Python-List

List

---

序列是Python中最基本的数据结构。序列中的每个元素都分配了一个数字，称为位置或索引。第一个索引是0，第二个是1…
Python中最常用的序列是List和Tuple;你可以对序列类型执行某些操作，包括索引、切片、添加、乘法和检查成员资格。此外，Python还有用于查找序列长度、最大和最小元素的内置函数。

List

List是Python中最通用的数据类型，仅需要用逗号分隔开列表元素，放在[]内。最重要的是List中的各项不必具有相同类型。
例：

list1 = ['physics', 'chemistry', 1997, 2000]
list2 = [1, 2, 3, 4, 5 ]
list3 = ["a", "b", "c", "d"]

获取List中的值

通过索引可以获取list的值

list1 = ['physics', 'chemistry', 1997, 2000]
list2 = [1, 2, 3, 4, 5, 6, 7 ]
print("list1[0]: ", list1[0])
print("list2[1:5]: ", list2[1:5])

执行结果

list1[0]:  physics
list2[1:5]:  [2, 3, 4, 5]

更新List

可以通过赋值运算符给左侧的索引来更新List元素，并且可以使用append方法向List中添加元素
例：

list = ['physics', 'chemistry', 1997, 2000]
print "Value available at index 2 : "
print(list[2])
list[2] = 2001
print("New value available at index 2 : ")
print(list[2])

执行结果

Value available at index 2 :
1997
New value available at index 2 :
2001

删除List元素

如果知道要删除的元素，可以使用del语句，如果不知道可以使用remove()方法
例：

list1 = ['physics', 'chemistry', 1997, 2000]
print(list1)
del list1[2]
print("After deleting value at index 2 : ")
print(list1)

执行结果

['physics', 'chemistry', 1997, 2000]
After deleting value at index 2 :
['physics', 'chemistry', 2000]

List基本操作

与string类似，+ *于List表示结合与重复操作，与string不同的是结果是一个新List

Python Expression	Results	Description
len([1,2,3])	3	Length
[1,2,3]+[4,5,6]	[1,2,3,4,5,6]	Concatenation
[‘Hi!’] * 4	[‘Hi!’, ‘Hi!’, ‘Hi!’, ‘Hi!’]	Repetition
3 in [1, 2, 3]	True	Membership
for x in [1, 2, 3]: print x	1 2 3	Iteration

索引、切片、矩阵

因为List是序列，因此对string适用的索引切片也可以用于List

L = ['spam', 'Spam', 'SPAM!']

Python Expression	Results	Description
L[2]	SPAM!	Offsets start at zero
L[-2]	Spam	Negative: count from the right
L[1:]	[‘Spam’, ‘SPAM!’]	Slicing fetches sections

List的内置函数与方法

Sr.No.	Function with Description
1	cmp(list1,list2) Compares elements of both lists
2	len(list) Gives the total length of the list
3	max(list) Returns item from the list with max value
4	min(list) Returns item from the list with min value
5	list(seq) Converts a tuple into list

List Method

Sr.No.	Methods with Description
1	list.append(obj) Appends object obj to list
2	list.count(obj) Returns count of how many times obj occurs in list
3	list.extend(seq) Appends the contents of seq to list
4	list.index(obj) Returns the lowest index in list that obj appears
5	list.insert(index,obj) Inserts object obj into list at offset index
6	list.pop(obj=list[-1]) Removes and returns last object or obj from list
7	list.remove(obj) Removes object obj from list
8	list.reverse() Reverses objects of list in place
9	list.sort(key=function,reverse=False) Sorts objects of list, key=function optional, reverse default is False