Lesson Six TEXT Structured Programming Much time and effor has been dev
oted to finding the best way to write computer programs. The outcome of
 that research has give rise to a discipline known as structured progra
mming, which applies to any language, be it dBASE, BASIC, Pascal, COBOL
, and so on. The discipline seeks to produce programs that are logicall
y correct and that can be easily read and maintained by someone other t
han the original author. A structured program is one which uses only th
ree types of elementary building blocks to solve a problem. These are s
equence, selection, and iteration, and they are depicted in Figure 1 (A
 fourth building block case, is sometimes included in an extended defin
ition and is also shown.) The fact that these structures are sufficient
 to express any desired logic was first postulated in a now-classic pap
er by Bohm and Jacopini. In Figure 1, each buiding block (or structure)
 is drawn as a flowchart, or pictorial representation of programming lo
gic. GFlowcharts use special symbols to communicate information. A rect
angle indicates a decision, and a small circle connects portions of the
 flowchart. All of the elementtary building blocks have one key  featur
e in common, namely, a single enteryoint. The sequence structure in Fig
ure 1a implies that program statements are executed sequentially, that 
is, one after another. In other words, block A is executed first, and t
hen block B. The selection structure of Figure 1b shows a conditional s
tatement, which provides a choice between two actions. If the condition
 is true. block A is executed; if it is false, block B is executed. The
 condition is a single entry point to the structure and both paths meet
 in a single exit point. The iteration (or loopint) structure in Figure
 1c calls for the repeated execution of one or more statments. A condit
ion is    tested and, if it is true, block A is executed. If, however, 
the condition is false, control passes to the next sequential statement
 after the iteration structure. Agin, there is exactly one entry and on
e exit point from the structure. The case (or multibranch) structure in
 Figure 1d is an extension of selection, as it pressents a choice betwe
en more than two statements, only one of which can be true. A condition
 is evaluated and a branch to one of several paths is taken, depending 
on the value of the condition. As with the other fundamenteral building
 blocks, there is one entry point and one exit point.