FOR and WHILE loops in PLC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of a FOR loop incrementing a counter

What is the value of the counter variable after this FOR loop runs in a PLC structured text program?

PLC Programming

VAR
  counter : INT := 0;
  i : INT;
END_VAR

FOR i := 1 TO 5 DO
  counter := counter + 2;
END_FOR

A 0

B 5

C 10

D 15

💡 Hint

Think about how many times the loop runs and how much the counter increases each time.

The FOR loop runs from 1 to 5, so 5 times. Each time, counter increases by 2. So 2 * 5 = 10.

💻 Command Output

intermediate

WHILE loop execution count

How many times does the WHILE loop execute in this PLC structured text code?

PLC Programming

VAR
  count : INT := 0;
  x : INT := 3;
END_VAR

WHILE x < 7 DO
  count := count + 1;
  x := x + 2;
END_WHILE

A 2

B 3

C 4

D 1

💡 Hint

Check how x changes each loop and when the condition becomes false.

x starts at 3, increments by 2 each loop: 3, 5, 7. Loop runs while x < 7, so runs twice (x=3 and x=5).

📝 Syntax

advanced

Identify the syntax error in this FOR loop

Which option shows the correct syntax for a FOR loop in PLC structured text?

PLC Programming

FOR i := 1 TO 5
  counter := counter + i;
END_FOR

A

FOR i IN 1 TO 5 DO
  counter := counter + i;
END_FOR

B

FOR i = 1 TO 5 DO
  counter := counter + i;
END_FOR

C

FOR i := 1 TO 5 THEN
  counter := counter + i;
END_FOR

D

FOR i := 1 TO 5 DO
  counter := counter + i;
END_FOR

💡 Hint

Look for the required keyword after the FOR statement.

In PLC structured text, FOR loops require 'DO' after the range declaration.

🔧 Debug

advanced

Why does this WHILE loop never end?

What is the reason this WHILE loop runs forever in PLC structured text?

PLC Programming

VAR
  x : INT := 0;
END_VAR

WHILE x < 5 DO
  x := x - 1;
END_WHILE

A WHILE loops require a break statement to end

B x decreases each loop, so condition x < 5 is always true, causing infinite loop

C x is not initialized, causing runtime error

D x should be incremented, not decremented, to reach 5

💡 Hint

Think about how the variable changes and the loop condition.

x starts at 0 and decreases by 1 each loop, so it stays less than 5 forever, never ending the loop.

🚀 Application

expert

Calculate sum of even numbers using FOR loop

Which code correctly calculates the sum of even numbers from 2 to 10 inclusive in PLC structured text?

A

VAR
  sum : INT := 0;
  i : INT;
END_VAR
FOR i := 1 TO 10 DO
  IF i MOD 2 = 0 THEN
    sum := sum + i;
  END_IF
END_FOR

B

VAR
  sum : INT := 0;
  i : INT;
END_VAR
FOR i := 2 TO 10 BY 2 DO
  sum := sum + i;
END_FOR

C

VAR
  sum : INT := 0;
  i : INT := 2;
END_VAR
WHILE i <= 10 DO
  sum := sum + i;
  i := i + 2;
END_WHILE

D

VAR
  sum : INT := 0;
  i : INT;
END_VAR
FOR i := 2 TO 10 DO
  IF i MOD 2 = 1 THEN
    sum := sum + i;
  END_IF
END_FOR

💡 Hint

Check which code sums only even numbers correctly.

Option A loops from 1 to 10, adds i only if i is even (i MOD 2 = 0). This sums 2,4,6,8,10 correctly.

Practice

1. Which statement best describes a FOR loop in PLC programming?

easy

A. It repeats a set of instructions a fixed number of times.

B. It repeats instructions only while a condition is true.

C. It runs instructions once and then stops.

D. It waits for an input before running.

Solution

1. Step 1: Understand the purpose of a FOR loop

   A FOR loop repeats instructions a specific number of times, controlled by a counter.

2. Step 2: Compare with other loop types

   WHILE loops repeat based on a condition, not a fixed count. The other options describe different behaviors.

