1. Sequence
The sequence control structure is
the straightforward execution of one processing step after another. In Pseudocode
we represent this as a sequence of Pseudocode statements
Statement a
Statement b
Statement c
The sequence control structure can
be used to represent the first four basic computer operations listed
previously: to receive information, put out information, perform arithmetic and
assign values. For example, a typical sequence of statements in an algorithm
might read:
Add 1 to page-count
Print heading line
Set linecount to zero
Read customer record
These instructions illustrate the
sequence control structure as a straightforward list of steps written one after
the other, in a top-to-bottom fashion. Each instruction will be executed in the
order in which it appears.
2. Selections
The selection control structure is
the presentation of a condition and the choice between two actions, the choice
depending on whether the condition is true or false. This constructs represent
the decision-making abilities of the computer and is used to illustrate the
fifth basic computer operation, namely to compare two variables and select one
of two alternate actions.
In Pseudocode, selection is
represented by the keywords IF, THEN, ELSE and ENDIF
IF condition p is true THEN
statements in true case
ELSE
statements false case
ENDIF
If condition P is true then the
statement or statements in the true case will be executed and the statements in
the false case will be skipped. Otherwise (the Else statement) the statements
in the true case will be skipped and statements in the false case will be executed.
In either case, control then passes to the next processing step after the
delimiter Endif. A typical Pseudocode example might read:
IF student is part-time THEN
add 1 to part-time count
ELSE
add 1 to full-time count
ENDIF
A variation of the selection control
structure is the null Else structure, which is used when a task is performed
only if a particular condition is true. The null Else construct is written in Pseudocode as:
IF condition is true THEN
statement in true case
ENDIF
Note that the keyword Else is
omitted. This construct tests the condition in the If clause and if that is
found to be true, performs the statement or statements listed in the Then
clause. However, if the initial condition is found to be false, no action will
be taken and processing will proceed to the next statement after the ENDIF.
3. Repetition
The repetition control structure can be defined as the
presentation of a set of instructions to be performed repeatedly, as long as a
condition is true. The basic idea of repetitive code is that a block of
statements is executed again and again, until a terminating condition occurs. This
construct represents the sixth basic computer operation, namely to repeat a
group of actions. It is written in Pseudocode as:
DO WHILE condition p is true
statement block
END DO
The DO
WHILE loop is a leading decision loop; that is, the condition is tested before
any statements are executed. If the condition in the DO WHILE statement is
found to be true, the block of statements following the statement is executed
once. The delimiter END DO then triggers a return of control to the retesting
of the condition. If the condition is still true, the statements are repeated,
and so the repetition process continues until the condition is found to be false.
Control then passes to the statement, which follows the END DO statement. It is
imperative that at least one statement within the statement block can alter the
condition and eventually render it false, because otherwise the logic may
result in an endless loop.
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