Collection Data Types I - Python Lecture Slides PDF

Lecture 4: Collection Data Types I Hrishav Tandukar [email protected] Pranchee singh...

Lecture 4: Collection Data Types I Hrishav Tandukar [email protected] Pranchee singh [email protected] CS4051 Fundamentals of Computing Last week branching - if/elif/else iteration - for loops iteration - while loops CS4051 Fundamentals of Computing 2 Today Have seen primitive data types: int,float,bool,string Introduce compound data types Lists Tuples CS4051 Fundamentals of Computing 3 Iteration/Control Flow – for loops iterate (loop) through numbers in a sequence >>> 0 for i in range(5): 1 print(i) 2 # for in range(n): 3 # 4 #... # range generates numbers from 0 to n-1 CS4051 Fundamentals of Computing 4 range(start,stop,step) default values are start = 0 and step = 1, both are optional, but can be overwritten loop until value is stop – 1 mysum = 0 >>> for i in range(5,10): 35 mysum = mysum + i print(mysum) 5+6+7+8+9 = 35 CS4051 Fundamentals of Computing 5 range(start,stop,step) >>> for i in range(5,10,2): 5 print(i) 7 9 for i in range(5,11,2): print(i) >>> 5 for i in range(5,12,2): 7 print(i) 9 11 CS4051 Fundamentals of Computing 6 range(start,stop,step) >>> 5 for i in range(5,0,-1): 4 print(i) 3 2 >>> 1 5 for i in range(5,-1,-1): 4 print(i) 3 2 1 0 CS4051 Fundamentals of Computing 7 Lists ordered sequence of information, accessible by index a list is denoted by square brackets, [] a list contains elements usually of the same type (e.g., all integers) can contain mixed types list elements can be changed so a list is mutable a = [1, 2, 3, 4, 5, 6] # a list b = [1, ”Hello”, 2.0, True] # another list CS4051 Fundamentals of Computing 8 List indices and ordering list indices start at 0 a = [34,22,54,99,45] # a list of numbers 0 1 2 3 4 print(a) # prints out 34 print(a) # prints out 45 print(a) # gives an error CS4051 Fundamentals of Computing 9 List indices and ordering lists support negative indexing too, starting at -1 from the end a = [34,22,54,99,45] # a list of numbers -5 -4 -3 -2 -1 print(a[-1]) # prints out 45 print(a[-5]) # prints out 34 print(a[-6]) # gives an error CS4051 Fundamentals of Computing 10 len() function len() returns the length(the number of items) of a list a = [“h”, “e”, “l”, “l”, “o”] print(len(a)) # prints out 5 b = [] # an empty list print(len(b)) # prints out 0 CS4051 Fundamentals of Computing 11 len() function len() returns the length(the number of items) of a list a = [“h”, “e”, “l”, “l”, “o”] print(len(a)) # prints out 5 b = [] # an empty list print(len(b)) # prints out 0 notice that here a function is taking another function as a parameter, the print function is taking the len function as a parameter which is taking a parameter itself and returning some value CS4051 Fundamentals of Computing 12 Changing elements lists are mutable, their structure and content can be changed assigning to an element at an index changes the value L = [2, 1, 3] L = 5 # puts 5 at index 1 L is now [2, 5, 3] CS4051 Fundamentals of Computing 13 Operations on lists - Add add elements to the end of the list with L.append(element) mutates the list! L = [2,1,3] L.append(5) # L is now [2,1,3,5] CS4051 Fundamentals of Computing 14 Operations on lists - Add add elements to the end of the list with L.append(element) mutates the list! L = [2,1,3] L.append(5) # L is now [2,1,3,5] What is this dot? lists are python objects, everything in Python is an object objects have data, methods/functions (remember OOP) access these functions by object_name.function_name() we will learn more about these later CS4051 Fundamentals of Computing 15 Operations on lists - Add to combine lists together use concatenation, + operator, gives a new list mutate lists with