L2-Python-Data-Structures

Transcription

1 L2-Python-Dt-Structures Mrch 19, Principl built-in types in Python (Python ) numerics: int, flot, long, complex sequences: str, unicode, list, tuple, byterry, buffer, xrnge mppings: dict files: clsses: instnces: exceptions: 2 Lists The list dt type hs some more methods. Here re ll of the methods of list objects in Python 3: list.ppend(x) Add n item to the end of the list. Equivlent to [len():] = [x]. In [1]: = [1, 2, 4, 5,8, 100,1005] In [2]:.ppend(2) Out[2]: [1, 2, 4, 5, 8, 100, 1005, 2] list.extend(l) Extend the list by ppending ll the items in the given list. Equivlent to [len():] = L. In [3]: b = ["Feng", "Li", "Love"].extend(b) Out[3]: [1, 2, 4, 5, 8, 100, 1005, 2, 'Feng', 'Li', 'Love'] list.insert(i, x) Insert n item t given position. The first rgument is the index of the element before which to insert, so.insert(0, x) inserts t the front of the list, nd.insert(len(), x) is equivlent to.ppend(x). 1

2 In [4]:.insert(1,"Hello") Out[4]: [1, 'Hello', 2, 4, 5, 8, 100, 1005, 2, 'Feng', 'Li', 'Love'] list.remove(x) Remove the first item from the list whose vlue is x. It is n error if there is no such item. list.pop([i]) Remove the item t the given position in the list, nd return it. If no index is specified,.pop() removes nd returns the lst item in the list. (The squre brckets round the i in the method signture denote tht the prmeter is optionl, not tht you should type squre brckets t tht position. You will see this nottion frequently in the Python Librry Reference.) list.cler() Remove ll items from the list. Equivlent to del [:]. In [5]:.pop(2) Out[5]: [1, 'Hello', 4, 5, 8, 100, 1005, 2, 'Feng', 'Li', 'Love'] In [6]:.pop() Out[6]: [1, 'Hello', 4, 5, 8, 100, 1005, 2, 'Feng', 'Li'] In [7]:.remove("Hello") Out[7]: [1, 4, 5, 8, 100, 1005, 2, 'Feng', 'Li'] list.index(x) Return the index in the list of the first item whose vlue is x. It is n error if there is no such item. list.count(x) Return the number of times x ppers in the list. list.sort() Sort the items of the list in plce. In [5]:.ppend("Love") In [6]:.index("Love") Out[6]: 11 In [7]:.count("Love") Out[7]: 2 2

3 In [8]: b = [1,7,2,14] b.sort() b Out[8]: [1, 2, 7, 14] list.reverse() Reverse the elements of the list in plce. list.copy() Return shllow copy of the list. Equivlent to [:]. In [12]:.reverse() Out[12]: ['Love', 'Li', 'Feng', 2, 1005, 100, 8, 5, 4, 1] In [13]: b =.copy() b Out[13]: ['Love', 'Li', 'Feng', 2, 1005, 100, 8, 5, 4, 1] The del sttement There is wy to remove n item from list given its index insted of its vlue: the del sttement. This differs from the pop() method which returns vlue. The del sttement cn lso be used to remove slices from list or cler the entire list In [9]: Out[9]: [1, 'Hello', 2, 4, 5, 8, 100, 1005, 2, 'Feng', 'Li', 'Love', 'Love'] In [10]: del [2] Out[10]: [1, 'Hello', 4, 5, 8, 100, 1005, 2, 'Feng', 'Li', 'Love', 'Love'] del cn lso be used to delete entire vribles In [16]: del # will not be there 2.1 Vribles nd pss-by-reference When ssigning vrible (or nme) in Python, you re creting reference to the object on the right hnd side of the equls sign. In [14]: = [1,2,3,4] b = print() print(b) 3

4 [1, 2, 3, 4] [1, 2, 3, 4] In [15]:.ppend(100) print() print(b) [1, 2, 3, 4, 100] [1, 2, 3, 4, 100] Understnding the semntics of references in Python nd when, how, nd why dt is copied is especilly criticl when working with lrger dt sets in Python. If you relly need to mke hrd copy, use method like list.copy() In [18]: c1 =.copy() c2 = [:].ppend(23232) print() print(c1) print(c2) [1, 2, 3, 4, 100, 23232] [1, 2, 3, 4, 100] [1, 2, 3, 4, 100] 2.2 Using Lists s Stcks The list methods mke it very esy to use list s stck, where the lst element dded is the first element retrieved ( lst-in, first-out ). To dd n item to the top of the stck, use ppend(). To retrieve n item from the top of the stck, use pop() without n explicit index In [19]: stck = [3, 4, 5] In [20]: stck.ppend(2) stck Out[20]: [3, 4, 5, 2] In [21]: stck.pop() stck Out[21]: [3, 4, 5] 4

