FOR and WHILE loops in PLC Programming - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a FOR loop that counts from 1 to 5.

PLC Programming

FOR i := 1 TO [1] DO
    // loop body
END_FOR;

Drag options to blanks, or click blank then click option'

A5

B0

C10

D6

Attempts:

3 left

💡 Hint

Common Mistakes

Using 6 as the upper limit causes the loop to run one extra time.

Using 0 as the upper limit makes the loop not run at all.

The FOR loop counts from 1 to 5, so the upper limit must be 5.

2fill in blank

medium

Complete the code to create a WHILE loop that runs while the variable count is less than 10.

PLC Programming

count := 0;
WHILE count [1] 10 DO
    count := count + 1;
END_WHILE;

Drag options to blanks, or click blank then click option'

A<=

B<

C>=

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>=' causes the loop to never run.

Using '<=' makes the loop run one extra time.

The loop should run while count is less than 10, so the operator is '<'.

3fill in blank

hard

Fix the error in the FOR loop that counts down from 5 to 1.

PLC Programming

FOR i := 5 TO [1] BY -1 DO
    // loop body
END_FOR;

Drag options to blanks, or click blank then click option'

A1

B6

C0

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 causes the loop to run one extra time.

Using 6 causes the loop to not run correctly.

To count down from 5 to 1, the loop must end at 1 with a step of -1.

4fill in blank

hard

Fill both blanks to create a FOR loop that sums numbers from 1 to 10.

PLC Programming

sum := 0;
FOR i := [1] TO [2] DO
    sum := sum + i;
END_FOR;

Drag options to blanks, or click blank then click option'

A1

B5

C10

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Starting at 0 includes an extra number.

Ending at 5 sums only half the range.

The loop starts at 1 and ends at 10 to sum all numbers in that range.

5fill in blank

hard

Fill all three blanks to create a WHILE loop that counts down from 10 to 1.

PLC Programming

count := [1];
WHILE count [2] 0 DO
    // do something
    count := count [3] 1;
END_WHILE;

Drag options to blanks, or click blank then click option'

A10

B>

C-

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' makes the loop never run.

Using '+' increases count and causes an infinite loop.

The loop starts at 10, runs while count is greater than 0, and decreases count by 1 each time.

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