4.3 Conditional Control

When more than one flow option exists in the control flow, it is called a conditional control flow. Whenever a certain condition is met, its respective flow option is followed. A condition is an expression defined using relational and Boolean operators that evaluate to either true or false. The list and interpretation of the supported relation operators is presented in Table 4-1.

An example of the use of relational operators to form relational expressions is shown in Example 4-1.

irb(main):001:0> 5 == 5
=> true
irb(main):002:0> 5 == 6
=> false
irb(main):003:0> 5 <= 5
=> true
irb(main):004:0> 5 != 5
=> false

To create complex expressions, simple relational expressions are combined by using Boolean operators. Two Boolean variable operators available in Ruby are and and or. The operator and evaluates to true only if both operands are true. In contrast, the operator or evaluates to true if either or both operands are true. To ensure the precedence you desire, a good programming practice is to use parentheses. A truth table is presented in Table 4-2.

Recall the negation operator (!) that appears in the not equal to relational operator. This operator is a simple negation and works on any true or false expressions or conditionals, as shown in Example 4-2. Sometimes the negation operator ! is denoted by the word not. In fact, the fifth example in the figure, regardless of the Boolean values you set for first and second, will always evaluate to true. This is called a tautology.

irb(main):001:0> !false
=> true
irb(main):002:0> !(true or false)
=> false
irb(main):003:0> first=true
=> true
irb(main):004:0> second=false
=> false
irb(main):005:0> (first and second) or !(first and second)
=> true

Control Flow

Control flow is logic flow in the context of a program. Just as algorithms start somewhere and show an order of steps until the end of the algorithm, so does a program. Programs have a starting instruction, and then instructions are executed in a specific order, and eventually the program ends. We use a flowchart to illustrate control flow. Figure 4-3 revisits the names example to illustrate control flow with flowcharts. Note that the start and endpoints of the algorithms are indicated by circular shapes (circles or ovals depending simply on the length of text contained).

This type of flow is known as one-directional control flow. Programs are not limited to one-directional control flow. In this chapter, we discuss programming constructs that allow us to change program control flow. Understanding this control flow is essential to being able to create and test (debug) an implementation of an algorithm.

Figure 4-3. Control flowchart