List has no items to pop however there was something there?

Question

Trying to make a brainfuck interpreter in python from scratch just for fun, I'm almost done with the code however this small error is coming up whenever I try interpreting ",[>++++<]" Which is supposed to input a number, and multiply it by 4. It stops and gets this error

File ".\bfInterpreter.py", line 42, in endWhile
    if self.stack[self.pointer]: self.place = self.inWhile.pop(-1)
IndexError: pop from empty list

Any ideas?

here is the code that the error is in:

import sys
import time

class Operator:
    def __init__(self) -> None:
        self.pointer = 0
        self.place = 0
        self.inWhile = []
        self.stack = [0]

    def plus(self):
        self.stack[self.pointer]+=1

    def minus(self):
        self.stack[self.pointer]-=1

    def left(self):
        if self.pointer > 0: self.pointer -= 1
        else: raise IndexError

    def right(self):
        self.pointer += 1
        try: self.stack[self.pointer] = 0
        except: self.stack.append(0)

    def output(self):
        print(ascii(self.stack[self.pointer])) 

    def inp(self):
        val = input("Num: ")
        try: self.stack[self.pointer] = int(val)
        except: 
            print("Please input a number")
            self.inp()

    def startWhile(self):
        self.inWhile.append(self.place)
        print(self.inWhile)

    def endWhile(self):
        print(self.inWhile)
        if self.stack[self.pointer]: self.place = self.inWhile.pop(-1)

    def interpret(self, bf):
        self.place = 0
        while self.place < len(bf):
            if bf[self.place] == "+": self.plus()
            elif bf[self.place] == "-": self.minus()
            elif bf[self.place] == "<": self.left()
            elif bf[self.place] == ">": self.right()
            elif bf[self.place] == "[": self.startWhile(bf)
            elif bf[self.place] == "]": self.endWhile()
            elif bf[self.place] == ",": self.inp()
            elif bf[self.place] == ".": self.output()
            else: raise SyntaxError
            self.place += 1
        print(self.stack)           

def main():
    start = time.time()
    op = Operator()
    op.interpret(",[>++++<]")
    print(f"Ended in {time.time() - start} second(s)")

if __name__ == "__main__": main()

Edit: self.stack is basically the memory that brainfuck is allowed to access and edit, so the "[" starts a while loop, checking if a certain spot in the stack has reached 0 every cycle, and if it's not 0, it repeats the code in the brackets

Answer 1

The main problem is self.inWhile.pop(-1) in command ] - it removes value from list and in next loop this list is empty - so pop(-1) raises your error message.

But there is more mistakes and problems in [ and ].

All commands runs self.place += 1 but ] shouldn't do this because this way it jump to first command after [ but it should run again [. Maybe if it would run again [ then it could add again value to inWhile and self.inWhile.pop(-1) whouldn't make problem.

But there is other problem with [ - it should check if value is 0 and jump to first command after ]. It would need another list to remember position of ] but it seems more complicated - because if at start value is 0 then it couldn't yet run ] and it couldn't add position to list. Better is to search matching ] in string with commands. The same method I used to search [ (instead of using inWhile).

Other problem is > you always try to set stack[place] = 0 but if you run > and next < and again > then stack[place] may already exists and it may have some value and stack[place] = 0 may remove this value. I rather use while loop with pointer >= len(stack) to append values only if stack is really too short.

---

Other problem is that program in branfuck runs loop forever because it doesn't have - to decreate inputed value. It needs also >. after loop to display result.

,[->++++<]>.

---

My version without inWhile but with loop which search matching [ or matching ].

import sys
import time

class Operator:
    
    def __init__(self):
        self.place = 0    # command place
        self.pointer = 0  # stack pointer
        self.stack = [0]  

    def input(self):
        #print('[DEBUG] input')
        while True:
            val = input("Number: ")
            try:
                self.stack[self.pointer] = int(val)
                break
            except: 
                print("Please input a integer number")

    def output(self):
        #print('[DEBUG] output')
        print(ascii(self.stack[self.pointer])) 

    def plus(self):
        #print('[DEBUG] plus')
        self.stack[self.pointer] += 1

    def minus(self):
        #print('[DEBUG] minus')
        self.stack[self.pointer] -= 1

    def left(self):
        #print('[DEBUG] left')
        if self.pointer > 0:
            self.pointer -= 1
        else:
            raise IndexError

    def right(self):
        #print('[DEBUG] right')
        self.pointer += 1
        while len(self.stack) <= self.pointer:
            self.stack.append(0)

    def start_loop(self):
        #print('[DEBUG] start_loop')
        #print('self.place:', self.place)

        if self.stack[self.pointer] == 0:
            # find matching `]` to jump to next command after `]`
            counter = 1
            while counter > 0:
                self.place += 1
                char = self.bf[self.place]
                if char == '[':
                    counter += 1
                elif char == ']':
                    counter -= 1
                #print('counter:', counter)    

    def end_loop(self):
        #print('[DEBUG] end_loop')
        #print('self.place:', self.place)

        # find matching `[` to run again `[` and all loop
        counter = 1
        while counter > 0:
            self.place -= 1
            char = self.bf[self.place]
            if char == '[':
                counter -= 1
            elif char == ']':
                counter += 1
            #print('counter:', counter)    

    def interpret(self, bf):
        self.bf = bf
        self.place = 0
        while self.place < len(bf):

            char = self.bf[self.place]

            #print('[DEBUG] interpret:', self.place, bf[self.place])

            if   char == ",":
                self.input()
                self.place += 1
            elif char == ".":
                self.output()
                self.place += 1
          
            elif char == "+":
                self.plus()
                self.place += 1
            elif char == "-":
                self.minus()
                self.place += 1
                
            elif char == "<":
                self.left()
                self.place += 1
            elif char == ">":
                self.right()
                self.place += 1
                
            elif char == "[":
                self.start_loop()
                self.place += 1
                
            elif char == "]":
                self.end_loop()
                # it has to be without `self.place += 1`
                
            else:
                raise SyntaxError

        print('stack:', self.stack)           

def main():
    start = time.time()

    op = Operator()
    op.interpret(",[->++++<]>.")  # input*4
    #op.interpret(",[->+>++>+++>++++<<<<]>.>.>.>.")  # input*1, input*2, input*3, input*4
    #op.interpret(",[->+<>>++<<>>>+++<<<>>>>++++<<<<]>.>.>.>.")  # input*1, input*2, input*3, input*4

    end = time.time()
    print(f"Ended in {end - start:.2f} second(s)")

if __name__ == "__main__":
    main()