Python Slicing Usage Example tutorial

This article describes the usage of slicing operations in Python in the form of examples, and shares them for reference, as follows:

Taking a list or tuple element is a very common operation. For example, a list is as follows:

>>> L = [' Michael ', ' Sarah ', ' Tracy ', ' Bob ', ' Jack ']

What should I do if I take the first 3 elements?

The more stupid way is as follows:

>>> [l[0], l[1], l[2]][' Michael ', ' Sarah ', ' Tracy ']

The reason why is stupid is because of the extension, take the first n elements will be out of the way.

Take the first N elements, which are the elements indexed as 0-(N-1), and you can use loops:

>>> r = []>>> n = 3>>> for I in Range (n):   ... R.append (L[i]) ... >>> r[' Michael ', ' Sarah ', ' Tracy ']

It is cumbersome to use loops for operations that often take a specified index range, so Python provides a slice (Slice) operator that can greatly simplify this operation .

Corresponding to the above problem, take the first 3 elements, a line of code to complete the slice:

>>> l[0:3][' Michael ', ' Sarah ', ' Tracy '

L[0:3] Indicates that the fetch starts at index 0 until index 3, but does not include index 3. That is, index 0,1,2, which is exactly 3 elements.

If the first index is 0, you can also omit:

>>> l[:3][' Michael ', ' Sarah ', ' Tracy '

You can also start with index 1 and take out 2 elements:

>>> l[1:3][' Sarah ', ' Tracy '

Similarly, since Python supports l[-1] to take the first element of the countdown, it also supports the inverse slice, try it:

>>> l[-2:][' Bob ', ' Jack ']>>> l[-2:-1][' Bob '

Remember that the index of the last element of the countdown is-1.

Slicing operations are useful. Let's start by creating a 0-99 series:

>>> L = range (+) >>> l[0, 1, 2, 3, ..., 99]

You can easily remove a segment of a sequence by slicing it. such as the first 10 numbers:

>>> l[:10][0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

10 Number of second:

>>> l[-10:][90, 91, 92, 93, 94, 95, 96, 97, 98, 99]

Number of first 11-20:

>>> l[10:20][10, 11, 12, 13, 14, 15, 16, 17, 18, 19]

First 10 numbers, each of two fetch one:

>>> l[:10:2][0, 2, 4, 6, 8]

All numbers, fetch one per 5:

>>> l[::5][0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95]

Don't even write anything, just write [:] to copy a list as it is:

>>> l[:][0, 1, 2, 3, ..., 99]

Tuple is also a list, the only difference is that the tuple is immutable. Therefore, a tuple can also be used for slicing operations, but the result of the operation is still a tuple:

>>> (0, 1, 2, 3, 4, 5) [: 3] (0, 1, 2)

The string ' xxx ' or the unicode string u ' xxx ' can also be viewed as a list, each element being a character. Therefore, a string can also be manipulated with a slice, but the result of the operation is still a string:

>>> ' ABCDEFG ' [: 3] ' ABC ' >>> ' ABCDEFG ' [:: 2] ' ACEG '

In many programming languages, there are many kinds of interception functions available for strings, in fact, the purpose is to slice the strings. Python does not have an intercept function for a string, it can be done simply by slicing one operation.

Summarize:

With the slicing operation, many local loops are no longer needed. Python's slices are very flexible, and one line of code can implement many lines of work to complete .

It is hoped that the examples mentioned in this paper will help you to master Python program design.

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