Python Data Structures 101: 1. How to LIST??

In this post, I'm going to talk about the list, one of the most common but powerful data structures in Python. Let's get started.

Q1. What is a python list?
 Ans: List is a part of core Python Language. It is implemented as a dynamic array. It is heterogeneous(can hold elements of different data types) and is mutable unlike string and tuples in Python.


Q2. How to declare a list?
Ans: We can use list() to make a list

l = list()

Alternatively, we can use

l = []

Just to be sure, we can view the type of the object by writing

print(type(l))

Q3. How to add items in a list?
Ans: 

• Use append() method: 

  l.append(1) 

  This adds 1 to the list.
• Use insert() method: 

  # Using insert we can insert an element at particular index
  # But this increments the size of list by 1 by shifting all the elements towards right
  l = [1, 2, 3, 4, 5]
  l.insert(2, 0)
  print(l)

Q4.  How to find the length of the list?

length_of_list = len(l)

Q5. How to combine two lists?

A = [1, 2, 3, 4, 5]
B = [6, 7, 8, 9]

Non Pythonic Way
combined_list = list()
for a in A:
    combined_list.append(a)
    
for b in B:
    combined_list.append(b)
print(combined_list)


Using extend()

# extend() is inplace i.e modifies the original list 
A.extend(B)
print(A)


Using + operator
combined_list = A + B
print(combined_list)




Q6. How to modify item in a list? Suppose



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


and we want to set element at index 2 = modified"?


l[2] = "modified"





Q7.  How to remove item from a list?
Ans: 

Using pop() method
# pop() removes the last elemeent from the list
popped_item = l.pop()
print(popped_item)
#Trying to pop from an empty list throws an IndexError
empty_list = []
empty_list.pop()

Using pop() method to delete element at particular index
# We can use pop() method with arguments to delete element at particular index
l = [1, 2, 3, 4]
# Delete element at index 2
l.pop(2)
# Note that pop returns the value of deleted element
Using del
l = [1, 2, 3, 2, 3]
# deletes element ar index 2
del l[2]
# We can use slicing to delete elements
l = [1, 2, 3, 2, 3]
# Deletes elements from index 2 till the end of the list
del l[2: ]
Using remove() method to delete all element by value
# remove() remove the element by value
l = [1, 2, 3, 2, 2]
# removes 2 from the list
# Note that there are multiple occurence of 2 in the list
# remove deletes the first occurence of the 2
print("Before Removing {}".format(l))
l.remove(2)
print("After removing {}".format(l))



Q8. How to reverse the list?
Ans:


using reverse() method:
# reverse() method reverses the list
# It is an inplace operation i.e the original list is modified
l = [1, 2, 3]
l.reverse()
print(l)
using reversed function:
# reversed function returns an reverse iterator to access the list

l = [1, 2, 3]
print(type(reversed(l)))

# TO obtain the reverse list we must first obtain the list from iterator using list(reversed(l)) and assign it to some variable
reversed_list = list(reversed(l))
print(reversed_list)
Using slicing:
l  = [1, 2, 3]
reversed_list = l[: :-1]
print(reversed_list)

Q9. How to find index of particular element?

Ans:

# index() returns the index of irst foccurence of  the element
l = [1, 2, 2, 3, 4, 5]
l.index(2)

Q10. How to count number of occurence of element in list?

Ans:

l = [1, 2, 2, 4, 3, 2, 6]
l.count(2)

Q11. How to clear the list?

ANS:

l = [1, 2, 2]
l.clear()

l = [1, 2, 3]
l = []

Q12. How to copy list?

A = [1 ,2 ,3]
B = A[:]

A = [1, 2, 3]
B = A.copy()

Note: Do not use the below snippet to copy as the memory location is copied instead of actually creating copies of values.

A = [1, 2, 3]
B = A

What if we do?

B[0] = "1"

Does it change the contents of only B or both A and B?


Q13. How to see what methods are available in list?
Ans: 

print(dir(list())

Note: You can use the dir() function for any objects to see what's available

     

Q14. How to make a list of size n initialized with 0?
Ans:

l = [0] * n 

Q15. How to make a list of size m * n initialized with 0?
Ans:

l = [[0] * n] * m