Lists in Python
·Mar 30, 2021·
3 min read
# ============================ # 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 ( * 10) # this will print [0,0,0,0,0,0,0,0,0,0] # concating lists zeros =  * 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
 # 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
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) print ("element is ", ch) # 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 # 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))
fre = g.count(12) # this will count how many times 12 occurred in the 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 # 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 ) # we do not need the return keyword
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