Python slice index usage

This article mainly introduces the usage of Python slice index, and analyzes the common usage methods and operation precautions of Python slice index in detail in the form of examples. Friends in need can refer to it

The examples in this article describe Python slices Index usage. Share it with everyone for your reference, the details are as follows:

In Python, you can access each element of the sequence by using simple square brackets plus a subscript. This method is called the slicing operator, slicing The operator has three forms:

[], [:], [ ::]

The syntax for accessing a certain data element is as follows:

sequence[index]

sequence is the name of the sequence, index is the corresponding offset of the accessed element, which is a positive number, 0; use negative When indexing, the range is -len(sequence)

Since Python is object-oriented, you can also add it directly after the sequence An index is accessed, as follows

print ('a','b','c','d')[2]

For the case of accessing multiple elements

sequence[starting_index:ending_index]

The following are some examples of access methods:

sequence="abcdefgh"
print len(sequence)  #显示序列长度
print sequence    #打印完整序列
print sequence[:]
print sequence[2:3]  #切片显示，不指定步长默认为1，指定了步长(这里是setp为2)按照步长进行显示
print sequence[1:6:2]
print sequence[3]   #元素访问
print sequence[0:3]  #从首元素开始访问，访问区间为[0,3)，左开you
print sequence[:3]
print sequence[2:8]  #从第二个元素一直访问到最后一个元素
print sequence[2:]
print sequence[::-1] #从最后一个元素开始访问，逆序访问，可以视为“翻转”操作
print max(sequence)
print min(sequence)
print sequence.index('c')

What should be noted here is the repetition operator *

sequence * copies_int

When multiple copies of a sequence are required, the repeat operator comes into play, copies_int must For integer

print sequence*3   #使用重复操作符

Connection operator

sequence1 sequence2

It is allowed to connect two sequences of the same type

print sequence + sequence

But note that it seems very convenient, but this operation is not the fastest or most efficient. For strings, this operation is not as good as putting all substrings into a list or iterable object, and then using the join() method to join all the contents together to save memory; for lists Generally speaking, it is recommended to use the extend() method of the list type to merge two or more list objects

str.join(sequence) #方法用于将序列中的元素以指定的字符连接生成一个新的字符串

str = "-";
seq = ("a", "b", "c"); # 字符串序列
print str.join( seq );

The output is:

a-b-c

list.extend(seq)  #函数用于在列表末尾一次性追加另一个序列中的多个值（用新列表扩展原来的列表）

aList = [123, 'xyz', 'zara', 'abc', 123];
bList = [2009, 'manni'];
aList.extend(bList)
print "Extended List : ", aList ;

The output is :

Extended List :  [123, 'xyz', 'zara', 'abc', 123, 2009, 'manni']

The syntax of slice index is much more flexible than the simple single element index method. The start and end index values ​​​​can exceed the length of the string. That is, the starting index value can be less than 0, and the ending index value can be greater than the length of the sequence, such as:

print ('Faye','Leanna','Daylen')[-100:100]

The output is:

('Faye', 'Leanna', 'Daylen')

If there is a string, and you want to display it through a loop in this way: cut off the last character every time, how to achieve this?

sequence = 'abcdef'
i = -1
for i in range(-1,-len(sequence),-1):
  print sequence[:i]

The output is:

abcde
abcd
abc
ab
a

It is found that the first one is not displayed, that is, the complete string is not displayed, unless another separate one is defined sequence[:0], use None as the index value here

sequence = 'abcdef'
for i in [None] + range(-1,-len(sequence),-1):
  print sequence[:i]

The output is:

abcdef
abcde
abcd
abc
ab
a

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