L.extend(some_list), adds a list of numbers to the end L1 = [2, 1] L2 = [4, 5] L3 = L1 + L2 # L3 is [2, 1, 4, 5], L1 & L2 unchanged L1.extend([0, 6]) # L1 is now [2, 1, 0, 6] CS4051 Fundamentals of Computing 16 Operations on lists - Remove delete element at a specific index with del(L[index]) remove element at end of list with L.pop(), returns the removed element remove a specific element with L.remove(element) looks for the element and removes it if element occurs multiple times, removes first occurrence if element not in list, gives an error CS4051 Fundamentals of Computing 17 Operations on lists - Remove L = [2, 1, 3, 6, 3, 7, 0] # a list of numbers L.remove(2) # mutates L = [1, 3, 6, 3, 7, 0] L.remove(3) # mutates L = [1, 6, 3, 7, 0] del(L) # mutates L = [1, 3, 7, 0] L.pop() # returns 0 and mutates L = [1, 3, 7] CS4051 Fundamentals of Computing 18 Iterating over a list remember, list indices start at 0 so any list would have indices from 0 to len(L)-1 L = [2, 43, 21, 5, 46] print(len(L)) # 5 indices 0 1 2 3 4 element 2 43 21 5 46 s the indices of the list need to be generated sequentially in order to access the elements iteratively CS4051 Fundamentals of Computing 19 Iterating over a list- using for loop L = [2, 43, 21, 5, 46] print(len(L)) # 5 indices 0 1 2 3 4 element 2 43 21 5 46 s for i in range(len(L)): print(L[i]) # print the element at index i range(len(L)) -> range(5) -> 0 to 4 which are the indices of the list 20 CS4051 Fundamentals of Computing Iterating over a list – for each loop a more simpler way using for each loop for in :... >>> 2 L = [2, 43, 21, 5, 46] 43 for each in L: print(each) 21 5 46 CS4051 Fundamentals of Computing 21 Iterating over a list – for each loop a more simpler way using for each loop note that each is just a for in : variable here, any other variable name can be used... >>> 2 L = [2, 43, 21, 5, 46] 43 for each in L: print(each) 21 5 46 CS4051 Fundamentals of Computing 22 Iterating through a list – for each loop fruits = [“apple”, “orange”, “mango”] for fruit in fruits: print(fruit) >>> apple orange mango CS4051 Fundamentals of Computing 23 Membership operators can check if an element is a member of a list with in fruits = [“apple”, “orange”, “mango”] if “apple” in fruits: if “potato” in fruits: print(“fruit”) print(“fruit”) else: else: print(“not fruit”) print(“not fruit”) CS4051 Fundamentals of Computing 24 Iterating through a list compute the sum of elements of a list L = [22, 33, 44, 55] L = [22, 33, 44, 55] total = 0 total = 0 for i in range(len(L)): for number in L: total += L[i] total += number print(total) print(total) both would give out the same result here CS4051 Fundamentals of Computing 25 Tuples an ordered sequence of elements can mix element types tuples once created cannot be changed, tuples are immutable represented with parentheses () t = () # empty tuple t = (“python”, 3, 5) # a tuple with 3 elements print(len(t)) # 3 CS4051 Fundamentals of Computing 26 Indices in tuples Indexing is same as in lists, but tuples are immutable t = (1, 23, 43, 4) print(t) # 1 print(t[-1]) # 4 t = 4 # gives error, can’t modify tuples! t.append(12) # error, can’t modify tuples! CS4051 Fundamentals of Computing 27 Iterating through tuples works the same way as for lists T = (22, 33, 44, 55) T = (22, 33, 44, 55) for number in T: for i in range(len(T)): print(number) print(T[i]) since tuples are immutable and cannot be modified, they are useful for storing constants CS4051 Fundamentals of Computing 28 Tuple uses example conveniently used to swap variable values x = y temp = x (x, y) = (y, x) y = x x = y y = temp CS4051 Fundamentals of Computing 29 End of Lecture 4 CS4051 Fundamentals of Computing 30  Thank you ! Any questions ? CS4051 Fundamentals of Computing 31