Summary Programming Logic and Design Ch 4

Chapter 4

Describe the selection structure
 Boolean expressions can only be true or false (yes or no / 1 or 0) - used in every selection
structure
 Dual-alternative (binary) selection structure- action for each possible outcome-
true/false (if-then-else)
 Single-alternative (unary) selection structure- actions for only one outcome (if-then) -
otherwise null
 if then clause- holds action executed when condition in decision is true
 else clause- executes only when tested condition in decision is false


List the relational comparison operators
 Relational comparison operators
 Two values compared could be variables or constants
 Variables have to be the same data type

= > < >= <= <>
equal greater less than greater or less or not equal
(==) than equal equal (! =)

  Restrict use of "if without else" - only take action when comparison is false (best to use
positive (true) outcome)
 Sometimes code is clearest in negative "if without an else"
if customerZipCode <> LOCAL_ZIP_CODE then
total = total + deliveryCharge
endif
 Trivial expression (currentTotal = previousTotal)
 Will always evaluate to the same result, never use
 Example: true for 20 = 20 / false for 30 = 40
 Common mistakes when using relational operators
 missing boundary for selection, double check – saying over 65, is it >65 or >= 65?


Use AND logic
 Compound condition - ask multiple questions before outcome is determined
 AND decision
 Requires that both of the two tests evaluate to true
 Requires a nested if or a cascading if statement


 Nesting AND decision for efficiency
 With nesting decisions, either selection can come first
 Performance time can be improved by asking questions in the right order