Intro Python Data Structures.pptx Intro Python Data Structures.pptx

Lists, Tuples, Sets, Dicts 5/23/2015

List General purpose Most widely used data structure Grow and shrink size as needed Sequence type Sortable Tuple Immutable (can’t add/change) Useful for fixed data Faster than Lists Sequence type Set Store non-duplicate items Very fast access vs Lists Math Set ops (union, intersect) Unordered Dict Key/Value pairs Associative array, like Java HashMap Unordered

SEQUENCES (String, List, Tuple) • indexing: x[6] • slicing: x[1:4] • adding/concatenating: + • multiplying: * • checking membership: in/not in • iterating for i in x: • len(sequence1) • min(sequence1) • max(sequence1) • sum(sequence1[1:3]]) • sorted(list1) • sequence1.count(item) • sequence1.index(item)

• indexing – Access any item in the sequence using its index x = 'frog' print (x[3]) # prints 'g' String x = ['pig', 'cow', 'horse'] print (x[1]) # prints 'cow' List SEQUENCES String List Tuple

• slicing – Slice out substrings, sublists, subtuples using indexes [start : end+1 : step] Code Result Explanation x[1:4] 'omp' Items 1 to 3 x[1:6:2] 'opt' Items 1, 3, 5 x[3:] 'puter' Items 3 to end x[:5] 'compu' Items 0 to 4 x[-1] 'r' Last item x[-3:] 'ter' Last 3 items x[:-2] 'comput' All except last 2 items x = 'computer' SEQUENCES String List Tuple

• adding / concatenating – Combine 2 sequences of the same type using + x = 'horse' + 'shoe' print (x) # prints 'horseshoe' String x = ['pig', 'cow'] + ['horse'] print (x) # prints ['pig', 'cow', 'horse'] List SEQUENCES String List Tuple

• multiplying – Multiply a sequence using * x = ‘bug' * 3 print (x) # prints ‘bugbugbug' String x = [8, 5] * 3 print (x) # prints [8, 5, 8, 5, 8, 5] List SEQUENCES String List Tuple

• checking membership – Test whether an item is in or not in a sequence x = 'bug' print ('u' in x) # prints True String x = ['pig', 'cow', 'horse'] print ('cow' not in x) # prints False List SEQUENCES String List Tuple

• iterating – Iterate through the items in a sequence x = [7, 8, 3] for item in x: print (item * 2) # prints 14, 16, 6 Item x = [7, 8, 3] for index, item in enumerate(x): print (index, item) # prints 0 7, 1 8, 2 3 Index & Item SEQUENCES String List Tuple

• number of items – Count the number of items in a sequence x = 'bug' print (len(x)) # prints 3 String x = ['pig', 'cow', 'horse'] print (len(x)) # prints 3 List SEQUENCES String List Tuple

• minimum – Find the minimum item in a sequence lexicographically – alpha or numeric types, but cannot mix types x = 'bug' print (min(x)) # prints 'b' String x = ['pig', 'cow', 'horse'] print (min(x)) # prints 'cow' List SEQUENCES String List Tuple

• maximum – Find the maximum item in a sequence – alpha or numeric types, but cannot mix types x = 'bug' print (max(x)) # prints 'u' String x = ['pig', 'cow', 'horse'] print (max(x)) # prints 'pig' List SEQUENCES String List Tuple

• sum – Find the sum of items in a sequence – entire sequence must be numeric type x = [5, 7, 'bug‘] print (sum(x)) # error! String -> Error x = [2, 5, 8, 12] print (sum(x)) # prints 27 print (sum(x[-2:])) # prints 20 List SEQUENCES String List Tuple

• sorting – Returns a new list of items in sorted order – Does not change the original list x = 'bug' print (sorted(x))# prints ['b', 'g', 'u'] String x = ['pig', 'cow', 'horse'] print (sorted(x)) # prints ['cow', 'horse', 'pig'] List SEQUENCES String List Tuple

• count (item) – Returns count of an item x = 'hippo' print (x.count('p')) # prints 2 String x = ['pig', 'cow', 'horse', 'cow'] print (x.count('cow')) # prints 2 List SEQUENCES String List Tuple

• index (item) – Returns the index of the first occurrence of an item x = 'hippo' print (x.index('p')) # prints 2 String x = ['pig', 'cow', 'horse', 'cow'] print (x.index('cow')) # prints 1 List SEQUENCES String List Tuple

• unpacking – Unpack the n items of a sequence into n variables x = ['pig', 'cow', 'horse'] a, b, c = x # now a is 'pig' # b is 'cow', # c is 'horse' Note: The number of variables must exactly match the length of the list. SEQUENCES String List Tuple

