# ============================
# Lits
# in lists every element can be of different type
letters = ["a", "b", "c"]
matrix = [[1,2], [3,4]]
# repeating items in a list
print ([0] * 10)
# this will print [0,0,0,0,0,0,0,0,0,0]
# concating lists
zeros = [0] * 5
combined = zeros + letters
# the lists function
numbers = list(range(20)) # as the range function gives an itterable
# this will create a list from 0 to 19
char = list("harshit saini")
Accessing the elements
[0] # access 0th element
[-1] # access 1st element from the last
[0:3] # from elements from 0 to 2
[:3] # from 0 to 2
[0:] # from 0 to end
[:] # from 0 to end
[::2] # from 0 to end and every 2nd element only
[::-1] # get all items in reverse order
List Unpacking
like object destructuring in js
numbers = [1,2,3]
first, second, third = numbers
# the number of variables on the left
# should be equal to numbers of elements in the list
first, *others = numbers
# we get the first and every thing
# else is packed inside others list
# this is called packing
first, *other, last = numbers
# this is how we can take the first and the last element
Looping over the list
l = list("harshit-saini")
for ch in l:
print(ch)
# this way the ch will have only elements in the list
# and it will print only the element
l = list("harshit-saini")
for ch in enumerate(l):
print(ch)
print ("index is" , ch[0])
print ("element is ", ch[1])
# this way ch will be a tuple which contains the
# index and the element itself
Adding and removing items
# adding and removing items
g = [1,2,3,4,5]
g.append(10) # adding to the last
g.insert(0, 10) # inserting 10 at index 0
g.pop() # remove the last element
g.pop(0) # remove element fron index 0
g.remove(1) # remove 1 from the list (first occurance)
# to remove all occurance we have to loop over the list
del g[0:3] # remove a range of items from 0 to 2
del g[0] # delete the item at index 0
Finding elements in a list
index = g.index(1) # get the index of 1
# note : in python if the element is not in the list and we try to print the index, we get error
# so fist we should check if the element is there and then we should find the index
if 1 in g:
print (g.index(1))
Counting elements
fre = g.count(12)
# this will count how many times 12 occurred in the list
Sorting List
numbers.sort() # this sorts in ass order
numbers.sort(reverse="True") # dess order
we can also use the built in function sorted( ). This function takes iterables as arguments. This returns a new sorted list.
new_sorted_list= sorted (numbers_list, reverse="True"
Sometimes we have to create our own sorting function to sort list like these
items = [
("Product", 10),
("Product", 9),
("Product", 12),
]
items.sort() # this will do nothing
# we have to make a function
def sort_item(item):
return item[1]
# now give this function to the sort function
items.sort(key=sort_item) # we do not have to call this function
Using Lamda Function
syntax is => lamda parameters:expression
items.sort( key=lamda item:item[1] )
# we do not need the return keyword