检查列表中是否存在字符

Question

I'm trying to write a recursive function that checks whether a certain character exists in a list(the list can contain sublists).

So far I've come up with this but it doesn't seem to be working. What am I doing wrong?

    def exists(str, seq):
        if not seq:
            return 0
        elif isinstance(seq[0], list):
            return exists(seq[0]) + exists(seq[1:])
        elif str == seq[0]:
            return True
        else:
            return exists(seq[1:])

If I try

    print(exists("c", [a,[b,c],d]))

I get name "a" is not defined

Answer 1

def exists(str, seq):

You should not use the built-in type str as the name of a variable.

        return 0

For binary functions you should return False , not 0.

        return exists(seq[0]) + exists(seq[1:])

Calls to exist must have two arguments, not one. And you should use or , not + .

        return exists(seq[1:])

Calls to exist must have two arguments, not one.

print(exists("c", [a,[b,c],d]))

[a,[b,c],d] Means "a nested list containing the variables named a, b, c, and d". If you don't have variables with those names, this will crash with a NameError. Perhaps you meant to use string literals.

def exists(s, seq):
    if not seq:
        return False
    elif isinstance(seq[0], list):
        return exists(s, seq[0]) or exists(s, seq[1:])
    elif s == seq[0]:
        return True
    else:
        return exists(s, seq[1:])

print(exists("c", ["a",["b","c"],"d"]))

Answer 2

"I get name "a" is not defined".

That is because it is not defined, assuming this is your full code. The same applies to b , c and d .

You need to either define those variables beforehand (1.) with some string objects, or write the letters as string literals themselves (2.):

1. a, b, c, d = "my_string1", "my_string2", "my_string3", "my_string4"
2. print(exists('c', ['a', ['b', 'c'], 'd']))

Apart from that, you define your function as def exists(str, seq): , but you invoke it as exists(seq[0]) which means you are missing a parameter.

Then you are trying to add boolean values, which might work, but I am not sure if it is what you want to achieve: return exists(seq[0]) + exists(seq[1:]) . If converted to a boolean, 2 will also be True . But it will also be True if one of the statements is 0 . To check whether both or none are True , you should multiply the values. Also, try to use the same return type for every return case.

You are also using the first character (or item) of your seq, while you actually wanted to use the first character of the string str . Careful! str is a type class, do NOT use it as a variable name.