为什么print（x [0：10：-1]）与print（x [::-1]）不同？

Question

If you define a string in Python like:

x = "1234567890"

print(x[::-1]) prints the string in reverse order, but print(x[0:10:-1]) prints nothing.

Why don't they print the same thing?

Answer 1

See note 5 of the relevant docs :

The slice of s from i to j with step k is defined as the sequence of items with index x = i + n*k such that 0 <= n < (ji)/k . In other words, the indices are i , i+k , i+2*k , i+3*k and so on, stopping when j is reached (but never including j )

Accordingly, when you specify i = 0, j = 10, k = -1 in [0:10:-1] , there are no values of n which satisfy the equation 0 <= n < (ji)/k , and therefore the range is empty.

On the other hand, in [::-1] , i == j == None . Refer to the next part of note 5:

If i or j are omitted or None, they become “end” values (which end depends on the sign of k ).

Since k is negative, i becomes len(s) - 1 , which evaluates to 9 . j cannot be represented by any number (since 0 would cut off the last character).

Answer 2

You should know about the basic rule x[start:stop:step] . In your code, you put two different rules: print(x[0:10:-1]) and print(x[::-1]) .

When you put nothing in the start and stop index, your code will be executed using the default depends on the step. If the step is positive, then the start index will be from 0 and the stop is the length of the string. Here, you may customize the start and stop value as long as they fulfilling start < stop . On the other hand, if the step is negative, then the start index will be from the last index of the string while the stop is 0 . Here, you may also customize the start and stop value as long as they fulfilling start > stop .

In your case, you wrote print(x[0:10:-1]) that gives you nothing back. This clearly because you set 0 as the start and 10 as the stop index while the step is negative.

Reference: https://www.digitalocean.com/community/tutorials/how-to-index-and-slice-strings-in-python-3

Answer 3

You are looking for print(x[10:0:-1]) .

>>> x = list(range(10))
>>> x[10:0:-1]
[9, 8, 7, 6, 5, 4, 3, 2, 1]

We can model x[start:end:step] with the following pseudocode:

i = start
if start < end:
  cond = lambda: i < end
else:
  cond = lambda: i > start

while cond():
  SELECT x[i]
  i += step