CASE statement in PLC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of a simple CASE statement

What will be the output of the following PLC CASE statement when the input value is 3?

PLC Programming

VAR
  inputValue : INT := 3;
  outputValue : STRING;
END_VAR

CASE inputValue OF
  1: outputValue := 'One';
  2: outputValue := 'Two';
  3: outputValue := 'Three';
  ELSE outputValue := 'Other';
END_CASE

// Output the value of outputValue

A "Three"

B "Other"

C "One"

D "Two"

💡 Hint

Look at the case that matches the inputValue 3.

The CASE statement matches the inputValue 3 and assigns 'Three' to outputValue.

💻 Command Output

intermediate

CASE statement with ELSE branch

What will be the output of the CASE statement if inputValue is 5?

PLC Programming

VAR
  inputValue : INT := 5;
  outputValue : STRING;
END_VAR

CASE inputValue OF
  1: outputValue := 'One';
  2: outputValue := 'Two';
  3: outputValue := 'Three';
  ELSE outputValue := 'Other';
END_CASE

// Output the value of outputValue

A "Other"

B "Five"

C "Three"

D "Error"

💡 Hint

Check what happens when no case matches and ELSE is present.

Since 5 does not match any case, the ELSE branch assigns 'Other'.

📝 Syntax

advanced

Identify the syntax error in CASE statement

Which option contains a syntax error in the CASE statement?

A

CASE inputValue OF
  1: outputValue := 'One';
  2: outputValue := 'Two';
  3: outputValue := 'Three';
  ELSE outputValue := 'Other';
END_CASE

B

CASE inputValue OF
  1: outputValue := 'One';
  2: outputValue := 'Two';
  ELSE outputValue := 'Other';
END_CASE

C

CASE inputValue OF
  1: outputValue := 'One';
  2: outputValue := 'Two';
END_CASE

D

CASE inputValue OF
  1 outputValue := 'One';
  2: outputValue := 'Two';
  ELSE outputValue := 'Other';
END_CASE

💡 Hint

Look for missing colon after case value.

Option D is missing a colon after '1', causing a syntax error.

🚀 Application

advanced

Using CASE statement for multiple outputs

Given the following CASE statement, what will be the value of outputValue after execution if inputValue is 4?

PLC Programming

VAR
  inputValue : INT := 4;
  outputValue : STRING;
END_VAR

CASE inputValue OF
  1, 2: outputValue := 'Low';
  3, 4: outputValue := 'Medium';
  5: outputValue := 'High';
  ELSE outputValue := 'Unknown';
END_CASE

// Output the value of outputValue

A "Low"

B "Medium"

C "High"

D "Unknown"

💡 Hint

Check which case group includes 4.

The CASE statement groups 3 and 4 together, so inputValue 4 sets outputValue to 'Medium'.

🧠 Conceptual

expert

Behavior of CASE statement without ELSE branch

What happens if a CASE statement in PLC code does not include an ELSE branch and the input value does not match any case?

A The program enters an infinite loop waiting for a valid case.

B The program throws a runtime error and stops execution.

C The program continues without changing the output variable, possibly leaving it uninitialized.

D The program automatically assigns a default value to the output variable.

💡 Hint

Think about what happens when no case matches and no ELSE is provided.

Without an ELSE branch, if no case matches, the CASE statement does nothing, so the output variable remains unchanged.

Practice

1. What is the main purpose of a CASE statement in PLC programming?

easy

A. To stop the program execution immediately

B. To repeat a set of instructions multiple times

C. To select one action from many based on a variable's value

D. To declare variables for use in the program

Solution

1. Step 1: Understand the role of CASE statement

   The CASE statement evaluates a variable and chooses one matching action from multiple options.

2. Step 2: Compare with other options

   Repeating instructions is done by loops, declaring variables is separate, and stopping execution is a different command.

3. Final Answer:

   To select one action from many based on a variable's value -> Option C

4. Quick Check:

   CASE statement = Select action by value [OK]

Hint: CASE chooses action based on value, like a traffic signal [OK]

Common Mistakes:

• Confusing CASE with loops
• Thinking CASE declares variables
• Assuming CASE stops program

2. Which of the following is the correct syntax to start a CASE statement in PLC structured text?

easy

A. CASE variable OF

B. SWITCH(variable) {

C. IF variable THEN

D. SELECT variable FROM

Solution

1. Step 1: Recall PLC structured text CASE syntax

   The correct syntax to start a CASE statement is CASE variable OF.

2. Step 2: Eliminate other options

   SWITCH(variable) { is JavaScript style, C is IF statement, D is invalid syntax.

