treated as a single statement for purposes of controlling the flow of a program. They are usually tagged by some language-specific symbol, e.g. {... }.

General control issues Compound statements (blocks):  Write pairs of braces together. Fill in the middle after you write both the opening and closing parts of a block.  Use braces to clarify conditionals. Use blocks to clarify your intentions regardless of whether the conde inside the block is 1 line or 20 lines long.

General control issues Simplifying deep nesting:  Deep nesting = more than three levels of nesting. Deep nesting should be avoided by restructuring the code.  Simplify a nested if by re-testing part of the condition.  Convert a nested if to a set of if-then- else's.  Convert a nested if to a case statement.

General control issues Simplifying deep nesting:  Factor deeply nested code into its own routine. Create specific routines for more internal tests, and leave general structure in root routine.  Entirely redesign deeply nested code.