LISTS All operations from Sequences, plus: • constructors: • del list1[2] delete item from list1 • list1.append(item) appends an item to list1 • list1.extend(sequence1) appends a sequence to list1 • list1.insert(index, item) inserts item at index • list1.pop() pops last item • list1.remove(item) removes first instance of item • list1.reverse() reverses list order • list1.sort() sorts list in place LISTS

• constructors – creating a new list LISTS x = list() x = ['a', 25, 'dog', 8.43] x = list(tuple1) List Comprehension: x = [m for m in range(8)] resulting list: [0, 1, 2, 3, 4, 5, 6, 7] x = [z**2 for z in range(10) if z>4] resulting list: [25, 36, 49, 64, 81]

• delete – Delete a list or an item from a list x = [5, 3, 8, 6] del(x[1]) # [5, 8, 6] del(x) # deletes list x LISTS

• append – Append an item to a list x = [5, 3, 8, 6] x.append(7) # [5, 3, 8, 6, 7] LISTS

• extend – Append an sequence to a list x = [5, 3, 8, 6] y = [12, 13] x.extend(y) # [5, 3, 8, 6, 7, 12, 13] LISTS

• insert – Insert an item at given index x.insert(index, item) x = [5, 3, 8, 6] x.insert(1, 7) # [5, 7, 3, 8, 6] x.insert(1,['a','m']) # [5, ['a', 'm'], 7, 3, 8, 6] LISTS

• pop – Pops last item off the list, and returns item x = [5, 3, 8, 6] x.pop() # [5, 3, 8] # and returns the 6 print(x.pop()) # prints 8 # x is now [5, 3] LISTS

• remove – Remove first instance of an item x = [5, 3, 8, 6, 3] x.remove(3) # [5, 8, 6, 3] LISTS

• reverse – Reverse the order of the list x = [5, 3, 8, 6] x.reverse() # [6, 8, 3, 5] LISTS

• sort – Sort the list in place x = [5, 3, 8, 6] x.sort() # [3, 5, 6, 8] Note: sorted(x) returns a new sorted list without changing the original list x. x.sort() puts the items of x in sorted order (sorts in place). LISTS

TUPLES • Support all operations for Sequences • Immutable, but member objects may be mutable • If the contents of a list shouldn’t change, use a tuple to prevent items from accidently being added, changed or deleted • Tuples are more efficient than lists due to Python’s implementation TUPLES

• constructors – creating a new tuple TUPLES x = () # no-item tuple x = (1,2,3) x = 1, 2, 3 # parenthesis are optional x = 2, # single-item tuple x = tuple(list1) # tuple from list

• immutable – But member objects may be mutable x = (1, 2, 3) del(x[1]) # error! x[1] = 8 # error! x = ([1,2], 3) # 2-item tuple: list and int del(x[0][1]) # ([1], 3) TUPLES

• constructors – creating a new set SETS x = {3,5,3,5} # {5, 3} x = set() # empty set x = set(list1) # new set from list # strips duplicates Set Comprehension: x = {3*x for x in range(10) if x>5} resulting set: {18, 21, 24, 27} but in random order

SETS Description Code Add item to set x x.add(item) Remove item from set x x.remove(item) Get length of set x len(x) Check membership in x item in x item not in x Pop random item from set x x.pop() Delete all items from set x x.clear() • basic set operations

• standard mathematical set operations SETS Set Function Description Code Intersection AND set1 & set2 Union OR set1 | set2 Symmetric Difference XOR set1 ^ set2 Difference In set1 but not in set2 set1 – set2 Subset set2 contains set1 set1 <= set2 Superset set1 contains set2 set1 >= set2

• constructors – creating a new dict DICTIONARIES x = {'pork':25.3, 'beef':33.8, 'chicken':22.7} x = dict([('pork', 25.3),('beef', 33.8),('chicken', 22.7)]) x = dict(pork=25.3, beef=33.8, chicken=22.7)

Description Code Add or change item in dict x x['beef'] = 25.2 Remove item from dict x del x['beef'] Get length of dict x len(x) Check membership in x (only looks in keys, not values) item in x item not in x Delete all items from dict x x.clear() Delete dict x del x • basic dict operations DICTIONARIES

• accessing keys and values in a dict DICTIONARIES x.keys() # returns list of keys in x x.values() # returns list of values in x x.items() # returns list of key-value tuple pairs in x item in x.values() # tests membership in x: returns boolean

• iterating a dict DICTIONARIES for key in x: # iterate keys print(key, x[key]) # print all key/value pairs for k, v in x.items(): # iterate key/value pairs print(k, v) # print all key/value pairs Note: Entries in a dict are in random order.

Intro Python Data Structures.pptx Intro Python Data Structures.pptx

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