In [11]:
#1. 1. You have given a Python list. Write a program to find value 20 in the list, and if it is present, replace it with 200. Only update the first occurrence of an item.

list1=[]
n= int(input("Enter number of elements in the list:"))

for i in range(0,n):
   lst=input()
   list1.append(lst)
print("first list:",list1)
if '20' in list1:
    index1=list1.index('20')
    list1[index1]='200'
else:
    print("Element 20 is not available in the list" )
print("updated list:",list1)    

Enter number of elements in the list:4
23
34
20
20
first list: ['23', '34', '20', '20']
updated list: ['23', '34', '200', '20']
In [12]:
#2. Given a Python list, write a program to remove all occurrences of item 20.
list1=[]
n= int(input("Enter number of elements in the list:"))

for i in range(0,n):
   lst=input()
   list1.append(lst)
print("first list:",list1)

for i in range(0,list1.count('20')):
    list1.remove('20')
print("updated list:",list1)    

Enter number of elements in the list:4
23
20
34
20
first list: ['23', '20', '34', '20']
updated list: ['23', '34']
In [19]:
#3. Given a two Python list. Write a program to iterate both lists simultaneously and display items from list1 in original order and items from list2 in reverse order.

#Given input : 
#list1 = [10, 20, 30, 40]
#list2 = [100, 200, 300, 400]

list1=[10,20,30,40]
list2=[100,200,300,400]

list2.reverse()
for i,j in zip(list1,list2):
    print(i," ",j)
    print(" ")

10   400
 
20   300
 
30   200
 
40   100
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