比较字符时字符串索引超出范围

Question

I am simply trying to compare one character in a string to its neighbor, but I keep getting a string index out of range error. I can't figure this out.

s = 'azcbobobegghakl'
current = ""
previous = ""
for letter in range(0,len(s)):
    if s[letter] <= s[letter+1]:
        current += s[letter]
print(current)

Thanks

Answer 1

The if statement was unnecessary. On the last iteration of the loop, s[letter+1] was throwing an error because that index of the string didn't exist. At that point, letter was equal to the length of your string, s . letter+1 was one higher than the length, so the index didn't exist.

There is no need to compare the value of letter to the length of s in the if statement because the loop would end before letter is greater than the length of s .

Solution:

s = 'azcbobobegghakl'
current = ""
previous = ""
for letter in range(0,len(s)):
    current += s[letter]
print(current)

Answer 2

while @sedavidw is completely right pointing out the indexing issue, it is not a very pythonic way of doing such a comparison. I'd do something like:

current = ''.join(letter for letter, next_letter in zip(s, s[1:]) 
                  if letter <= next_letter)

Answer 3

I think you want to print a letter only if the previous one is less than the current one. The index out of bounds occur because the last index points to (l) but index + 1 points to nothing.

Here is my solution:

s = 'azcbobobegghakl' 
current = "" 
previous = "" 
for letter in range(0,len(s)): 
   if letter + 1 < len(s): # this is important to prevent the index out of bounds error
       if s[letter] <= s[letter+1]: 
           current += s[letter] 
 print(current)

Output: abbbeggak