My Python Notes-->(List & List Comprehension)

What is a list ?

List is a collection of similar numbers , strings or both.

List is a container which hold elements in a particular order.

   

Creating a list 

To create an empty list

 lst = []

To create a list of number from 1 to 5

num =[1,2,3,4,5]

To create a list of alphabets

alpha = ['a','b','c','d','e']

To create a list  of words     

seasons = ['Summer','Winter','Autumn','Spring']

To create a list of word containing number

pythonReleases = ['python3.4.2','python 2.7.0','python 2.7.9']

To create a list within a list

bigList =[[], 

                [1,2,3,4,5],

                ['a','b','c','d','e'],

                ['Summer','Winter','Autumn','Spring'],

                ['python3.4.2','python 2.7.0','python 2.7.9']

               ]

Note-----Like string elements we can access each and every list element using the index value.

alpha = ['a','b','c','d','e']

To access the 1st element

firstElement = alpha[0]

To access the last element

lastElement = alpha[-1]

Example 1
num = [1,2,3,4,5] seasons = ['Winter','Summer','Autumn','Spring'] pythonReleases = ['python3.4.2','python2.7.0','python2.7.6','python3.3.2'] bigList = [[], ['Winter','Summer','Autumn','Spring'], ['python3.4.2','python2.7.0','python2.7.6','python3.3.2'], [1,2,3,4,5] ] #print list elements print("The seasons are") print(seasons) firstElement = pythonReleases[0] print("The first element in list pythonRelease is %s "%firstElement) lastElement = pythonReleases[-1] print("The last element of the list pythonRelease is %s"%lastElement) #We can find the length of the list using len()--method print("The length of list bigList is %d"%len(bigList)) print("The bigList elements") print(bigList)

Python List Methods

List in python is a class.

Note:List can also be created by using list()--method.
Eg ---list_name = list()

.append(value) ---adds "value" to the end of the list.

Lets consider an empty list

lst = []

To add an element to the list

lst.append(1)

.pop()---remove element from the list

pop()--by default remove the last element

seasons = ['summer', 'winter', 'autumn', 'spring']

seasons.pop()---will remove 'spring' from the list "seasons".

We can remove a particular element based on the index

Syntax-------seasons.pop([index])

seasons.pop(1)--- will remove 'winter' from the list "seasons".

.insert()---Now if we need to insert a value at a particular index we can use insert.

List are Mutable

browsers = ['IE6','Chrome','firefox','UC']

Now the element at index 2 is firefox,  if we what to insert "phantomjs" at index 2 we can do it using insert().

Syntax-------browsers.insert([index],value)

.remove()---remove an element based on its value and not based on the index value.

pythonContainters  = ['list','sets','tuple','dictionary']

pythonContainters.remove('list')---will remove 'list' value from the list "pythonContainers".

If the element to be remove if not found , an error is throw.

Lets try to remove 'jude' from the list 'pythonContainters'

  .count()---count the number of time an element occurs in a given list

lets us create two list

lst = [1,2,1,4]

lst_char = ['a','a','d','e']

Note:List can store duplicate values.

Python String Methods

.joint()---is a string method we can use it to join a list of strings

words = ['my','name' ,'is ','jude ']

If we want to join the above list with a space between each list element

' '.join(words)

If we want to join the above list with '_' between each list element

'_'.join(words)

.split()---is a string method we can convert string into list.

string ="my name is jude"

' '.split(string)-----> will convert the string to a list.

names = "jude job james john joseph"

'j'.split(names)---->will return a list contain word without the letter "j".

