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Exercise #36: Reverse StringreverseString('Hello') → 'olleH'
Strings are immutable in the Python language, meaning you can’t modify their characters the way you can modify the items in a list. For example, if you tried to change 'Rat' to 'Ram' with the assignment statement 'Rat'[2] = 'm', you would receive a TypeError: 'str' object does not support item assignment error message. On the other hand, if you store a string 'Rat' in a variable named animal, the assignment statement animal = 'Ram' isn’t modifying the 'Rat' string but rather making animal refer to an entirely new string, 'Ram'.
We can modify an existing string is to create a list of single-character strings, modify the list, and then create a new string from the list. Enter the following into the interactive shell:
>>> animal = 'Rat'
>>> animal = list(animal)
>>> animal
['R', 'a', 't']
>>> animal[2] = 'm'
>>> animal
['R', 'a', 'm']
>>> animal = ''.join(animal)
>>> animal
'Ram'
We’ll use this technique to reverse the characters in a string.
Exercise Description
Write a reverseString() function with a text parameter. The function should return a string with all of text’s characters in reverse order. For example, reverseString('Hello') returns 'olleH'. The function should not alter the casing of any letters. And, if text is a blank string, the function returns a blank string.
These Python assert statements stop the program if their condition is False. Copy them to the bottom of your solution program. Your solution is correct if the following assert statements’ conditions are all True:
assert reverseString('Hello') == 'olleH'
assert reverseString('') == ''
assert reverseString('aaazzz') == 'zzzaaa'
assert reverseString('xxxx') == 'xxxx'
Try to write a solution based on the information in this description. If you still have trouble solving this exercise, read the Solution Design and Special Cases and Gotchas sections for additional hints.
Prerequisite concepts: lists, list(), for loops, range(), len(), integer division, indexes, swapping values, join()
Solution Design
Instead of building up a string from the characters in text in reverse order, let’s make our function first convert the string in text into a list of single-character strings. Python’s list() function does when we pass it a string. For example, list('Hello') returns the list value ['H', 'e', 'l', 'l', 'o']. We can assign this list as the new value of text.
Once we have the characters of text in a list, create a for loop that loops over the first half of the list’s indexes. This can be calculated from the expression len(text) // 2. We want to replace the character at each index with the character at the “mirror” index in the second half of the list. The mirror of the first index is the last index, the mirror of the second index is the second to last index, the mirror of the third index is the third to last index, and so on.
To calculate the mirror of an index i in text, you would want len(text) - 1 - i. For example, the mirror of index 0 is len(text) - 1 - 0, the mirror of index 1 is len(text) - 1 - 1, the mirror of index 2 is len(text) - 1 - 2, and so on.
Python’s assignment statement allows you to swap two values simultaneously. For example, the assignment statement myList[0], myList[5] = myList[5], myList[0] swaps the values at indexes 0 and 5 in a hypothetical myList variable.
Figure 36-1 shows the characters of a hypothetical 12-item list made from 'Hello, world' being swapped until the string is reversed.
Figure 36-1: The process of reversing a list of single-character strings by swapping their mirror indexes.
Finally, the join() string method creates a string from the text list with the instruction ''.join(text). This is the string the reverseString() function should return.
Special Cases and Gotchas
You may think that you need two different algorithms depending on if the string to reverse has an odd or even number of characters. However, it turns out this doesn’t matter. If the text string has an odd number of characters, like the 5 characters in the list ['H', 'e', 'l', 'l', 'o'], then swapping the middle character at index 2 with itself doesn’t change the list.
Now try to write a solution based on the information in the previous sections. If you still have trouble solving this exercise, read the Solution Template section for additional hints.
Solution Template
Try to first write a solution from scratch. But if you have difficulty, you can use the following partial program as a starting place. Copy the following code from https://invpy.com/reversestring-template.py and paste it into your code editor. Replace the underscores with code to make a working program:
def reverseString(text):
text = ____(text)
for i in range(len(____) // ____):
mirrorIndex = len(text) - ____ - ____
text[i], text[mirrorIndex] = text[____], text[____]
return ''.join(text)
The complete solution for this exercise is given in Appendix A and https://invpy.com/reversestring.py. You can view each step of this program as it runs under a debugger at https://invpy.com/reversestring-debug/.
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