Selection Control Structures

The Selection Control Structure

 You can use the selection control structure in pseudocode to illustrate a choice between two or more actions, depending on whether a condition is true or false. 

 

General Format:

 IF [condition]

THEN

          [true actions]

ELSE

          [false actions]

ENDIF

 

The condition in the IF statement is based on a comparison of two items, and is usually expressed with one of the following relational operators:

 

<   less than                         >       greater than

=   equal to                           <=     less than or equal to

>= greater than or equal to  < >    not equal to

 

When the condition following the IF is true only the actions following the THEN and before the ELSE are performed.

When the condition following the IF is false only the actions following the ELSE and before the ENDIF are performed.

 

There are a number of variations of the IF statement.

 


Simple Selection (simple IF statement)

 

Simple selection occurs when a choice is made between two alternate paths, depending on the result of a condition being true or false. 

 The structure is represented in pseudocode using the keywords IF, THEN, ELSE and ENDIF. 

 

Example:

 IF Gender = “M”

THEN

          Add 1 to Male-Count

ELSE

          Add 1 to Female-Count

ENDIF

 

Simple Selection with Null False Branch (null ELSE statement)

 It is used when a task is performed only when a particular condition is true. 

 If the condition is false, then no processing will take place and the IF statement will be bypassed.   

 

The ELSE clause is not used. 

 

Example:

 IF Age > 18

THEN

          Add 1 to Adult-Count

ENDIF

 

 

Combined Selection (combined IF statement)

 

A combined IF statement is one that contains multiple conditions, each connected with the logical operators AND or OR. 

If the connector AND is used to combine two conditions, then both conditions must be true for the combined condition to be true. 

If the connector OR is used to combine two conditions, then only one of the conditions needs to be true for the combined condition to be considered true.

 

Examples:

IF Gender = "M" THEN
    Add 1 to Male-Count
ELSE
    Add 1 to Female-Count
ENDIF

The NOT operator

The NOT operator can be used for the logical negation of a condition as follows:

 IF NOT (Record-Code = ’23’)

THEN

          Update customer record

ENDIF

 

Nested Selection (nested IF statement)

 A nested selection occurs when the word IF appears more than once within an IF statement. 

 A nested IF statements can be classified as linear or non-linear. 

 

Linear nested IF statements

The linear nested IF statement is used when a field is being tested for various values and a different action is to be taken for each value. 

This form of nested IF is called linear, because each ELSE immediately follows the IF condition to which it corresponds. 

         

Example:

Transaction Code

Action

 

A

Add a new customer record

 

C

Change customer record

 

D

Delete customer record

 

Any other code

Display error message

 

IF Tran-Code = “A”

THEN

          Add a new customer record

ELSE

          IF Tran-Code = “C”

          THEN

                   Change customer record

          ELSE

                   IF Tran-Code = “D”

                   THEN

                             Delete customer record

                   ELSE

                             Display error message

                   ENDIF

          ENDIF

ENDIF


Nested Selection (nested IF statement) – continued

 A non-linear nested IF occurs when a number of different conditions need to be satisfied before a particular action can occur.  It is termed non-linear because the ELSE statement may be separated from the IF statement with which it is paired. 

 

Example:

Condition 1

Condition 2

Action

 

Salaried Employee

 

Pay base salary

 

Hourly Employee

Hours <= 40

Calculate pay at hourly rate

 

Hourly Employee

Hours > 40

Calculate pay with overtime

 

IF Employee-Code = “H”

THEN

          IF Hours-Worked <= 40

          THEN

                   Calculate pay at hourly rate

          ELSE

                   Calculate pay with overtime

          ENDIF

ELSE

          Pay base salary

ENDIF

 

 

The Case Structure

 The CASE control structure in pseudocode is another way of expressing a linear nested IF statement. 

It is used in pseudocode for two reasons:

  1. It can be directly translated into many high-level languages, and
  2. It makes the pseudocode easier to write and understand. 

Using nested IF statements in pseudocode often look cumbersome and depend on correct structure and indentation for readability. 

 

General format:

CASE OF variable-name

          VALUE-1: statement block-1

          VALUE-2: statement block-2

          VALUE-3: statement block-3

          .

          .

          .

          VALUE-n: statement block-n

          VALUE-OTHER: statement block-other

ENDCASE

 

The value of the “variable-name” is compared to each VALUE-n from the top down. When there is an equal condition the corresponding “statement block” is executed. If there is no match the VALUE-OTHER “statement block” is executed.

 

Example:

Condition

Action

 

Age <   3

Ticket Price =   25% of standard fare

 

Age < 13

Ticket Price =   50% of standard fare

 

Age < 20

Ticket Price = 150% of standard fare

 

Age > 64

Ticket Price =   50% of standard fare

 

Other

Ticket Price = 100% of standard fare

 

 

CASE OF age

          <   3: Calculate Ticket Price =   25% of standard fare

          < 13: Calculate Ticket Price =   50% of standard fare

          < 20: Calculate Ticket Price = 150% of standard fare

          > 64: Calculate Ticket Price =   50% of standard fare

          VALUE_OTHER: Calculate Ticket Price = 100% of standard fare

ENDCASE