How to check whether a character comes before another character in a string in Python

Question

I'm writing a program that determines whether a math equation input by user has matching parentheses. I have it to where it counts whether the amount of right parentheses are equal to the amount of left parentheses, but the user could input 2) (3+x)(4+x) (6. To avoid telling the user his parentheses match in this case, I want to check if a ')' comes before a '('.

left_parenth = math.count("(")
right_parenth = math.count(")")

total_parenth = right_parenth + left_parenth

if total_parenth % 2 != 0:
    if right_parenth > left_parenth:
        print("\nYou are missing a left parentheses '('")
    elif right_parenth < left_parenth:
        print("\nYou are missing a right parentheses ')'")

elif total_parenth % 2 == 0:
    print("\nYour equation has the correct amount of parentheses")

Answer 1

General idea here is to use stack. When you encounter a opening parentheses, PUSH it on stack When you encounter a closing parentheses, POP cooresponding opening parentheses from stack

Answer 2

Using a stack is definitely not necessary. Just keep a count of how many parentheses are open. If that count ever is negative, then they aren't matched, and if the count ends up positive, they aren't matched:

def valid_parens(string):
    open_count = 0
    for char in string:
        if char == "(":
            open_count += 1
        elif char == ")":
            open_count -= 1
            if open_count < 0:
                return False
    return open_count == 0

Test cases:

assert valid_parens("((2+2)-4)*2") == True
assert valid_parens(")(2/2)(") == False
assert valid_parens("()()()") == True
assert valid_parens("((())") == False

Answer 3

Although the problem can be solved with a stack, it's a little wasteful of memory. In normal-sized inputs this wouldn't ever be a problem, but in really large sequences it would become a problem. You don't need to keep track of the parens themselves, but just a count of how many open ones there are -- total memory usage is a single integer, rather than one entry on a stack per paren encountered.

def proper_paren(seq):
    open_parens = 0
    for char in seq:
        if char == '(':
            open_parens += 1
        elif char == ')':
            open_parens -= 1
            if open_parens < 0:
                return False
    return open_parens == 0