Python List

Q. 1 What is list in python? Also give an example.
Ans.

• • A list is an ordered collection of items separated by commas and enclosed within square brackets ([]).
  • It can contain data of any type.
  • List is mutable, means we can change any data inside the list unlike string.
  • An empty list is denoted by empty square brackets[]
  • g. list = [ ‘abcd’, 26 , ‘j’, 3.14 ]

Q. 2 How to create a python list?
Ans.

• • To create a new Python list, you can either assign empty square brackets L=[ ] or you can use the list constructor L= list().
  • You can also assign any values directly inside square brackets separated by comma.
    L1= L = [ val1, val2,val3, val4….]
  • You can also create nested lists.
    L2 = [val1, val2, [val31, val32], val4]
  • You can create lists from other sequences such as string or tuple.
    L1 = list(“String”)
    L2 = list( (1,2,3,4))
  • Do note that creating a new list by inserting a string in square brackets, behaves differently than using a constructor which takes in a string.

Q. 3 What is the difference between creating two lists like this L1 = [‘Simply’] and L1 = list(‘Simply’)?
Ans. In the first option a list will be created and it will have only 1 element “Simply”. In second one a list will be created from string “Simply” and each individual character will be separate element of the list. So it will give output as [‘S’,’i’, ‘m’, ‘p’, ‘l’ , ‘y’ ]

Q. 4 Give 2 similarities and dis-similarities between lists and Strings.
Ans.

     Similarities

• • Indexing and Slicing work the same
  • In, not in + and * operation works the same

     Dis-Similarities

• • Strings are non-mutable, lists are mutable
  • Each character is stored directly in memory in String. In list each list element is stored as a reference (its address is stored)

Q.5  What is the output when following operators are applied on these list?
             a =  [ ‘Simply’, 2 ]
             b = [ ‘Coding’, 3.14]

1. 1. print (a+b)
   2. print (a*2)
   3. print (2 in a)
   4. print (‘S’ in a)
   5. ‘C’ not in b
   6. 3 not in b

Ans. 

1. 1. [ ‘Simply’, 2, ‘Coding’, 3.14]
   2. [ ‘Simply’, 2, ‘Simply’, 2]
   3. True
   4. False
   5. True
   6. True

Q. 6 Consider following 2 lists, what is the output of following List indexing and Slicing?
               a =  [ ‘Simply’, 2, 4, 10 ]
               b = [ ‘Coding’, 3.14, 55, 14]

1. 1. print (a[0], b[0])
   2. a[-1]
   3. a [10]
   4. a[ :-1]
   5. a[0: ]
   6. a[:2] + a[2:]
   7. a[0:10]

Ans. 

1. 1. Simply Coding
   2. 10
   3. Error
   4. [‘Simply’, 2, 4]
   5. [ ‘Simply’, 2, 4, 10 ]
   6. [ ‘Simply’, 2, 4, 10 ]
   7. [ ‘Simply’, 2, 4, 10 ]

Q. 7 What is the output of List editing commands on following lists?
              a =  [ ‘Simply’, 2, 4, 10 ]
              b = [ ‘Coding’, 3.14, 55, 14]

1. 1. a[0:2] = [0,1]
   2. a[0:2] = [40]
   3. a[1] = 20
   4. del a[0]
   5. a[0: ]
   6. a.pop()
   7. a.pop(1)
   8. a.remove(2)
   9. a.clear()
   10. a.insert(2,40)
   11. a.append(2)
   12. a.extend([3,4])

Ans. 

1. 1. [ 0, 1, 4, 10 ]
   2. [ 40, 4, 10 ]
   3. [ ‘Simply’, 20, 4, 10 ]
   4. [ 2, 4, 10 ]
   5. [ 4, 10 ]
   6. [ ‘Simply’, 2, 4 ]
   7. [ ‘Simply’, 4, 10 ]
   8. [ ‘Simply’, 4, 10 ]
   9. []
   10. [ ‘Simply’, 2, 40, 4, 10]
   11. [ ‘Simply’, 2, 4, 10, 2 ]
   12. [ ‘Simply’, 2, 4, 10, 3, 4 ]

Q. 8 List method ___ is used to add element at the end of the list.

1. 1. insert()
   2. append()
   3. extend()
   4. pop()

Ans. b

Q. 9 extend() Method use to add a ___ to the end of the list

1. 1. List
   2. Number
   3. String
   4. None of above.

Ans. a

Q. 10 Which List Method is used to add element at exact position? Also give Syntax for
Ans. Insert to add a element at specified position.
Syntax: list.insert(index, elmnt)
           elmnt can be anything like string, number, list, etc.

Q. 11 Pop Function is used to ____ element at specific position.

1. 1. Remove
   2. Add
   3. Copy
   4. None of above

Ans. a

Q. 12 _____ List function is used to display elements in ascending or descending order.
Ans. sort()

Q. 13 What is the difference between second and third statement below?
             L1 = [2,3]
             L2 = L1
             L3 = L1.copy()

Ans. The second statement does not creates a copy of L1. IT just assigns another name to L1. So if you make change in L1 it reflects in L2 too.
However third statement makes a true copy of L1. If L1 is changed, it does not impact L3.

Q. 14 Consider the following List and give the output after evaluating the code.

Lst = ['These',"are","a",["few","words"],"that","we","will","use"]
Lst[3:4][0]
[L[1]] + L[3]
remove('a')
print(Lst)
L2= ["simply",'Coding']
Lst.append(L2)
print(Lst)

Ans.

[‘few’, ‘words’]
[‘are’, ‘few’, ‘words’]
[‘These’, ‘are’, [‘few’, ‘words’], ‘that’, ‘we’, ‘will’, ‘use’]
[‘These’, ‘are’, ‘a’, [‘few’, ‘words’], ‘that’, ‘we’, ‘will’, ‘use’, [‘simply’, ‘Coding’]]

Q. 15 Give the output of following code.

a,b,c = [1,2],[1,2],[1,2]
print (a == b)
print (a is b)
d = a
print (a is d)

Ans. output.

True
False
True

Q. 16 Write a Python program to remove duplicates from a list.
Ans.

a = [10,20,30,20,10,50,60,40,80,50,40]
dup_items = set()
uniq_items = []
for x in a:
   if x not in dup_items:
      uniq_items.append(x)
      dup_items.add(x)
print(dup_items)

Q.17  Write a Python program which accepts a sequence of comma-separated numbers from user and generate a list and a tuple with those numbers.
Ans.

values = input("Input some comma separated numbers : ")
list = values.split(",")
tuple = tuple(list)
print('List : ',list)
print('Tuple : ',tuple)

Q. 18 Write a Python program to create a histogram from a given list of integers.
Ans.

def histogram( items ):
  for n in items:
    output = ''
    times = n
    while( times > 0 ):
      output += '*'
      times = times - 1
    print(output)
histogram([2, 3, 6, 5])

Q. 19 Write a Python program to filter the positive numbers from a list.
Ans.

nums = [34, 1, 0, -23]
print("Original numbers in the list: ",nums)
new_nums = list(filter(lambda x: x >0, nums))
print("Positive numbers in the list: ",new_nums)

Q. 20 Write a Python program to remove and print every third number from a list of numbers until the list becomes empty.
Ans.

def remove_nums(int_list):
   #list starts with 0 index
   position = 3 - 1
   idx = 0
   len_list = (len(int_list))
   while len_list>0:
     idx = (position+idx)%len_list
     print(int_list.pop(idx))
     len_list -= 1
nums = [10,20,30,40,50,60,70,80,90]
remove_nums(nums)

Some List methods :