As sequences, strings support operations that assume a positional ordering among items.
For example, if we have a four-character string coded inside quotes, we can verify its length with the built-in len function.
We can fetch its components with indexing expressions:
S = 'test' # Make a 4-character string, and assign it to a name print( len(S) ) # Length print( S ) # The first item in S, indexing by zero-based position print( S ) # The second item from the left # from w ww. j a va2 s . c o m
In Python, indexes are coded as offsets from the front, and so start from 0: the first item is at index 0, the second is at index 1, and so on.
In Python, we can index backward, from the end-positive indexes count from the left.
The negative indexes count back from the right:
S = 'test' # Make a 4-character string, and assign it to a name print( S[-1] ) # The last item from the end in S print( S[-2] ) # The second-to-last item from the end # w ww . j a v a2s.co m
A negative index is simply added to the string's length, so the following two operations are equivalent:
S = 'test' # Make a 4-character string, and assign it to a name print( S[-1] ) # The last item in S print( S[len(S)-1] ) # Negative indexing, the hard way # w w w .j ava 2s. c om
We can use an arbitrary expression in the square brackets, not just a hard-coded number literal.
Sequences support a more general form of indexing known as slicing.
It is a way to extract an entire section (slice) in a single step. For example:
S = 'test' # Make a 4-character string, and assign it to a name print( S[1:3] ) # Slice of S from offsets 1 through 2 (not 3)
In a slice, the left bound defaults to zero, and the right bound defaults to the length of the sequence being sliced.
This leads to some common usage variations:
S = 'test' # Make a 4-character string, and assign it to a name # from ww w . j a v a 2s.co m print(S[1:]) # Everything past the first (1:len(S)) # S itself hasn't changed print( S[0:3] ) # Everything but the last print( S[:3] ) # Same as S[0:3] print( S[:-1] ) # Everything but the last again, but simpler (0:-1) print( S[:] ) # All of S as a top-level copy (0:len(S))