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Making Decisions

In this chapter:

  • Describe the term selection​
  • Learn to use relational operators and logical operators​
  • Write if…else if…else statements​
  • Use switch statements
  • Use flowcharts to describe and algorithm

In each of the short programs thus far each instruction has been executed sequentially, one after the other. Each statement will alter the state of the program so the order in which they are executed does matter. This order is also known as the flow of control. Designing the right algorithm to solve a problem is all about planning the steps needed to arrive at the solution, and ensuring those steps are in the right order.

This flow of control can be altered through the use of selection, iteration, methods and events.

In this chapter we will see how this flow of control can be changed using selection. That is, either one statement (or block of statements) will be executed or another statement (or block of statements).

We will be making extensive use of the comparison operators from Chapter 3 (<, >=, != etc.) as these are used in the conditional statements that are used to determine the outcome of a decision. Depending on the result of this decision (or condition), one path or another is followed in a program.

For example, in the following extract, the Console.WriteLine() statement will only be executed if the value of the variable, score, is greater than or equal to \(75\):

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    if (score >= 75)
    {
        Console.WriteLine("Congratulations! You got an A*");
    }

Comparison operators can be used to compare expressions, such as two values, two expressions, or a number and a variable.1 The result will be a boolean value (true or false). These expressions may also make use of one or more logical operators (AND, OR etc).

  1. Not every pair of data types can be compared with each other directly e.g. you cannot compare a string to a number. ↩