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:
- It can be directly translated into many high-level languages, and
- 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