Conditional statements (IF-THEN) in CNC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of a simple IF-THEN block

What will be the output of this CNC program snippet if the variable #100 is set to 5?

CNC Programming

IF [#100 GT 3] THEN
  #101 = 1
ENDIF
M30

A Runtime error due to undefined variable

B Program ends with #101 = 0

C Syntax error due to missing ELSE

D Program ends with #101 = 1

💡 Hint

Check if the condition #100 GT 3 is true when #100 is 5.

Since #100 is 5, which is greater than 3, the condition is true, so #101 is set to 1.

💻 Command Output

intermediate

Behavior of nested IF-THEN statements

Given #200 = 10 and #201 = 5, what is the value of #202 after running this code?

CNC Programming

IF [#200 GT 5] THEN
  IF [#201 LT 10] THEN
    #202 = 100
  ENDIF
ENDIF
M30

A #202 = 0

B #202 = 100

C Syntax error due to nested IF

D #202 is undefined

💡 Hint

Both conditions must be true for #202 to be set.

Since #200 is 10 (greater than 5) and #201 is 5 (less than 10), both conditions are true, so #202 is set to 100.

📝 Syntax

advanced

Identify the syntax error in IF-THEN usage

Which option contains a syntax error in the IF-THEN statement?

A IF [#10 EQ 5] #20 = 1 ENDIF

B IF #10 EQ 5 THEN #20 = 1 ENDIF

C IF [#10 EQ 5] THEN #20 = 1 ENDIF

D IF [#10 EQ 5] THEN #20 = 1 ELSE #20 = 0 ENDIF

💡 Hint

Check if the THEN keyword is present after the condition.

Option A is missing the THEN keyword after the condition, causing a syntax error.

🚀 Application

advanced

Using IF-THEN to control spindle speed

You want to set spindle speed to 1000 if the tool number (#1) is 5, otherwise set it to 500. Which code snippet achieves this?

A IF [#1 EQ 5] THEN S1000 ELSE S500 ENDIF

B

IF [#1 EQ 5] THEN S1000 ENDIF
S500

C

IF [#1 EQ 5] THEN S1000
ELSE S500
ENDIF

D IF #1 EQ 5 THEN S1000 ELSE S500 ENDIF

💡 Hint

Check the correct placement of ELSE and ENDIF keywords.

Option C correctly uses THEN, ELSE, and ENDIF to set spindle speed conditionally.

🧠 Conceptual

expert

Understanding IF-THEN evaluation order

Consider this code snippet: IF [#10 GT 0] THEN IF [#20 LT 5] THEN #30 = 1 ELSE #30 = 2 ENDIF ELSE #30 = 3 ENDIF If #10 = 0 and #20 = 3, what is the value of #30 after execution?

A #30 = 3

B #30 = 1

C #30 = 2

D #30 is undefined

💡 Hint

The outer IF condition controls whether the inner IF runs.

Since #10 is 0 (not greater than 0), the outer IF condition is false, so the ELSE block sets #30 to 3.

Practice

1. What does an IF-THEN statement do in CNC programming?

easy

A. It repeats a set of commands multiple times.

B. It stops the machine immediately.

C. It defines the speed of the machine.

D. It makes the program choose actions based on conditions.

Solution

1. Step 1: Understand the purpose of IF-THEN

   IF-THEN checks a condition and runs commands only if the condition is true.

2. Step 2: Compare with other options

   Repeating commands is done by loops, speed is set by other commands, and stopping is a different command.

3. Final Answer:

   It makes the program choose actions based on conditions. -> Option D

4. Quick Check:

   IF-THEN controls flow based on conditions [OK]

Hint: IF-THEN means "if this, then do that" [OK]

Common Mistakes:

• Confusing IF-THEN with loops
• Thinking IF-THEN sets machine speed
• Assuming IF-THEN stops the machine

2. Which of the following is the correct syntax for an IF statement in CNC programming?

easy

A. WHEN condition DO
commands
END

B. IF (condition) {
commands
}

C. IF [condition] THEN
commands
ENDIF

D. if condition:
commands

Solution

1. Step 1: Identify CNC IF syntax

   CNC uses IF [condition] THEN ... ENDIF structure.

2. Step 2: Check other options

   Options B and C use programming languages like C or Python syntax, A is not standard CNC syntax.

3. Final Answer:

   IF [condition] THEN commands ENDIF -> Option C

4. Quick Check:

   CNC IF syntax uses THEN and ENDIF [OK]

Hint: Look for THEN and ENDIF keywords in CNC IF syntax [OK]

Common Mistakes:

• Using parentheses or braces like other languages
• Omitting THEN or ENDIF keywords
• Confusing IF with WHEN or DO

3. What will be the output of this CNC code snippet?

IF #100 GT 50 THEN
  #101 = 1
ELSE
  #101 = 0
ENDIF
PRINT #101

Assuming variable #100 is 60.

medium

A. 0

B. 1

C. Syntax Error

D. No output

Solution

1. Step 1: Evaluate the condition #100 GT 50

   #100 is 60, which is greater than 50, so condition is true.

2. Step 2: Determine the assigned value to #101

   Since condition is true, #101 is set to 1, then printed.

3. Final Answer:

   1 -> Option B

4. Quick Check:

   60 > 50 is true, so #101 = 1 [OK]

Hint: Check condition truth to find which branch runs [OK]

Common Mistakes:

• Confusing GT (greater than) with LT (less than)
• Ignoring ELSE branch
• Assuming syntax error due to ELSE

4. Identify the error in this CNC IF statement:

IF #200 EQ 10
  G01 X100 Y100
ENDIF

medium

A. Missing THEN keyword after condition

B. Incorrect variable name #200

C. Missing ENDIF keyword

D. G01 command is invalid inside IF

Solution

1. Step 1: Check IF statement syntax

   CNC IF requires THEN after the condition, missing here.

2. Step 2: Verify other parts

   Variable name is valid, ENDIF is present, G01 is valid command inside IF.

3. Final Answer:

   Missing THEN keyword after condition -> Option A

4. Quick Check:

   IF must have THEN before commands [OK]

Hint: IF condition must be followed by THEN [OK]

Common Mistakes:

• Forgetting THEN after IF condition
• Thinking ENDIF is missing
• Assuming commands can't be inside IF

5. You want the CNC machine to move to X=50 only if the tool number (#1) is 3 or higher. Which IF statement correctly does this?

hard

A. IF #1 GE 3 THEN
G01 X50
ENDIF

B. IF #1 GT 3 THEN
G01 X50
ENDIF

C. IF #1 LE 3 THEN
G01 X50
ENDIF

D. IF #1 EQ 3 THEN
G01 X50
ENDIF

Solution

1. Step 1: Understand the condition "3 or higher"

   This means #1 is greater than or equal to 3 (GE 3).

2. Step 2: Check each option's condition

   IF #1 GE 3 THEN G01 X50 ENDIF uses GE (greater or equal), B uses GT (greater than) which excludes 3, C uses LE (less or equal), D uses EQ (equal only).

3. Final Answer:

   IF #1 GE 3 THEN G01 X50 ENDIF -> Option A

4. Quick Check:

   GE means "3 or higher" [OK]

Hint: Use GE for "greater or equal" conditions [OK]

Common Mistakes:

• Using GT excludes the value 3
• Using LE or EQ does not cover all values 3 or higher
• Confusing GE with GT