Editing Jackson structured programming (section)

== The basic method ==
JSP uses semi-formal steps to capture the existing structure of a program's inputs and outputs in the structure of the program itself.

The intent is to create programs which are easy to modify over their lifetime.  Jackson's major insight was that requirement changes are usually minor tweaks to the existing structures.  For a program constructed using JSP, the inputs, the outputs, and the internal structures of the program all match, so small changes to the inputs and outputs should translate into small changes to the program.

JSP structures programs in terms of four component types:

* fundamental operations
* sequences
* iterations
* selections

The method begins by describing a program's inputs in terms of the four fundamental component types.  It then goes on to describe the program's outputs in the same way.  Each input and output is modelled as a separate [[Data structure diagram|Data Structure Diagram]] (DSD).  To make JSP work for compute-intensive applications, such as digital signal processing (DSP) it is also necessary to draw algorithm structure diagrams, which focus on internal data structures rather than input and output ones.

The input and output structures are then unified or merged into a final program structure, known as a Program Structure Diagram (PSD).  This step may involve the addition of a small amount of high level control structure to marry up the inputs and outputs.  Some programs process all the input before doing any output, whilst others  read in one record, write one record and iterate.  Such approaches have to be captured in the PSD.

The PSD, which is language neutral, is then implemented in a programming language.  JSP is geared towards programming at the level of control structures, so the implemented designs use just primitive operations, sequences, iterations and selections.  JSP is not used to structure programs at the level of classes and objects, although it can helpfully structure [[control flow]] within a class's methods.

JSP uses a diagramming notation to describe the structure of inputs, outputs and programs, with diagram elements for each of the fundamental component types.

A simple operation is drawn as a box.

{{center|[[Image:Element Jackson.png|A box labeled 'A']]<br>An operation}}

A sequence of operations is represented by boxes connected with lines.  In the example below, A is a sequence consisting of operations B, C and D.

{{center|[[Image:JSP Sequence.png|A box labeled 'A' connected to three boxes below it labeled 'B', 'C' and 'D']]<br>A sequence}}

An iteration is again represented with joined boxes.  In addition the iterated operation has a star in the top right corner of its box.  In the example below, A is an iteration of zero or more invocations of operation B.

{{center|[[Image:JSP Iteration.png|A box labeled 'A' connected to a box labeled 'B' below it with a star in the top right corner]]<br>An iteration}}

Selection is similar to a sequence, but with a circle drawn in the top right hand corner of each optional operation.  In the example, A is a selection of one and only one of operations B, C or D.

{{center|[[Image:JSP Selection.png|A box labeled 'A' connected to three boxes below it labeled 'B', 'C' and 'D' each with a circle in the top right hand corner]]<br>A selection}}Note that it in the above diagrams, it is element A that is the sequence or iteration, not the elements B, C or D (which in the above diagrams are all elementary).  Jackson gives the 'Look-down rule' to determine what an element is, i.e. look at the elements below an element to find out what it is.