Introduction To Python Lists

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pythonlistdatastructures

List is the most versatile python data structure and stores an ordered sequence of elements just like your shopping list or to-do-list.In Python, Lists are mutable,meaning that the elements can be altered unlike tuples or even strings.These elements of a list are called items and can be any data type.

Creating Lists in Python

Creating lists in python is quite simple, use square brackets [] and separate items in the list with commas. A list can be empty or contain any number of items with different data types (integers,float,strings etc.).

mylist = [ ]  #empty list
mylist2 = [1,2]  # list containing 2 items with integers
mylist3 = [1, 2, 'hello']  # list with 3 items,mixed data types,integers and a string

Note : a list can also hold another list as an item,these are called nested lists. as show below.

mylist4 = [['python','php'],[1,2,3]  # a nested list

Accessing items in a list

We can access items in a list by

  1. Indexing. We use the index operator [ ].Each item in a list has an assigned index value. It is important to Note that the index starts from 0 in python and must be an integer
    Languages = ['java', 'python', 'perl' 'ruby', 'c#' ]   # define a list
    print(Languages[0]) # Access the first item of a list at index 0
    # Output:java which is the first item on the list.

2. Negative Indexing Python also supports negative indexing.The negative indexing is useful when you want to get the last item in a list because it start accessing a list from the end.

    # define a list 
    Languages = ['java', 'python', 'perl' 'ruby', 'c#' ] 
     print(Languages[-1) #Access the last item of a list
    # Output:  C#

length of list

Python has useful in-built functions that work with list.We will discuss them later but for now, len() function help us in returning the total number of elements in a list.

# define a list
languages = ['java', 'python' ,'perl', 'ruby', 'c#']
# print the length of the list
print(len(languages)) 
# output 5

Slicing of a List

Slices are good for getting a subset of items in a list.It uses the slicing operator : (colon) to extract part of the sequence.

Languages = ['java', 'python', 'perl','ruby', 'c#' ] # define a list 
# [:3] slicing everything up to but not including index 3
print(languages[:3])  
#Outputs: ['java', 'python', 'perl' ]

# define a list 
Languages = ['java', 'python', 'perl','ruby', 'c#' ]
# [3:] slicing everything from index 3 to the last item
print(languages[3:])
#Output: ['ruby', 'c#' ]


Languages = ['java', 'python', 'perl','ruby', 'c#' ]
#elements from beginning to end
print(languages[:])
#Output: ['java', 'python', 'perl','ruby', 'c#' ]

# define a list 
Languages = ['java', 'python', 'perl','ruby', 'c#' ]
# elements from 1st to 3rd
print(languages[0:3])
Output # ['java', 'python', 'perl']

How to change/add elements in a list

list.append(item)

The method list.append(item) will add the item at the end of a list.

   # define a list
   languages = ['java', 'python' ,'perl', 'ruby', 'c#']
   # append c
   languages.append('c') 
   print (languages)
  # Output : ['java', 'python', 'perl', 'ruby', 'c#', 'c']

list.insert(i, item)

This method will insert an item at the ith position in a list , shifting elements to the right.

# define a list
languages = ['java', 'python' ,'perl', 'ruby', 'c#']
# insert c
languages.insert( 0, 'php') 
print languages 
# Output :  ['php', 'java', 'python', 'perl', 'ruby', 'c#']

list.extend(items)

The extend method concatenates lists. Note that you do not call extend with multiple arguments;it takes in a second list as its argument.

# define a list
languages = ['java', 'python' ,'perl', 'ruby', 'c#']

Languages2=['c++', 'c'] # define a second list

languages.extend(Languages2)

print(languages)

# Output ['java', 'python', 'perl', 'ruby', 'c#', 'c++', 'c']

extend method does not return any value,instead it modifies the original list by adding the content of the second list

Deleting List Elements

Use the del keywords to delete an item at specific index

  • single item

       languages = ['java', 'python' ,'perl', 'ruby', 'c#'] # define a list
   del languages[2] # use del keyword
   print(languages)
   # Output removes perl ['java', 'python', 'ruby', 'c#']

  • Multiple items

       languages = ['java', 'python' ,'perl', 'ruby', 'c#'] # define a list
   del languages[1:3]  #delete Multiple items by slicing
   print(languages)

   # Output : ['java', 'ruby', 'c#']

list.remove(item)

It will search and remove only the first occurrence of an item

Languages = ['java', 'python', 'perl','python','ruby', 'c#' ]
Languages.remove('python')  # the first occurrence of python
print(languages) # outputs ['java', 'perl', 'python', 'ruby', 'c#']

It removes the python after java'.Note that the 'python' after perl is still present

list.pop()

Removes and returns the last item of a list

languages = ['java', 'python' ,'perl', 'ruby', 'c#']

print(languages.pop()) # print c#

print(languages)
# output ['java', 'python', 'perl', 'ruby']

list.pop(i)

Removes and returns the ith item of a list

Languages = ['java', 'python', 'perl','ruby', 'c#' ]
print(languages.pop(1)) # removes  index 1 item
#  Output returns Python

list.index(item)

When lists start to get long, it becomes more difficult for us to count out our items to determine at what index position a certain value is located. We can use list.index(item), to return the index in the list where that item is located. If there is more than one item with value item, this method will return the first occurrence.

Summary of list methods

Common List Operations

Concatenation

Concatenation uses the + Operator. It Combines lists.

# define a list
languages = ['java', 'python' ,'perl', 'ruby', 'c#']
print(languages +(['c++', 'c']))
# Output:  ['java', 'python' ,'perl', 'ruby', 'c#', c++, c]

Repetition

uses the * Operator. Concatenates a list a repeated number of times

 mylist = ['strings are  cool'] * 2
 print (mylist)
# output:  ['strings are  cool', 'strings are  cool']

Membership

Keyword In is used to test if an item is a member of a list.

languages = ['java', 'python' ,'perl', 'ruby', 'c#']
if 'python' in languages:
    print('right')

Iteration

For Loop is used to iterate through each element on a list with the keyword in.The for loop allows you to to perform an action for every element in the list.

illustration 1

languages = ['java', 'python' ,'perl', 'ruby', 'c#']
for i in languages:
   print(i)

This will print the all the items in the list one per line as shown below:

java
python
perl
ruby
c#

illustration 2

random_sum = [2,7,8,9]
total = 0
for i in random_sum:
    total +=i
print total

The above code will print 26,the sum of all the items of the list. The for loop requires a variable to hold the items being iterated and the source.

While Loop will first check the condition .If the condition is true,it will keep iterating and terminates the loop once the condition turns false.

languages = ['java', 'python' ,'perl', 'ruby', 'c#']
i = 0
while i <len(languages):
    print(languages[i])
    i = i+3

The above codes will print

java
ruby

Conclusion

Now you know what lists are and how to manipulate them.For further understanding on list, read about list comprehension.