Notes

def validate_dfa(input_string):
# Define states
q0 = 0 # Start state
q1 = 1 # State after reading the first character
q_accept = 2 # Accepting state
q_reject = 3 # Rejecting state

# Initialize the current state to q0 (start state)
current_state = q0

# Check if the string length is exactly 2
if len(input_string) != 2:
return "Reject: String length is not 2"

# Define transitions
for char in input_string:
if current_state == q0:
# Transition from q0 to q1 on reading the first character
current_state = q1
elif current_state == q1:
# Transition from q1 to q_accept on reading the second character
current_state = q_accept
else:
# If we reach any state not defined in the transition
current_state = q_reject
break

# After processing the input string
if current_state == q_accept:
return "Accept: String is of length 2"
else:
return "Reject: Invalid input or incorrect string length"

# Test cases
print(validate_dfa("ab")) # Expected: Accept: String is of length 2
print(validate_dfa("abc")) # Expected: Reject: String length is not 2
print(validate_dfa("a")) # Expected: Reject: String length is not 2
print(validate_dfa("")) # Expected: Reject: String length is not 2