如何使用if语句使for循环遍历字符串中的每个项目？

Question

I'm trying to make a function that takes in a string from a user and then outputs the same string. However for each letter in an even position it outputs the corresponding lower case letter, and for each letter in an odd position it outputs the corresponding uppercase letter. Keep in mind only one word will be passed through it at a time.

I've tried to create a for loop with an if statement nested within it, but so far, the for loop stops after iterating through the first letter. My code is below:

def converter(string):
    for letters in string:
        if len(letters) % 2 == 0:
            return letters.lower()
        elif len(letters)% 2 != 0:
            return letters.upper()

When I run the code:
converter('app')

The output I get is 'A'
The expected output should be 'aPp'

Answer 1

The first thing you need to know is that in Python, strings are immutable. So "modifying" a string means you have to build a new string from scratch in (here, I call that newstring ).

Second, you are misunderstanding the loop. You are saying for letters in string . This loop iterates over each letter of the string. On the first iteration, letters is the first letter of the strong. You then convert it to upper case (since the length of a single letter is always 1), and return it. You aren't reaching the rest of the letters! In the code below, I change the plurality to just letter to make this idea clear.

This amends all of those problems:

def converter(string):
    newstring = ""
    for i, letter in enumerate(string):
        if i % 2 == 0:
            newstring += letter.lower()
        elif i % 2 != 0:
            newstring += letter.upper()
    return newstring

This can be boiled down to a nice list comprehension:

def converter(string):
    return "".join([letter.lower() if i % 2 == 0 else letter.upper()
                    for i, letter in enumerate(string)])

Answer 2

In [1]: def converter(string):
   ...:     return ''.join([j.upper() if i % 2 == 1 else j.lower() for i, j in enumerate(string)])


In [2]: converter('apple')
Out[2]: 'aPpLe'

Answer 3

''.join([s.lower() if c % 2 == 0 else s.upper() for c, s in enumerate('apple')])
# returns 'aPpLe'

first check for the condition, then iterate through the string using the nice old enumerate built-in.