Last update on February 28 2020 12:07:32 (UTC/GMT +8 hours)
List
A list is a container which holds comma-separated values (items or elements) between square brackets where items or elements need not all have the same type.
In general, we can define a list as an object that contains multiple data items (elements). The contents of a list can be changed during program execution. The size of a list can also change during execution, as elements are added or removed from it.
Note: There are much programming languages which allow us to create arrays, which are objects similar to lists. Lists serve the same purpose as arrays and have many more built-in capabilities. Traditional arrays can not be created in Python.
<list>=<list>[from_inclusive : to_exclusive : ±step_size] <list>.append(<el>)# Or: <list> += [<el>]<list>.extend(<collection>)# Or: <list> += <collection> <list>.sort()<list>.reverse()<list>=sorted(<collection>)<iter>=reversed(<list>)sum_of_elements =sum(<collection>)elementwise_sum = [sum(pair)for pair inzip(list_a, list_b)]sorted_by_second =sorted(<collection>, key=lambdael: el[1])sorted_by_both =sorted(<collection>, key=lambdael: (el[1], el[0]))flatter_list =list(itertools.chain.from_iterable(<list>))product_of_elems = functools.reduce(lambdaout, x: out * x, <collection>)list_of_chars =list(<str>)# Returns number of occurrences. Also works on strings.<int>=<list>.count(<el>)# Returns index of first occurrence or raises ValueError. index =<list>.index(<el>)# Inserts item at index and moves the rest to the right.<list>.insert(index, <el>)# Removes and returns item at index or from the end. <el>=<list>.pop([index])# Removes first occurrence of item or raises ValueError. <list>.remove(<el>)# Removes all items. Also works on dictionary and set. <list>.clear()
Create a Python list
Following list contains all integer values:
my_list1 = [5,12,13,14] # the list contains all integer valuesprint(my_list1)[5,12,13,14]
Following list contains all string:
my_list2 = ['red','blue','black','white'] # the list contains all string# valuesprint(my_list2)['red','blue','black','white']
Following list contains a string, an integer and a float values:
my_list3 = ['red',12,112.12] # the list contains a string, an integer and# a float valuesprint(my_list3)# ['red', 12, 112.12]
A list without any element is called an empty list. See the following statements.
my_list=[]print(my_list)# []
Use + operator to create a new list that is a concatenation of two lists and use * operator to repeat a list. See the following statements.
List indices work the same way as string indices, list indices start at 0. If an index has a positive value it counts from the beginning and similarly it counts backward if the index has a negative value. As positive integers are used to index from the left end and negative integers are used to index from the right end, so every item of a list gives two alternatives indices. Let create a list called color_list with four items.
color_list=["RED","Blue","Green","Black"]
Item
RED
Blue
Green
Black
Index (from left)
0
1
2
3
Index (from right)
-4
-3
-2
-1
If you give any index value which is out of range then interpreter creates an error message. See the following statements.
color_list=["Red","Blue","Green","Black"] # The list have four elements# indices start at 0 and end at 3color_list[0]# Return the First Element# 'Red'print(color_list[0],color_list[3])# Print First and Last Elements# Red Blackcolor_list[-1]# Return Last Element# 'Black'print(color_list[4])# Creates Error as the indices is out of range# Traceback (most recent call last):# File "<stdin>", line 1, in <module># IndexError: list index out of range
Lists can be sliced like strings and other sequences.
Syntax:
sliced_list = List_Name[startIndex:endIndex]
This refers to the items of a list starting at index startIndex and stopping just before index endIndex. The default values for list are 0 (startIndex) and the end (endIndex) of the list. If you omit both indices, the slice makes a copy of the original list.
Cut first two items from a list:
See the following statements:
color_list=["Red","Blue","Green","Black"] # The list have four elements # indices start at 0 and end at 3print(color_list[0:2])# cut first two items# ['Red', 'Blue']
Cut second item from a list:
See the following statements:
color_list=["Red","Blue","Green","Black"] # The list have four elements # indices start at 0 and end at 3print(color_list[1:2])# ['Blue']print(color_list[1:-2])# ['Blue']
Cut second and third elements from a list:
See the following statements:
color_list=["Red","Blue","Green","Black"] # The list have four elements # indices start at 0 and end at 3print(color_list[1:-1])# ['Blue', 'Green']
Cut first three items from a list:
See the following statements:
color_list=["Red","Blue","Green","Black"] # The list have four elements # indices start at 0 and end at 3print(color_list[:3])# cut first three items# ['Red', 'Blue', 'Green']
Creates copy of original list:
See the following statements:
color_list=["Red","Blue","Green","Black"] # The list have four elements # indices start at 0 and end at 3print(color_list[:])# Creates copy of original list# ['Red', 'Blue', 'Green', 'Black']
Remove the item at the given position in the list, and return it
See the following statements:
color_list=["Red","Blue","Green","Black"]print(color_list)# ['Red', 'Blue', 'Green', 'Black']color_list.pop(2)# Remove second item and return it# 'Green'print(color_list)# ['Red', 'Blue', 'Black']
Return the index in the list of the first item whose value is x
Items in the list are mutable i.e. after creating a list you can change any item in the list. See the following statements.
color_list=["Red","Blue","Green","Black"]print(color_list[0])# Redcolor_list[0]="White"# Change the value of first item "Red" to "White"print(color_list)# ['White', 'Blue', 'Green', 'Black']print(color_list[0])# White
listx=[1,5,7,3,2,4,6]print(listx)# [1, 5, 7, 3, 2, 4, 6]sublist=listx[2:7:2]#list[start:stop:step], #step specify an increment# between the elements to cut of the list.print(sublist)# [7, 2, 6]sublist=listx[::3]#returns a list with a jump every 2 times.print(sublist)# [1, 3, 6]sublist=listx[6:2:-2]#when step is negative the jump is made backprint(sublist)# [6, 2]
Find the largest and the smallest item in a list
listx=[5,10,3,25,7,4,15]print(listx)# [5, 10, 3, 25, 7, 4, 15]print(max(listx))# the max() function of built-in allows to know the highestvalue in the list.# 25print(min(listx))#the min() function of built-in allows to know the lowestvalue in the list.# 3
Compare two lists in Python
listx1, listx2=[3,5,7,9], [3,5,7,9]print (listx1 == listx2)# Truelistx1, listx2=[9,7,5,3], [3,5,7,9] #create two lists equal, but unsorted.print(listx1 == listx2)# Falselistx1, listx2 =[2,3,5,7], [3,5,7,9] #create two different listprint(listx1 == listx2)# Falseprint(listx1.sort() == listx2.sort())#order and compare# True
Nested lists in Python
listx = [["Hello","World"], [0,1,2,3,4,5]]print(listx)# [['Hello', 'World'], [0, 1, 2, 3, 4, 5]]listx = [["Hello","World"], [0,1,2,3,3,5]]print(listx)# [['Hello', 'World'], [0, 1, 2, 3, 3, 5]]print(listx[0][1])#The first [] indicates the index of the outer list.# Worldprint(listx[1][3])#the second [] indicates the index nested lists.# 3listx.append([True, False])#add new itemsprint(listx)# [['Hello', 'World'], [0, 1, 2, 3, 3, 5], [True, False]] listx[1][2]=4print(listx)# [['Hello', 'World'], [0, 1, 4, 3, 3, 5], [True, False]] #update value items
How can I get the index of an element contained in the list?
listy =list("HELLO WORLD")print(listy)# ['H', 'E', 'L', 'L', 'O', ' ', 'W', 'O', 'R', 'L', 'D']index = listy.index("L")#get index of the first item whose value is passed as parameterprint(index)# 2index = listy.index("L", 4)#define the index from which you want to searchprint(index)# 9index = listy.index("O", 3, 5)#define the segment of the list to be searchedprint(index)# 4
from collections import dequecolor_list =deque(["Red", "Blue", "Green", "Black"])color_list.append("White")# White arriveprint(color_list)# deque(['Red', 'Blue', 'Green', 'Black', 'White'])color_list.append("Yellow")# Yellow arriveprint(color_list)# deque(['Red', 'Blue', 'Green', 'Black', 'White', 'Yellow'])color_list.popleft()# The first to arrive now leaves# 'Red'print(color_list)# deque(['Blue', 'Green', 'Black', 'White', 'Yellow'])color_list.popleft()# The second to arrive now leaves# 'Blue'print(color_list)# deque(['Green', 'Black', 'White', 'Yellow'])print(color_list)# Remaining queue in order of arrival# deque(['Green', 'Black', 'White', 'Yellow'])
