One of the most important concepts of programming is the ability to control a program so that different lines of code are executed or that some lines of code are executed many times. The mechanisms that allow us to control the fow of execution are called control structures. Flowcharting is a method of documenting (charting) the fow (or paths) that a program would execute. There are four main categories of control structures:
- Sequence Very boring. Simply do one instruction then the next and the next. Just do them in a given sequence or in order listed. Most lines of code are this.
- Selection This is where you select or choose between two or more fows. The choice is decided by asking some sort of question. The answer determines the path (or which lines of code) will be executed.
- Iteration Also known as repetition, it allows some code (one to many lines) to be executed (or repeated) several times. The code might not be executed at all (repeat it zero times), executed a fxed number of times or executed indefnitely until some condition has been met. Also known as looping because the fowcharting shows the fow looping back to repeat the task.
- Branching A control structure that allows the fow of execution to jump to a different part of the program. This category is rarely used in modular structured programming.
All high-level programming languages have control structures. All languages have the first three categories of control structures (sequence, selection, and iteration). Most have the if thenelse structure (which belongs to the selection category) and the while structure (which belongs to the iteration category). After these two basic structures there are usually language variations.
The concept of structured programming started in the late 1960's with an article by Edsger Dijkstra. He proposed a "go to less" method of planning programming logic that eliminated the need for the branching category of control structures. The topic was debated for about 20 years. But ultimately "By the end of the 20th century nearly all computer scientists were convinced that it is useful to learn and apply the concepts of structured programming.