Understanding string reversal via slicing
I was looking for a way to print a string backwards, and after a quick search on google, I found this method:
Suppose a is a string variable. This will return the a string backwards:
a[::-1]
Can anyone explain how that works?
Answer
Sure, the [::] is the extended slice operator. It allows you to take substrings. Basically, it works by specifying which elements you want as [begin:end:step], and it works for all sequences. Two neat things about it:
- You can omit one or more of the elements and it does "the right thing"
- Negative numbers for begin, end, and step have meaning
For begin and end, if you give a negative number, it means to count from the end of the sequence. For instance, if I have a list:
l = [1,2,3]
Then l[-1] is 3, l[-2] is 2, and l[-3] is 1.
For the step argument, a negative number means to work backwards through the sequence. So for a list::
l = [1,2,3,4,5,6,7,8,9,10]
You could write l[::-1] which basically means to use a step size of -1 while reading through the list. Python will "do the right thing" when filling in the start and stop so it iterates through the list backwards and gives you [10,9,8,7,6,5,4,3,2,1].
I've given the examples with lists, but strings are just another sequence and work the same way. So a[::-1] means to build a string by joining the characters you get by walking backwards through the string.