Nassi-Shneiderman diagram, flowchart and program flow chart

In short: Structure charts (Nassi-Shneiderman) and program flow charts present algorithms graphically; they are common in training and IHK exams.

In more detail: Before writing code it helps to draw the flow of an algorithm. Two classic notations are the Nassi-Shneiderman diagram (boxes nested in each other) and the program flow chart with symbols and arrows.

In Depth

The basic building blocks

Every algorithm consists of three structures:

• Sequence: statements one after another.
• Selection (branch): if condition then … else ….
• Repetition (loop): with test at the head or foot (while, until, for). A structure chart knows only these three boxes, so jumps (goto) are not possible and the flow stays clear.

Nassi-Shneiderman diagram

+-------------------------------+
| read number                   |
+-------------------------------+
| number > 0 ?                  |
|   yes         |      no       |
| print "pos."  | print "not    |
|               |   positive"   |
+-------------------------------+

Repetitions appear as an indented box, for example “while i < 10: …”. Advantage: unambiguous, each structure has one entry and one exit.

Program flow chart

Symbols according to DIN 66001: oval for start/end, rectangle for processing, diamond for decision (yes/no branches), parallelogram for input/output, arrows for the flow. The flow chart is more flexible but can become confusing (“spaghetti”) if many jumps arise.

Practice and exam

Typical tasks: maximum of numbers, sum 1 to n, prime test, sorting, factorial. Approach: clarify inputs and outputs, describe the flow in words, then draw and play through with test values (desk check with a table of variable values). Check edge cases such as 0, empty input, negative numbers. Pseudocode helps too.

See also: Modelling (UML, EPC, BPMN), Pseudocode, Loops