Nassi-Schneiderman Flowcharts

The Loop Structure

The loop structure (also called the iterative structure) is used when you need to do something repetitively in an algorithm. Loops are characterized by two main components: a condition that must be tested each time through, and a body that is executed, depending on whether the condition is True or False. The body describes some action that must be done (perhaps) many times. The condition determines whether you must keep doing that repetitive action.

For example, consider English statement “I will mow my lawn while there is grass to be cut”. This statement implies the existence of a loop in the sense that you are (a) performing a repetitive action (cutting rows of grass) and (b) defining a condition that tells you whether to continue mowing (“while there is grass to be cut”).

In general, there are two versions of the iterative structure: the pretest loop and the posttest loop. The pretest loop is a construct that first tests the condition of the loop, then executes the body of the loop if the condition is met. The posttest loop is a construct that executes the body of the loop at least once and tests the condition at the end. In a pretest loop, you could specify a loop that never actually gets executed, if the condition is never satisfied. A posttest loop always gets executed at least once.

Pretest Loops

The pretest loop first tests the condition of the loop and then permits a program segment to be repeatedly executed, as long as the test condition is satisfied. There are at least two versions of the pretest loop: while and for.

While Loops

The while (aka while-do) loop is a general loop that repeats the body of the loop as long as its condition is true. It tells the computer “while some condition is true, perform some instructions repeatedly.” The diagram below illustrates the general format of the while loop:

In the case of the while loop (or any other loop), the statement “while some condition is true” is replaced by some question that determines whether to keep executing the loop. Thus, an If/Then style of decision structure is not necessary here, because it is considered by the “while” portion of the NS chart.

Thinking of the general statement “while X is true, repeatedly do the following…”, you might consider the previous real-world example: “While there is long grass, mow the lawn.” This statement can also be used to illustrate the “execute ZERO or more times” property: if you get the mower out and examine the lawn only to find that it has already been mowed, then the English statement is still valid (the “While” portion turned out to be false, so you didn’t have to do any mowing).

The diagram below illustrates a formal NS chart for that English statement:

A more complex example is given below. This example gives the logic for a program that will print the numbers from one to ten.

In the beginning of this program, a variable called count was initialized to 1. This illustrates an important requirement with pretest loops such as the while. A pretest loop always tests a condition before the loop body can be executed, and this condition must be a question that can be answered. Above, if the count variable had no initial value, then the question “while count is less than 11?” would make no sense.

The computer requires that this initial condition always be “set up” so that the question testing the loop is valid. Otherwise, the question won’t make sense to the computer. As a real-world example, when you are eating, you might be implicitly performing a loop: “while I am hungry, eat.” This means that there is an initial condition that exists: you are hungry. You don’t explicitly tell yourself to “be hungry” beforehand, but that is precisely the sort of thing that must be done with a computer, as the computer won’t know whether “it is hungry” or “count is 1” or whatever else.

For Loops

The for (aka for-do) loop is used when we know exactly how many repetitions there will be. A special variable is used for counting the number of loop iterations. The for loop tells the computer to essentially “execute this loop precisely N times, for some positive value of N. The diagram below illustrates the general format of the for loop:

The for loop is often used in mathematical situations or at any time in which you know in advance that you are dealing with a precise number of loop iterations. Consider a real-world example in which you have to mow 5 yards:

In this example, there is a while loop inside the for loop. The outer for loop determines that you will mow exactly 5 yards. The inner while loop determines that you will keep mowing (whatever yard) while long grass exists.

A final example shows an NS chart using a for loop. Here, a value K is read and the loop finds the sum of the first K integers: