12.2.2 Parameters, Interfaces and Control in Structure Charts

12.2.2 Parameters, Interfaces and Control in Structure Charts

A structure chart does more than show how a program is divided into modules. It can also show the information passed between those modules and indicate when a module is selected or repeated.

This section adds parameter and control notation to the hierarchical designs introduced in the previous section. The resulting chart provides a more precise plan for the procedures and functions that may later appear in pseudocode.

From Module Hierarchy to Module Communication

Decomposition identifies the modules needed by a solution. The next design question is how those modules obtain the information required to perform their tasks.

Consider a module called CheckLoanEligibility. The name tells us its responsibility, but it does not tell us:

• which member is requesting the loan;
• which item has been selected;
• how many days the item is needed;
• what decision the module produces.

Parameter labels and arrows make these relationships visible.

Parameters in a Structure Chart

Parameters allow a module to perform a general task using values provided by another part of the program. For example:

CheckLoanEligibility
    receives: MemberID, ItemCode, LoanDays
    produces: LoanAllowed, RejectionReason

The eligibility module does not need to collect the values itself. Its responsibility is to use the supplied values to make a decision.

Parameters and arguments

In a structure chart, a label normally identifies a parameter in the planned interface. When pseudocode calls the module, the actual value or expression supplied is an argument.

CheckLoanEligibility(CurrentMember, SelectedItem, RequestedDays)

Here, the interface may define parameters named MemberID, ItemCode and LoanDays, while CurrentMember, SelectedItem and RequestedDays are the arguments used in one particular call.

Interfaces Between Modules

A well-designed interface makes a module easier to understand because its required inputs and produced results are explicit.

An interface should expose the values needed for communication without exposing every local variable used inside the module.

Following Parameter Directions

The arrow direction shows whether a value is supplied to a lower-level module or returned to the higher-level module.

Data Values and Control Information

Most parameters represent data that a module processes, such as an identifier, quantity, date or calculated result. A structure chart can also show a control value that influences which module is called or whether work is repeated.

In some structure-chart conventions, an arrow with a filled circular end identifies a control flag. A question may provide its own symbol key, which should always be followed.

Showing Selection

Selection is needed when a condition determines which one of several module calls should occur.

In the equipment-loan system:

IF LoanAllowed = TRUE
    call CreateLoan
ELSE
    call DisplayRejection
ENDIF

A structure chart can represent this choice using a diamond or selection marker connected to the alternative modules. The condition should be labelled clearly.

Showing Repetition

Repetition notation indicates that one module or a group of modules is called more than once while, or until, a condition is satisfied.

An equipment desk may process requests until its queue is empty:

WHILE MoreRequests = TRUE
    ReadNextRequest
    ProcessToolLoan
    CheckForMoreRequests
ENDWHILE

A curved or semicircular arrow may be drawn over the repeated modules. Its label identifies the repetition condition.

Values That Are Updated by a Module

Sometimes a module receives a value, changes it and makes the updated value available to its parent. A double-headed arrow can be used to represent this two-way movement.

Suppose the system stores how many units of a tool remain available:

ReserveItem receives AvailableUnits
ReserveItem subtracts 1
ReserveItem returns the updated AvailableUnits

Worked Example: Community Equipment Loan

A community workshop lends tools to registered members. A loan request contains a member identifier, an item code and the number of requested days.

The system must:

• read the request;
• check the member, item and requested duration;
• create a loan when the request is allowed;
• show a rejection reason when it is not allowed;
• update the number of available units after a successful reservation.

Modules and interfaces

Control design

Text representation of the design

ProcessLoanQueue
└── WHILE MoreRequests
    ├── ReadLoanRequest
    │   └── returns MemberID, ItemCode, LoanDays
    ├── CheckLoanEligibility
    │   ├── receives MemberID, ItemCode, LoanDays
    │   └── returns LoanAllowed, RejectionReason
    └── IF LoanAllowed
        ├── ReserveItem
        │   └── updates AvailableUnits
        ├── CreateLoan
        │   └── returns LoanReference
        └── DisplayConfirmation
       ELSE
        └── DisplayRejection

How to Read an Annotated Structure Chart

Read a chart systematically rather than trying to understand every symbol at once.

1. Identify the parent module. Determine which broad task is coordinating the lower modules.
2. Identify its children. State the responsibilities of the called modules.
3. Trace downward parameters. Find the values supplied to each child.
4. Trace upward parameters. Find results returned to the parent.
5. Identify control flags. Determine how Boolean results influence the design.
6. Interpret selection and repetition. State which modules are conditional or repeated.
7. Locate updated values. Explain any double-headed parameter arrows.

Adding Interfaces and Control to a Chart

Begin with the module hierarchy. Then annotate it using the information required by each module.

1. List each module's required inputs. Ask what the module must know before it can perform its task.
2. List each result. Ask what information another module needs after the task is complete.
3. Draw and label parameter arrows. Show supplied values and returned results clearly.
4. Identify Boolean decisions. Decide whether a returned flag controls a selection or repetition.
5. Add selection notation. Label the condition and its alternative branches.
6. Add repetition notation. Identify the repeated group and its continuation or stopping condition.
7. Mark updated parameters. Use two-way notation where a supplied value is changed and returned.
8. Check the chart. Ensure that every parameter is created before it is passed and used by a module that needs it.

Interactive: Structure Chart Interface Explorer

Switch between data passing, selection, repetition and an exam-style challenge. Select any module to inspect its inputs, outputs and control role.

Common Mistakes and Misconceptions

Practice

Review