## remove from a list # L = [2,1,3,6,3,7,0] # L.remove(2) # L.remove(3) # del(L[1]) # print(L.pop()) ############# ## Removing elements ############# # L = [2,1,3,6,3,7,0] # L.pop(5) # print(L) # L.remove(3) # print(L) # L.pop() # print(L) ############ YOU TRY IT ############### # This one is similar to remove_elem from lec10 except that remove_elem # returns a new list and this one mutates the parameter L (and returns None) def remove_all(L, e): """ L is a list Mutates L to remove all elements in L that are equal to e Returns None. """ # your code here # Lin = [1,2,2,2] # remove_all(Lin, 2) # print(Lin) # prints [1] # Lin = [1,2,2,2] # remove_all(Lin, 1) # print(Lin) # prints [2, 2, 2] # Lin = [1,2,2,2] # remove_all(Lin, 0) # print(Lin) # prints [1, 2, 2, 2] ####################################### def remove_all(L, e): """ L is a list Mutates L to remove all elements in L that are equal to e Returns None. """ while e in L: L.remove(e) # Lin = [1,2,2,2] # remove_all(Lin, 2) # print(Lin) # prints [1] ## this function does not do the right thing def remove_all(L, e): """ L is a list Mutates L to remove all elements in L that are equal to e Returns None. """ for elem in L: if elem == e: L.remove(e) # Lin = [1,2,2,2] # remove_all(Lin, 2) # print(Lin) # INCORRECTLY prints [1,2] ############# ## TRICKY EXAMPLE 4: removing element as you are mutating a list ############# ## this is an incorrect way to do it def remove_dups(L1, L2): for e in L1: if e in L2: L1.remove(e) # L1 = [10, 20, 30, 40] # L2 = [10, 20, 50, 60] # remove_dups(L1, L2) # print(L1) ## this is an incorrect way to do it def remove_dups(L1, L2): L1_copy = L1 # not actually a copy, just an alias!! for e in L1: if e in L2: L1.remove(e) # L1 = [1, 2, 3, 4] # L2 = [1, 2, 5, 6] # remove_dups(L1, L2) # print(L1) ## this is the CORRECT way to do it def remove_dups(L1, L2): L1_copy = L1[:] # actually a copy aka clone for e in L1_copy: if e in L2: L1.remove(e) # L1 = [1, 2, 3, 4] # L2 = [1, 2, 5, 6] # remove_dups(L1, L2) # print(L1) ############################ ############################ ## Control copying, alises # old_list = [[1,2],[3,4],[5,'foo']] # new_list = old_list # new_list[2][1] = 6 # print("New list:", new_list) # print("Old list:", old_list) ## Control copying, shallow copy # import copy # old_list = [[1,2],[3,4],[5,6]] # new_list = copy.copy(old_list) # old_list.append([7,8]) # old_list[1][1] = 9 # print("New list:", new_list) # print("Old list:", old_list) ## Control copying, deep copy # import copy # old_list = [[1,2],[3,4],[5,6]] # new_list = copy.deepcopy(old_list) # old_list.append([7,8]) # old_list[1][1] = 9 # print("New list:", new_list) # print("Old list:", old_list) ## EXAMPLE: aliasing # a = 1 # b = a # print(a) # print(b) # warm = ['red', 'yellow', 'orange'] # hot = warm # hot.append('pink') # print(hot) # print(warm) ## EXAMPLE: cloning # cool = ['blue', 'green', 'grey'] # chill = cool[:] # chill.append('black') # print(chill) # print(cool) ## EXAMPLE: sorting with/without mutation # warm = ['red', 'yellow', 'orange'] # sortedwarm = warm.sort() # print(warm) # print(sortedwarm) # cool = ['grey', 'green', 'blue'] # sortedcool = sorted(cool) # print(cool) # print(sortedcool) ## EXAMPLE: lists of lists of lists... # warm = ['yellow', 'orange'] # hot = ['red'] # brightcolors = [warm] # brightcolors.append(hot) # print(brightcolors) # hot.append('pink') # print(hot) # print(brightcolors) ############ YOU TRY IT AT HOME ################### # Step through the code below without running it # Write down what values each variable has # Draw the memory diagram to help you keep track of aliases and clones # cool = ['blue', 'green'] # warm = ['red', 'yellow', 'orange'] # print(cool) # print(warm) # colors1 = [cool] # print(colors1) # colors1.append(warm) # print('colors1 = ', colors1) # colors2 = [['blue', 'green'], # ['red', 'yellow', 'orange']] # print('colors2 =', colors2) # warm.remove('red') # print('colors1 = ', colors1) # print('colors2 =', colors2) # for e in colors1: # print('e =', e) # for e in colors1: # if type(e) == list: # for e1 in e: # print(e1) # else: # print(e) # flat = cool + warm # print('flat =', flat) # print(flat.sort()) # print('flat =', flat) # new_flat = sorted(flat, reverse = True) # print('flat =', flat) # print('new_flat =', new_flat) # cool[1] = 'black' # print(cool) # print(colors1) ############################### ############################################ ################### AT HOME ###################### ############################################ def repeat(L, n): """ L is a list of ints n is a positive int Mutates L to contain whatever elements L has right now repeated n times. """ # your code here # Lin = [1,2,3] # repeat(Lin, 3) # print(Lin) # prints [1, 2, 3, 1, 2, 3, 1, 2, 3] # Think about why the following solution does not work! def repeat(L, n): """ L is a list of ints n is a positive int Mutates L to contain whatever elements L has right now repeated n times. """ # your code here Lnew = [] for i in range(n): for e in L: Lnew.append(e) Lin = Lnew # hint, even thought we reuse the name Lin here, we make it point to a NEW object! # Lin = [1,2,3] # repeat(Lin, 3) # print(Lin) # prints [1, 2, 3] which is wrong! ####################################### ########## ANSWERS TO YOU TRY IT ############### ########################################### def remove_all(L, e): """ L is a list Mutates L to remove all elements in L that are equal to e """ Lnew = L[:] L.clear() for elem in Lnew: if elem != e: L.append(elem) # L = [1,2,2,2] # remove_all(L, 1) # print(L) # prints [2, 2, 2] # L = [1,2,2,2] # remove_all(L, 2) # print(L) # prints [1] # L = [1,2,2,2] # remove_all(L, 0) # print(L) # prints [1, 2, 2, 2] ####################################### ########## ANSWERS TO AT HOME ############### ########################################### def repeat(L, n): """ L is a list of ints n is a positive int Mutates L to contain whatever elements L has right now repeated n times. """ # your code here rep = len(L) for i in range(n-1): for j in range(rep): L.append(L[j]) # Lin = [1,2,3] # repeat(Lin, 3) # print(Lin) # prints [1, 2, 3, 1, 2, 3, 1, 2, 3]