Structured programming can be seen as a subset or subdiscipline of procedural programming, one of the major programming paradigms. It is most famous for removing or reducing reliance on the GOTO statement.
Historically, several different structuring techniques or methodologies have been developed for writing structured programs. The most common are:
The two latter meanings for the term "structured programming" are more common, and that is what this article will discuss. Years after Dijkstra (1969), object-oriented programming (OOP) was developed to handle very large or complex programs (see below: Object-oriented comparison).
At a low level, structured programs are often composed of simple, hierarchical program flow structures. These are sequence, selection, and repetition:
do..until. Often it is recommended that each loop should only have one entry point (and in the original structural programming, also only one exit point), and a few languages enforce this.
Some languages, such as Dijkstra's original Guarded Command Language, emphasise the unity of these structures with a syntax which completely encloses the structure, as in
if..fi.In others, such as C, this is not the case, which increases the risk of misunderstanding and incorrect modification.
A language is described as "block-structured" when it has a syntax for enclosing structures between bracketed keywords, such as an if-statement bracketed by
if..fi as in ALGOL 68, or a code section bracketed by
BEGIN..END, as in PL/I. However, a language is described as "comb-structured" when it has a syntax for enclosing structures within an ordered series of keywords. A "comb-structured" language has multiple structure keywords to define separate sections within a block, analogous to the multiple teeth or prongs in a comb separating sections of the comb. For example, in Ada, a block is a 4-pronged comb with keywords
END, and the if-statement in Ada is a 4-pronged comb with keywords
Structured programming is often (but not always) associated with a "top-down" approach to design.
It is possible to do structured programming in any programming language, though it is preferable to use something like a procedural programming language. Since about 1970 when structured programming began to gain popularity as a technique, most new procedural programming languages have included features to encourage structured programming (and sometimes have left out features that would make unstructured programming easy). Some of the better known structured programming languages are ALGOL, Pascal, PL/I and Ada.
The structured program theorem provides the theoretical basis of structured programming. It states that three ways of combining programs - sequencing, selection, and iteration - are sufficient to express any computable function. This observation did not originate with the structured programming movement; these structures are sufficient to describe the instruction cycle of a central processing unit, as well as the operation of a Turing machine. Therefore a processor is always executing a "structured program" in this sense, even if the instructions it reads from memory are not part of a structured program. However, authors usually credit the result to a 1966 paper by Böhm and Jacopini, possibly because Dijkstra cited this paper himself. The structured program theorem does not address how to write and analyze a usefully structured program. These issues were addressed during the late 1960s and early 1970s, with major contributions by Dijkstra, Robert W. Floyd, Tony Hoare, and David Gries.
Us converts waved this interesting bit of news under the noses of the unreconstructed assembly-language programmers who kept trotting forth twisty bits of logic and saying, 'I betcha can't structure this.' Neither the proof by Böhm and Jacopini nor our repeated successes at writing structured code brought them around one day sooner than they were ready to convince themselves.
In 1967 a letter from Dijkstra appeared in Communications of the ACM with the heading "Goto statement considered harmful." The letter, which cited the Böhm and Jacopini proof, called for the abolishment of unconstrained GOTO from high-level languages in the interest of improving code quality. This letter is usually cited as the beginning of the structured programming debate.
Although, as Plauger mentioned, many programmers unfamiliar with the theorem doubted its claims, the more significant dispute in the ensuing years was whether structured programming could actually improve software's clarity, quality, and development time enough to justify training programmers in it. Dijkstra claimed that limiting the number of structures would help to focus a programmer's thinking, and would simplify the task of ensuring the program's correctness by dividing analysis into manageable steps. In his 1969 Notes on Structured Programming, Dijkstra wrote:
When we now take the position that it is not only the programmer's task to produce a correct program but also to demonstrate its correctness in a convincing manner, then the above remarks have a profound influence on the programmer's activity: the object he has to produce must be usefully structured.
…In what follows it will become apparent that program correctness is not my only concern, program adaptability or manageability will be another… 1
Donald Knuth accepted the principle that programs must be written with provability in mind, but he disagreed (and still disagrees) with abolishing the GOTO statement. In his 1974 paper, "Structured Programming with Goto Statements", he gave examples where he believed that a direct jump leads to clearer and more efficient code without sacrificing provability. Knuth proposed a looser structural constraint: It should be possible to draw a program's flow chart with all forward branches on the left, all backward branches on the right, and no branches crossing each other. Many of those knowledgeable in compilers and graph theory have advocated allowing only reducible flow graphs.
Structured programming theorists gained a major ally in the 1970s after IBM researcher Harlan Mills applied his interpretation of structured programming theory to the development of an indexing system for the New York Times research file. The project was a great engineering success, and managers at other companies cited it in support of adopting structured programming, although Dijkstra criticized the ways that Mills's interpretation differed from the published work.
As late as 1987 it was still possible to raise the question of structured programming in a computer science journal. Frank Rubin did so in that year with a letter, "'GOTO considered harmful' considered harmful." Numerous objections followed, including a response from Dijkstra that sharply criticized both Rubin and the concessions other writers made when responding to him.
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. High-level programming languages that originally lacked programming structures, such as FORTRAN, COBOL, and BASIC, now have them.
Although there is almost never a reason to have multiple points of entry to a subprogram, multiple exits are often used to reflect that a subprogram may have no more work to do, or may have encountered circumstances that prevent it from continuing.
A typical example of a simple procedure would be reading data from a file and processing it: open file; while (reading not finished)