Slicing a list

Taking a partial element of a list is a very common operation. For example, a list is as follows:

>>> L = [' Adam ', ' Lisa ', ' Bart ', ' Paul ']

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

Stupid way:

>>> [l[0], l[1], l[2]][' Adam ', ' Lisa ', ' Bart ']

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[' Adam ', ' Lisa ', ' Bart ']

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][' Adam ', ' Lisa ', ' Bart '

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][' Adam ', ' Lisa ', ' Bart '

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

>>> l[1:3][' Lisa ', ' Bart ' 

Use only one : to indicate from beginning to end:

>>> l[:][' Adam ', ' Lisa ', ' Bart ', ' Paul '

So l[:] Actually copied out a new list.

The slice operation can also specify a third parameter:

>>> l[::2][' Adam ', ' Bart ']

The third parameter indicates that each n takes one, and the top L[::2] takes one out of every two elements, that is, one at a intervals.

Change the list to a tuple, the slice operation is exactly the same, but the result of the slice becomes a tuple.

The range () function can create a sequence number:

>>> Range (1, 101) [1, 2, 3, ..., 100]

Please use the slices to remove:

1. The first 10 digits;
2. Multiples of 3;
3. A multiple of 5 not greater than 50.

Reference code:

L = Range (1, 101)

Print L[0:10]
Print L[2::3]
Print L[4:50:5]

Print L[0:10] #从第1个数元素开始取, to the end of the 11th element
Print L[2::3] #从第三元素开始取, take one element at every 2
Print L[4:50:5] #从第五个取, take one every 4, ' start element ': ' last element ': ' Take element interval '

Reverse sectioning

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

>>> L = [' Adam ', ' Lisa ', ' Bart ', ' Paul ']>>> l[-2:][' Bart ', ' Paul ']>>> l[:-2][' Adam ', ' Lisa ']& gt;>> l[-3:-1][' Lisa ', ' Bart ']>>> l[-4:-1:2][' Adam ', ' Bart '

Remember that the index of the first element of the countdown is-1. The reverse slice contains the starting index and does not contain the end index.

Task

Use the reverse section to remove the 1-100 sequence:

* last 10 digits;

* A multiple of the last 10 5.

Reference code:

L = Range (1, 101)
Print l[-10:]
Print l[4::5][-10:]//Get a multiple of 5 before taking 10

Slicing the string

The string ' xxx ' and 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 ' [-3:] ' EFG ' >>> ' 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.

Task

The string has a method upper () to turn the character into uppercase:

>>> ' abc '. UPPER () ' ABC '

But it will turn all the letters into uppercase. Design a function that takes a string and returns a string with only the first letter capitalized.

tip: use slice manipulation to simplify string manipulation.

Reference code:

def firstcharupper (s):
Return S[0].upper () +s[1:]

Print firstcharupper (' Hello ')
Print Firstcharupper (' Sunday ')
Print Firstcharupper (' September ')

Slicing a list