11.2 Constructs: IF Statements with ELSE and Nested IFs

Objective

This section focuses on using pseudo-code to represent conditional logic using 'IF' statements, including the 'ELSE' clause and nested 'IF' statements. Understanding these constructs is fundamental for creating programs that can make decisions and execute different blocks of code based on specific conditions.

IF Statement with ELSE Clause

An `IF` statement allows a program to execute a block of code only if a certain condition is true. The `ELSE` clause provides an alternative block of code to execute if the condition in the `IF` statement is false.

Pseudo-code:

    IF condition THEN
        // Code to execute if the condition is true
    ELSE
        // Code to execute if the condition is false
    ENDIF
    

Example: Determine if a number is positive, negative, or zero.

Step	Pseudo-code
1. Get the number	Let number = input_number
2. Check if the number is greater than 0	IF number > 0 THEN
3. If true, display "Positive"	Display "Positive"
4. Else, check if the number is less than 0	ELSE IF number < 0 THEN
5. If true, display "Negative"	Display "Negative"
6. Else, display "Zero"	ELSE
7. Display "Zero"	Display "Zero"
8. End the IF statement	ENDIF

Nested IF Statements

Nested `IF` statements allow you to create more complex decision-making structures. This involves placing an `IF` statement inside another `IF` statement. This is useful when you need to evaluate multiple conditions.

Pseudo-code:

    IF condition1 THEN
        // Code to execute if condition1 is true
        IF condition2 THEN
            // Code to execute if condition1 and condition2 are both true
        ELSE
            // Code to execute if condition1 is true but condition2 is false
        ENDIF
    ELSE
        // Code to execute if condition1 is false
    ENDIF
    

Example: Determine the grade based on a student's score.

Step	Pseudo-code
1. Get the student's score	Let score = input_score
2. Check if the score is greater than or equal to 90	IF score >= 90 THEN
3. If true, display "Grade: A"	Display "Grade: A"
4. Else, check if the score is greater than or equal to 80	ELSE IF score >= 80 THEN
5. If true, display "Grade: B"	Display "Grade: B"
6. Else, check if the score is greater than or equal to 70	ELSE IF score >= 70 THEN
7. If true, display "Grade: C"	Display "Grade: C"
8. Else, display "Grade: D"	ELSE
9. Display "Grade: D"	Display "Grade: D"
10. End the IF statement	ENDIF

Key Considerations

• Ensure that the conditions within the `IF` statements are boolean expressions (i.e., they evaluate to either true or false).
• Use appropriate comparison operators (e.g., =, >, <, >=, <=, !=).
• The `ELSE` clause is optional.
• When using nested `IF` statements, ensure that the conditions in each `IF` statement are independent.