3. Final Answer:

   CASE variable OF -> Option A

4. Quick Check:

   PLC CASE starts with 'CASE variable OF' [OK]

Hint: PLC CASE always uses 'CASE variable OF' to start [OK]

Common Mistakes:

• Using SWITCH instead of CASE
• Confusing IF with CASE
• Writing SELECT instead of CASE

3. Given the following PLC code snippet, what will be the value of output after execution?

CASE input OF
  1: output := 'One';
  2: output := 'Two';
  3: output := 'Three';
ELSE
  output := 'Other';
END_CASE;

If input is 2?

medium

A. 'One'

B. 'Two'

C. 'Three'

D. 'Other'

Solution

1. Step 1: Match input value with CASE options

   The input is 2, so the CASE matches the line 2: output := 'Two';.

2. Step 2: Determine output value

   Since input is 2, output is assigned the string 'Two'.

3. Final Answer:

   'Two' -> Option B

4. Quick Check:

   Input 2 matches case 2, output = 'Two' [OK]

Hint: Match input to CASE label to find output [OK]

Common Mistakes:

• Choosing ELSE output for matching case
• Confusing input 2 with 1 or 3
• Ignoring ELSE clause

4. Identify the error in this CASE statement snippet:

CASE status OF
  0: action := 'Stop';
  1: action := 'Start';
  2 action := 'Pause';
ELSE
  action := 'Unknown';
END_CASE;

medium

A. Missing colon after 2 in third case

B. ELSE clause is not allowed in CASE

C. END_CASE should be ENDCASE

D. Variable 'action' is not declared

Solution

1. Step 1: Check syntax of each CASE line

   The third case line is missing a colon after 2; it should be 2: action := 'Pause';.

2. Step 2: Verify other parts

   ELSE clause is valid, END_CASE is correct syntax, and variable declaration is not shown but not an error here.

3. Final Answer:

   Missing colon after 2 in third case -> Option A

4. Quick Check:

   Each case label needs a colon ':' [OK]

Hint: Each case label must end with ':' [OK]

Common Mistakes:

• Omitting colon after case value
• Thinking ELSE is invalid
• Confusing END_CASE syntax

5. You want to use a CASE statement to control a motor speed setting based on an integer input from 0 to 4, where 0 means 'Stop', 1 means 'Slow', 2 means 'Medium', 3 means 'Fast', and any other value means 'Error'. Which of the following CASE statements correctly implements this logic?

hard

A. CASE speed OF
0: motor_speed := 'Stop';
1: motor_speed := 'Slow';
2: motor_speed := 'Medium';
3: motor_speed := 'Fast';
END_CASE;

B. CASE speed OF
0: motor_speed := 'Stop';
1: motor_speed := 'Slow';
2: motor_speed := 'Medium';
3: motor_speed := 'Fast';
4: motor_speed := 'Error';
END_CASE;

C. CASE speed OF
0: motor_speed := 'Stop';
1: motor_speed := 'Slow';
2: motor_speed := 'Medium';
3: motor_speed := 'Fast';
4: motor_speed := 'Error';
ELSE motor_speed := 'Error';
END_CASE;

D. CASE speed OF
0: motor_speed := 'Stop';
1: motor_speed := 'Slow';
2: motor_speed := 'Medium';
3: motor_speed := 'Fast';
ELSE motor_speed := 'Error';
END_CASE;

Solution

1. Step 1: Check the required mapping

   Values 0 to 3 have specific motor speeds; any other value should set 'Error'.

2. Step 2: Analyze each option

   CASE speed OF 0: motor_speed := 'Stop'; 1: motor_speed := 'Slow'; 2: motor_speed := 'Medium'; 3: motor_speed := 'Fast'; ELSE motor_speed := 'Error'; END_CASE; uses ELSE for all other values, correctly handling unexpected inputs. CASE speed OF 0: motor_speed := 'Stop'; 1: motor_speed := 'Slow'; 2: motor_speed := 'Medium'; 3: motor_speed := 'Fast'; 4: motor_speed := 'Error'; END_CASE; treats 4 as 'Error' but misses ELSE for other values. CASE speed OF 0: motor_speed := 'Stop'; 1: motor_speed := 'Slow'; 2: motor_speed := 'Medium'; 3: motor_speed := 'Fast'; 4: motor_speed := 'Error'; ELSE motor_speed := 'Error'; END_CASE; incorrectly uses a colon after ELSE ('ELSE:'), which is invalid syntax. CASE speed OF 0: motor_speed := 'Stop'; 1: motor_speed := 'Slow'; 2: motor_speed := 'Medium'; 3: motor_speed := 'Fast'; END_CASE; lacks handling for values outside 0-3.

3. Final Answer:

   CASE speed OF 0: motor_speed := 'Stop'; 1: motor_speed := 'Slow'; 2: motor_speed := 'Medium'; 3: motor_speed := 'Fast'; ELSE motor_speed := 'Error'; END_CASE -> Option D

4. Quick Check:

   Use ELSE for unexpected values in CASE [OK]

Hint: Use ELSE to catch all unexpected values in CASE [OK]

Common Mistakes:

• Not using ELSE for unexpected values
• Assigning error to a specific case instead of ELSE
• Missing cases for valid inputs