Non-structured programming is the historically earliest programming paradigm capable of creating Turing-complete algorithms. It has been followed historically by procedural programming and then object-oriented programming, both of them considered as structured programming.
Unstructured programming has been heavily criticized for producing hardly-readable ("spaghetti") code and is sometimes considered a bad approach for creating major projects, but had been praised for the freedom it offers to programmers and has been compared to how Mozart wrote music.
There are both high- and low-level programming languages that use non-structured programming. These include early versions of BASIC (such as MSX BASIC and GW-BASIC), FORTRAN, JOSS, FOCAL, MUMPS, TELCOMP, COBOL, machine-level code, early assembler systems (without procedural metaoperators), assembler debuggers and some scripting languages such as MS-DOS batch file language.
Features and typical concepts
A program in a non-structured language usually consists of sequentially ordered commands, or statements, usually one in each line. The lines are usually numbered or may have labels: this allows the flow of execution to jump to any line in the program.
Non-structured programming introduces basic control flow concepts such as loops, branches and jumps. Although there is no concept of procedures in the non-structured paradigm, subroutines are allowed. Unlike a procedure, a subroutine may have several entry and exit points, and a direct jump into or out of subroutine is (theoretically) allowed. This flexibility allows realization of coroutines.
There is no concept of local variables in non-structured programming (although for assembly programs, general purpose registers may serve the same purpose after saving on entry), but labels and variables can have a limited area of effect (for example, a group of lines). This means there is no (automatic) context refresh when calling a subroutine, so all variables might retain their values from the previous call. This makes general recursion difficult, but some cases of recursion--where no subroutine state values are needed after the recursive call--are possible if variables dedicated to the recursive subroutine are explicitly cleared (or re-initialized to their original value) on entry to the subroutine. The depth of nesting also may be limited to one or two levels.
Non-structured languages allow only basic data types, such as numbers, strings and arrays (numbered sets of variables of the same type). The introduction of arrays into non-structured languages was a notable step forward, making stream data processing possible despite the lack of structured data types.
- William W. Cobern. A positive albeit ambiguous case for BASIC programming in secondary science teaching.
- Cobb, Gary W. (1978). "A measurement of structure for unstructured programming languages". ACM SIGSOFT Software Engineering Notes 3 (5): 140–147. doi:10.1145/953579.811114. ISSN 0163-5948.
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- BPStruct - A tool to structure concurrent systems (programs, process models)