Flowcharting

• Basic symbols
• Three Basic Control Structures
  • Sequence
  • Selection
  • Repetition
• Exercise in Flowchart implementation
  • Problem Statement
  • Defining Diagram
  • General Flowchart for file processing
  • Solution Development for this problem
• Flowchart vs. Pseudocode - Tradeoffs

Flowcharting is an alternative method for documenting the logic of a solution algorithm, compared to pseudocode. Flowcharting uses graphics symbols rather than text to describe the logic of the solution. It has advantages and disadvantages relative to pseudo-code.

The Basic Flowcharting Symbols and their usage are:

	Used to start and stop a code block. The text it contains is one of : 1. the name of the module it starts 2. the keyword 'End', which is used only once in a program 3. the keyword 'Return', used at the end of modules to transfer control back to the calling module NOTE: The text to be used here is NOT what you see in the Robertson textbook.
	Used to represent a process or a single step in an algorithm. The text it contains is the name or the description of a simple process. Use this only for simple processes.
	Used to represent a pre-defined process. The text it contains is always a descriptive name of the process it represents. The process that it represents must be described separately.
	The decision symbol represents a true / false question that controls the flow of the program logic. It should contain the text of the condition that it represents. The top vertex of the decision symbol represents its entry point. There should only be two lines exiting the symbol. These represent the logic paths to be followed for the true or false states as appropriate. Always used in selection or repetition structures described below.
	Flowlines connect the other symbols to show the flow of logic control from one part of the solution to another. The arrowhead is only necessary when lines meet or the flow of control is NOT from top to bottom, or left to right

The Three Basic Control Structures

There are some standard ways of connecting the symbols to represent the three basic structures. Notice that each of the three basic structures has a single entry flow and a single exit flow.

Sequence Steps that execute in sequence are represented by symbols that follow each other top to bottom or left to right. Top to bottom is the standard. Any one of the processes could be replaced by another structure (for example, a single process "box" could be replaced by a sequence of two process "boxes").

Selection Once the condition is evaluated, the control flows into one of two paths. Once the conditional execution is finished, the flows rejoin before leaving the structure. Once again the single entry, single exit nature of a structure is evident. Either process could be replaced with any structure (sequence, selection, or repetition) or it could be omitted (for example the process on the "NO" side might not exist). As an alternative, the "Question" could be replaced with aBoolean expression and the branches labelled "TRUE" and "FALSE".

Repetition Either the processing repeats or the control leaves the structure. (Again, the process could be replace another structure). Notice that the return line joins the entry line before the question. Never try to insert processing in the entry line between the join and the question.

Exercise in Flowchart Implementation

Problem Statement

The Student Information consists of records which contain:

• ID number (int)
• name
• age (int)
• gender (char)
• program (char[20])
• academic level (int)
• attendance pattern (char[3]),
     i.e. full time (F/T) or part time (P/T)

The report has the heading 'PART TIME FEMALE STUDENTS', and includes the student's ID number, name, program, and academic level for each part-time, female student.

IPO Diagram

GeneralFlowchart for file processing

This is a problem that involves processing a file.
The basic file processing algorithm includes these steps:

• Open the file
• Read data from the file into the record variable
• Process the record
• Check to see if this was the last record
• Close the file

We can start with the following flowchart to describe the basic file processing steps:

Solution Development for this problem

This diagram only covers our basic file processing steps. Obviously, there are still details to be added to the general flowchart, based on the specific requirements of this problem.

We need to add the processing steps concerning our report and the processing of the individual students.

There are now two selections as well as the file processing loop, and the selection of the Part-time female students for the report has been looked after.

Flowchart vs. Pseudocode - Tradeoffs

Pseudocode:

• is easy to develop and maintain. The most common tool is a simple text editor.
• encourages standard control sequences. Only a very few sequences are possible, because the terminology is not defined for general "unstructured" sequences.
• mimics programming code sequences, which makes the conversion into program code easier.
• permits a lot of activity to represented in a relatively small space (this is both an advantage and disadvantage).
• can be almost impossible to understand or analyze if not properly indented and aligned.

Flowcharts:

• provides a visual representation which is often easier to understand.
• forces the designer to simplify in order to "fit" the image on a page.
• can be easy to code from, provided only recognizable, standardized flow sequences are drawn.
• encourages the designer to work linearly "depth-first"; that is, to follow a single path from the beginning until the lowest level is reached, before going back "up" to follow alternate activity paths to their lowest levels (this is a bad thing).

A good flowchart presents a clear picture of the activities to be performed, using recognizable flow controls; but when "a flowchart goes bad"... it can go really, really bad! Compare the earlier (relatively "good") flowchart for processing "part-time, female students" with the following example:

This logic was probably developed, with a narrow focus: "Once I've done this, what do I need to do next?" approach, instead of trying to identify significant flow control structures and filling in the details later.

The resulting logic has two terminal points (invalidating a basic structured programming concept) and contains a "looping" structure which can not be implemented with any standard program sequence control statements.

(One more time... with feeling... The preceding flowchart is an example of HOW NOT TO DESIGN AN ALGORITHM!

	Advantages	Disadvantages
Pseudo-code	Quick Uses Text Processor Spatially dense	Spatially dense Difficult to understand complex logic Harder to see 'Big Picture' in complex solutions    especially if alignment is poor and/or    if "nesting" of structures is deep
Flowcharts	A picture is worth a 1000 words.... Intuitive Symbolic Good 'Big Picture' visibility    (if standard structures are used    and the logic can be represented in a single page)	Time consuming Space consuming Requires template or special software Permits development of logic sequences    which can NOT be coded    using valid structured code