3. Final Answer:

   It repeats a set of instructions a fixed number of times. -> Option A

4. Quick Check:

   FOR loop = fixed repetitions [OK]

Hint: FOR loops run fixed times; WHILE loops depend on conditions [OK]

Common Mistakes:

• Confusing FOR loops with WHILE loops
• Thinking FOR loops run only once
• Believing FOR loops wait for input

2. Which of the following is the correct syntax for a FOR loop in PLC structured text?

easy

A. for i in range(1,6): ...

B. FOR (i = 1; i <= 5; i++) { ... }

C. FOR i := 1 TO 5 DO ... END_FOR;

D. WHILE i < 5 DO ... END_WHILE;

Solution

1. Step 1: Identify PLC structured text syntax

   PLC uses FOR i := 1 TO 5 DO ... END_FOR; for loops.

2. Step 2: Recognize other languages' syntax

   The syntax FOR (i = 1; i <= 5; i++) { ... } is C/JavaScript. The syntax for i in range(1,6): ... is Python. WHILE i < 5 DO ... END_WHILE; is a WHILE loop.

3. Final Answer:

   FOR i := 1 TO 5 DO ... END_FOR; -> Option C

4. Quick Check:

   PLC FOR loop uses := and TO [OK]

Hint: PLC FOR loops use ':=' and 'TO' keywords [OK]

Common Mistakes:

• Using C or Python syntax in PLC code
• Confusing FOR and WHILE syntax
• Omitting END_FOR statement

3. What will be the output of this PLC structured text code?

VAR i: INT := 1; sum: INT := 0; END_VAR
FOR i := 1 TO 3 DO
  sum := sum + i;
END_FOR;
// Output sum

medium

A. Syntax error

B. 6

C. 0

D. 3

Solution

1. Step 1: Trace the FOR loop iterations

   The loop runs with i = 1, 2, 3. sum starts at 0.

2. Step 2: Calculate sum after each iteration

   sum = 0+1=1, then 1+2=3, then 3+3=6.

3. Final Answer:

   6 -> Option B

4. Quick Check:

   1+2+3 = 6 [OK]

Hint: Add loop variable each iteration to sum [OK]

Common Mistakes:

• Adding only last value of i
• Starting sum at 1 instead of 0
• Confusing loop range boundaries

4. Identify the error in this PLC code snippet:

VAR i: INT := 0; count: INT := 0; END_VAR
WHILE i < 5 DO
  count := count + 1;
END_WHILE;

medium

A. Missing increment of i inside the WHILE loop

B. Incorrect variable declaration syntax

C. WHILE loop should be FOR loop

D. count variable should be BOOL, not INT

Solution

1. Step 1: Analyze the WHILE loop condition and body

   The loop condition is i < 5, but i never changes inside the loop.

2. Step 2: Identify infinite loop risk

   Without incrementing i, the condition stays true forever, causing an infinite loop.

3. Final Answer:

   Missing increment of i inside the WHILE loop -> Option A

4. Quick Check:

   Loop variable must change to avoid infinite loop [OK]

Hint: Always update loop variable inside WHILE loops [OK]

Common Mistakes:

• Forgetting to increment loop variable
• Confusing FOR and WHILE loops
• Incorrect variable types

5. You want to use a loop to turn on 4 output coils named Q1 to Q4 in sequence using PLC structured text. Which code snippet correctly uses a FOR loop to do this?

hard

A. FOR i := 1 TO 4 DO
Q[i-1] := TRUE;
END_FOR;

B. FOR i := 1 TO 4 DO
Q + i := TRUE;
END_FOR;

C. FOR i := 1 TO 4 DO
Qi := TRUE;
END_FOR;

D. FOR i := 1 TO 4 DO
Q[i] := TRUE;
END_FOR;

Solution

1. Step 1: Understand coil addressing in arrays

   Using Q[i] accesses coil Q1 to Q4 correctly if Q is an array of outputs.

2. Step 2: Check other options for syntax errors

   FOR i := 1 TO 4 DO Q + i := TRUE; END_FOR; uses invalid syntax Q + i. FOR i := 1 TO 4 DO Qi := TRUE; END_FOR; treats Q1, Q2 as fixed names, not variable. FOR i := 1 TO 4 DO Q[i-1] := TRUE; END_FOR; uses i-1 which may cause off-by-one error if indexing starts at 1.

3. Final Answer:

   FOR i := 1 TO 4 DO Q[i] := TRUE; END_FOR; -> Option D

4. Quick Check:

   Use array indexing for sequential coil control [OK]

Hint: Use array indexing Q[i] for sequential outputs [OK]

Common Mistakes:

• Using invalid syntax for coil names
• Off-by-one errors in indexing
• Treating coil names as strings