Program Control Structures

Program Control Structures

program control structures are blocks of statements that determine how statements are to be executed, in structured programming languages, there are three control structures namely; sequence, selection and iteration (looping)

​Sequence

In this structure, the computer reads instructions from a program file starting from the first top line and proceeding downwards one -by -one to the end. This is called sequential programming execution i.e.

​Selection

In selection control, execution of statements depends on a condition which is either a true or false
There are four types of selection controls. Namely:

• IF.....THEN
• IF.....THEN.....ELSE
• NESTED IF
• CASE SECTION

IF....THEN
The IF....THEN selection is used if only one option is available. All other options are ignored
Format:
IF <condition> THEN
    Statements;
ENDIF

IF....THEN....ELSE
This type of selection is suitable where there are two available options
General Format
IF <condition> THEN
    Statements;
ELSE
    Statements;
EndIF

Nested IF Selection
This type of selection is used where two or more options have to be considered to make a selection.
General Format
IF <condition> THEN
            statements
    ELSE
    IF <condition> THEN
            statements
    ELSE
        IF<condition> THEN
            Statements
    ELSE
            Statements
        ENDIF
    ENDIF
ENDIF

CASE Construct
This is an alternative to the Nested IF especially where there are several options to choose from. Its preferred because it reduces the many lines of code. However, the boolean expression for the case selection can only be expressed using integers and alphabetic characters only. The General format should be
CASE integer OF or
CASE Char OF
Example:
Case x of
label 1: statement 1
label 2: statement 2
label 3: statement 3
.
.
.
label n: statement n-1
Else
statement
Endcase

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​Iteration (looping)

Iteration is also referred to as looping or repetition.  Its designed to execute the same block of code again and again until a certain condition is fulfilled.
The three main looping controls are:-

• The WHILE Loop
• The REPEAT....UNTIL Loop
• The FOR Loop

The WHILE Loop
The "WHILE" loop is used if a condition has to be met before the statements within the loop are executed.
General Format
WHILE condition DO
    statement
ENDWHILE

The REPEAT....UNTIL loop
The REPEAT....UNTIL loop allows the statements within it to be executed at least once since the condition is tested at the end of the loop.
General Format
REPEAT
statements
UNTIL <condition>

The FOR Loop
The FOR loop is used in circumstances where the execution of the chosen statements has to be repeated a predetermined number of times.
General Format
//pseudocode for 'FOR" loop that counts from the lower limit
FOR loop variable = lower limit to upper limit DO
statements
ENDFOR

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