Structured Programming - Low-level Structure Programming

Low-level Structure Programming

At a low level, structured programs are often composed of simple, hierarchical program flow structures. These are sequence, selection, and repetition:

  • "Sequence" refers to an ordered execution of statements.
  • In "selection" one of a number of statements is executed depending on the state of the program. This is usually expressed with keywords such as if..then..else..endif, switch, or case. In some languages keywords cannot be written verbatim, but must be stropped.
  • In "repetition" a statement is executed until the program reaches a certain state, or operations have been applied to every element of a collection. This is usually expressed with keywords such as while, repeat, for or 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).

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 - or the curly braces {...} of C and many later languages.

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