Nassi-Schneiderman Flowcharts

The Decision Structure

The decision structure is used when your program must make a choice of some sort. In this case, it allows your program to only execute sections of your algorithm if certain conditions are met.

There are two versions of the decision structure: the single alternative and double alternative. The single alternative structure is used when there is only one conclusion to be made by the decision. Instead of an either-or situation, it describe a simple condition that can be stated in general terms by thinking something like: “if condition X, then do Y”. For example, you might think “if it rains, then I use an umbrella.”

The NS form of the single alternative is created with the following general diagram:

In this case, you state the condition in the form of a true/false question (here, what happens if the question is false is simply not considered). Note that if the condition is true, then there is a list of one or more instructions that must be followed below the True triangle. The False triangle is empty, meaning that nothing is done (in terms of flow, you would simply exit at the bottom below the False triangle).

Example 1:

Think of the English statement “If today is Friday, then I will water the plants”. Notice that you really don’t need to include “else, I won’t water the plants”, because that is the same as saying “else, do nothing” in this situation. It simply doesn’t need to be said. This means that we have a single-alternative If-Then statement. The corresponding NS chart would look like this:

Example 2:

The second example combines a chart from the discussion on the sequence structure with a decision. Here, we read in the price and tax rate, compute the (overall) sale price, and print it out only if it is less than fifty dollars.