如何在Python字符串中设置字符？

Question

I have a function that decrements a whole number parameter represented by a string. For instance, if I pass in the string "100", it should return "99."

def dec(s):
    i = len(s) - 1
    myString = ""
    while (i >= 0):
        if s[i] == '0':
            s[i] = '9'
            i -= 1
        else:
            s[i] = chr(int(s[i]) - 1)
            break
    return s

However, Python issues this error.

s[i] = '9'
TypeError: 'str' object does not support item assignment 

I am assuming that s[i] cannot be treated as an lvalue. What is a solution around this?

Answer 1

You can do:

s = s[:i] + "9" + s[i+1:]

This takes the part of the string before character index i , appends a 9 , then appends the part of the string after character index i . However, doing a lot of string appends like this is not terribly efficient.

The other answer is that if you're dealing with numbers, why not actually use numbers instead of strings?

def dec(s):
    return str(int(s) - 1)

Answer 2

You can't. In Python, strings are immutable -- once created, they can't be changed.

You have two options without changing your function entirely.

Convert the string to a list and back:

def dec(s):
    s = list(s)
    i = len(s) - 1
    myString = ""
    while (i >= 0):
        if s[i] == '0':
            s[i] = '9'
            i -= 1
        else:
            s[i] = chr(int(s[i]) - 1)
            break
    return ''.join(s)

Create a new string each time you want to make a change:

def dec(s):
    i = len(s) - 1
    myString = ""
    while (i >= 0):
        if s[i] == '0':
            s = s[:i] + "9" + s[i+1:]
            i -= 1
        else:
            s = s[:i] + chr(int(s[i]) - 1) + s[i+1:]
            break
    return s

Answer 3

I'm not sure why you are playing with the string character by character. Isn't this simpler?

def decrement_string(s):
    try:
      i = int(s)
      i = i - 1
      return str(i)
    except:
      # do something else
      return "that's no number!"

while True:
  s = raw_input("give me a number and I'll decrement it for you: ")
  print decrement_string(s)

Answer 4

The solution to your specific problem of "decrementing" strings is to convert s to an int with int(s) , decrease that, and convert back to a str: str(int(s)-1) .

In [175]: str(int('100')-1)
Out[175]: '99'

The general solution is to not attempt to alter the elements of a string; use some other type to represent your work, and convert to a string as the last step.

Answer 5

Python strings are immutable so you cannot modify them. You have to create a new string that contains the value that you need. You are better off converting the string to an integer, decrementing it, and then converting it back to an integer again.

The reason for immutable strings is primarily efficiency.

Answer 6

Strings aren't mutable, but lists are. You can easily convert the string to a list of individual characters:

l = list(s)

Then convert it back:

s = ''.join(l)

Since you're working with a numeric string there are more direct approaches, but this answer works for the general case.