CODEWITHSUNDEEP
pythonstringslice
Thorson Kahn
Thorson Kahn

Reputation: 11

reverse slice of string [x:y:-1] prints blank

I have strings representing a DNA sequence and its complement and I am searching them for reverse palindromes.

The slice

seq1[i:z]

works great, but

seq2[i:z:-1]

prints nothing and returns nothing.

Further, seq[i:z][::-1] works fine in all cases, but is this normal for string slices?

Upvotes: 1

Views: 112

Answers (2)

Alex Riley
Alex Riley

Reputation: 176938

If you're stepping backwards through an iterable (i.e. step is negative), you need to swap the order of the start and end values so that start > end:

>>> seq = 'abcde'
>>> seq[1:4:-1]
''
>>> seq[4:1:-1]
'edc'

Otherwise, you'll get back an empty string (or empty list or empty tuple).

Do note that seq[4:1:-1] does not produce the same result as seq[1:4][::-1] however. The former starts at index 4, moves backwards and stops before index 1, whereas the latter starts at index 1, moves forward, stops before index 4 and then reverses the slice.

Instead we have for an iterable seq and for integers i < j:

seq[i:j][::-1] == seq[j-1:i-1:-1]

Upvotes: 6

GLHF
GLHF

Reputation: 4035

For palindroms,

a=input("entry something: ")
if a==a[::-1]:
    print ("It is a palindrom.")

So if a's reverse equal to a, it will print its a palindrom.

Upvotes: 0

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