Looping over List

For--loop--over single list

Syntax:
            for name in iterable:
                  statements
                  ........
                  ......
          iterable---produces a stream of values
         and assign each value to name.
         for each value of name the statements are executed.

names = ['john','jude','mary','james','joseph']

I want to print the name from the names list using for--loop

for name in names:
      print(name)

Output

Now using for--loop we can print the data and index value as well

enumerate()---help us to make useful pair.

for index,name in enumerate(names):

      print("names[%d] = %s "%(index,name))

 For--loop--over two list

zip()---makes a pair-wise loop

words = ['apple','boy','cat','dog']

alphabets = ['a','b','c','d']

Python List Comprehension

 List-Comprehension allow us to built sequence from other sequence.

Example 2.1--write a program which takes a list and return a new list with square of each element from the given input list(Without list - Comprehension)

def square_num(numbers): num = [] for number in numbers: num.append(number**2) return num print(square_num([1,2,3,4])) print(square_num([3])) print(square_num([]))

Example 2.2--A function which takes a list and return a new list with square of each element from the given list.(Using list-Comprehension)

def square_num(numbers): return [number**2 for number in numbers] print(square_num([1,2,3,4])) print(square_num([3])) print(square_num([]))

Output --  for ex2.1.py and ex2.2.py

List-Comprehension which filter input sequence based on a certain condition.
Note:Condition are optional.

Example 3.1--A function which takes a input sequence of number and return a sequence of numbers greater than zero.(Without List Comprehension)

 def positive_num(numbers): new_list =[] for number in numbers: if number > 0: new_list.append(number) return new_list print(positive_num([-1,-2,1,0,3,4])) print(positive_num([-1,-2]))

Example 3.2--A function which takes a input sequence of number and return a sequence of numbers
greater than Zero( Using List-Comprehension)

def positive_num(numbers): return [number for number in numbers if number>0] print(positive_num([-1,-2,1,0,3,4])) print(positive_num([-1,-2]))

Output -- is for both ex3.1.py and ex3.2.py

 List-Comprehension with multiple conditions

Example 4.1--A function which takes a input sequence of number and return a sequence of even-numbers.(WithOut list-Comprehension)

def even_num(numbers): even = [] for number in numbers: if number > 0 and number % 2 == 0: even.append(number) return even print(even_num([0])) print(even_num([1,3,5,-9])) print(even_num([2,1,4]))

Example 4.2--With List Comprehension def even_num(numbers): return [number for number in numbers if number % 2 == 0 and number > 0] print(even_num([1,2,-6,-9])) print(even_num([])) print(even_num([-4,-6])) Output for ex4.1.py & ex4.2.py

List-Comprehension for two loops

Example 5.1--Working with two loops def two_loops(p,q): com = [] for i in p: for j in q: com.append((i,j)) list1 = [1,2,3,4] list2 = ['a','b','c','d'] print(two_loops(list1,listt2)) Example 5.2--Using list-Comprehension def two_loops(p,q): return [(i,j) for i in p for j in q] list1 = [1,2,3,4] list2 = ['a','b','c','d'] print(two_loops(list1,list2))
Output

Example 6.1 Let us combine two list, string list and  number list(WithOut Comprehension)
Note we can convert an integer into a string using str()--method.

def add_str(a,b): com = [] for i in a: for j in b: com.append((str(i) + j)) return com lst1 = [1,2,3] lst2 = ['a','b','c'] print(add_str(lst1,lst2))

Example 6.2---Repeat example 6.1 Using List Comprehension def add_str(a,b): return [(str(i)+j) for i in a for j in b] lst1 = [1,2,3] lst2 = ['a','b','c'] print(add_str(lst1,lst2))
Output

 Example 7--generating an unity matrix with List Comprehension
A unity matrix =[
                             [1,0,0]
                             [0,1,0]
                             [0,0,1]
                          ]

Example 8--generate a diagonal matrix whose diagonal element are equal to the row number
diagonal_matrix =[
                               [1,0,0,]
                               [0,2,0],
                               [0,0,3]
                              ]

Example 9--generate a diagonal matrix whose diagonal element are equal to square of the row number.
square_matrix =[
                               [1,0,0]
                               [0,4,0]
                               [0,0,9]
                              ]