How to remove certain elements from an array that start with certain characters?

Question

I have a list of expressions, in an array known as literals :

literals = ['-PacketAt(A)', '+CarAt(B)', '-CarAt(A)', '-PacketInCar', '+PacketAt(B)', '+CarAt(A)', 'LoadA', '+PacketAt(A)', '+PacketInCar', 'LoadB', '-PacketAt(B)', 'DriveAB', '-CarAt(B)', 'DriveBA', 'ProduceA', 'DischargeA', 'DischargeB']

I would like to remove the elements of the array that do not start with a + and - sign.

I have written the following to try to do that:

for literal in literals:
  if not (literal.startswith('-')) and not (literal.startswith('+')):
    literals.remove(literal)

However, after I run this for loop, I receive the following output:

literals = ['-PacketAt(A)', '+CarAt(B)', '-CarAt(A)', '-PacketInCar', '+PacketAt(B)', '+CarAt(A)', '+PacketAt(A)', '+PacketInCar', '-PacketAt(B)', '-CarAt(B)', 'ProduceA', 'DischargeB']

The desired output is:

literals = ['-PacketAt(A)', '+CarAt(B)', '-CarAt(A)', ' -PacketInCar', '+PacketAt(B)',
'+CarAt(A)', '+PacketAt(A)', '+PacketInCar', '-PacketAt(B)', 'DriveAB', '-CarAt(B)']

From this, 'ProduceA' and 'DischargeB' should not be in the list known as literals , but they are. Why is this so, and how can I modify my code such that they do not appear? Here is some runnable test code I have provided:

literals = ['-PacketAt(A)', '+CarAt(B)', '-CarAt(A)', ' -PacketInCar', '+PacketAt(B)',
'+CarAt(A)', 'LoadA', '+PacketAt(A)', '+PacketInCar', 'LoadB', '-PacketAt(B)', 'DriveAB', '-CarAt(B)', 'DriveBA', 'ProduceA', 'DischargeA', 'DischargeB']

for literal in literals:
    if not (literal.startswith('-')) and not (literal.startswith('+')):
        literals.remove(literal)
        
print(literals)

and an online Python IDE to run it in: https://www.programiz.com/python-programming/online-compiler/

Answer 1

The issue you're having is that your are trying to remove elements from a list while iterating the list. Try to avoid this.

You can use list comprehension to get the results:

 literals = ['-PacketAt(A)', '+CarAt(B)', '-CarAt(A)', ' -PacketInCar', '+PacketAt(B)', '+CarAt(A)', 'LoadA', '+PacketAt(A)', '+PacketInCar', 'LoadB', '-PacketAt(B)', 'DriveAB', '-CarAt(B)', 'DriveBA', 'ProduceA', 'DischargeA', 'DischargeB']
 lst  = [x for x in literals if x[0] in ['-','+']]
 print(lst)

Output

['-PacketAt(A)', '+CarAt(B)', '-CarAt(A)', '-PacketInCar', '+PacketAt(B)', '+CarAt(A)', '+PacketAt(A)', '+PacketInCar', '-PacketAt(B)', '-CarAt(B)']

Answer 2

I will use a list comprehension:

literals = ['-PacketAt(A)', '+CarAt(B)', '-CarAt(A)', '-PacketInCar', '+PacketAt(B)', '+CarAt(A)', 'LoadA', '+PacketAt(A)', '+PacketInCar', 'LoadB', '-PacketAt(B)', 'DriveAB', '-CarAt(B)', 'DriveBA', 'ProduceA', 'DischargeA', 'DischargeB']
literals = [x for x in literals if not x.startswith(('-','+'))]
print(literals)