5 2.3 Using Lists s Queues It is lso possible to use list s queue, where the first element dded is the first element retrieved ( first-in, first-out ); however, lists re not efficient for this purpose. To implement queue, use collections.deque which ws designed to hve fst ppends nd pops from both ends. For exmple: In [22]: from collections import deque queue = deque(["eric", "John", "Michel"]) queue.ppend("terry") # Terry rrives queue.ppend("grhm") # Grhm rrives queue Out[22]: deque(['eric', 'John', 'Michel', 'Terry', 'Grhm']) In [23]: queue.popleft() queue.popleft() queue # The first to rrive now leves # The second to rrive now leves Out[23]: deque(['michel', 'Terry', 'Grhm']) 2.4 List Comprehensions List comprehensions provide concise wy to crete lists. Common pplictions re to mke new lists where ech element is the result of some opertions pplied to ech member of nother sequence or iterble, or to crete subsequence of those elements tht stisfy certin condition. In [16]: squres = [] # the usul wy for x in rnge(10): squres.ppend(x**2) squres Out[16]: [0, 1, 4, 9, 16, 25, 36, 49, 64, 81] We cn clculte the list of squres without ny side effects using the following. This is very similr to R progrmming lnguge s pply type functions. In [17]: squres = [x**2 for x in rnge(10)] squres Out[17]: [0, 1, 4, 9, 16, 25, 36, 49, 64, 81] which eventully did this In [18]: import mth squres = list(mp(lmbd x: mth.exp(x), rnge(10))) squres 5

6 Out[18]: [1.0, , , , , , , , , ] A list comprehension consists of brckets contining n expression followed by for cluse, then zero or more for or if cluses. The result will be new list resulting from evluting the expression in the context of the for nd if cluses which follow it. In [27]: [(x, y) for x in [1,2,3] for y in [3,1,4] if x!= y] Out[27]: [(1, 3), (1, 4), (2, 3), (2, 1), (2, 4), (3, 1), (3, 4)] which is equivlent to: In [28]: combs = [] for x in [1,2,3]: for y in [3,1,4]: if x!= y: combs.ppend((x, y)) combs Out[28]: [(1, 3), (1, 4), (2, 3), (2, 1), (2, 4), (3, 1), (3, 4)] 2.5 Nested List Comprehensions The initil expression in list comprehension cn be ny rbitrry expression, including nother list comprehension. In [29]: mtrix = [[1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12]] mtrix Out[29]: [[1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12]] The following list comprehension will trnspose rows nd columns: In [30]: [[row[i] for row in mtrix] for i in rnge(4)] Out[30]: [[1, 5, 9], [2, 6, 10], [3, 7, 11], [4, 8, 12]] the mgic behind this ws 6

7 In [31]: trnsposed = [] for i in rnge(4): trnsposed.ppend([row[i] for row in mtrix]) trnsposed Out[31]: [[1, 5, 9], [2, 6, 10], [3, 7, 11], [4, 8, 12]] In the rel world, you should prefer built-in functions to complex flow sttements. The zip() function would do gret job for this use cse: In [32]: list(zip(*mtrix)) Out[32]: [(1, 5, 9), (2, 6, 10), (3, 7, 11), (4, 8, 12)] 3 Tuples A tuple consists of number of vlues seprted by comms In [33]: t = 12345, 54321, 'hello!' t[0] Out[33]: In [34]: t Out[34]: (12345, 54321, 'hello!') Tuples my be nested nd tuples re immutble, but they cn contin mutble objects. In [35]: u = t, (1, 2, 3, 4, 5) u Out[35]: ((12345, 54321, 'hello!'), (1, 2, 3, 4, 5)) In [36]: v = ([1, 2, 3], [3, 2, 1]) v Out[36]: ([1, 2, 3], [3, 2, 1]) Note On output tuples re lwys enclosed in prentheses, so tht nested tuples re interpreted correctly; they my be input with or without surrounding prentheses, lthough often prentheses re necessry nywy (if the tuple is prt of lrger expression). It is not possible to ssign to the individul items of tuple, however it is possible to crete tuples which contin mutble objects, such s lists. 7

8 4 Dictionries Unlike sequences, which re indexed by rnge of numbers, dictionries re indexed by keys. Dictionries re sometimes found in other lnguges s ssocitive memories'' orssocitive rrys. The keys cn be ny immutble type; strings nd numbers cn lwys be keys. Tuples cn be used s keys if they contin only strings, numbers, or tuples; if tuple contins ny mutble object either directly or indirectly, it cnnot be used s key. You cn t use lists s keys. It is best to think of dictionry s n unordered set of key:vlue pirs, with the requirement tht the keys re unique (within one dictionry). A pir of brces cretes n empty dictionry: {}. Plcing comm-seprted list of key:vlue pirs within the brces dds initil key:vlue pirs to the dictionry; this is lso the wy dictionries re written on output. The min opertions on dictionry re storing vlue with some key nd extrcting the vlue given the key. In [19]: tel = {'jck': 4098, 'spe': 4139} tel['guido'] = 4127 print(tel) {'guido': 4127, 'jck': 4098, 'spe': 4139} The dict() constructor builds dictionries directly from sequences of key:vlue pirs: In [38]: dict([('spe', 4139), ('guido', 4127), ('jck', 4098)]) Out[38]: {'guido': 4127, 'jck': 4098, 'spe': 4139} When the keys re simple strings, it is sometimes esier to specify pirs using keyword rguments: In [39]: dict(spe=4139, guido=4127, jck=4098) Out[39]: {'guido': 4127, 'jck': 4098, 'spe': 4139} In [20]: list(tel.keys()) Out[20]: ['guido', 'jck', 'spe'] In [21]: sorted(tel.keys()) Out[21]: ['guido', 'jck', 'spe'] In [22]: 'guido' in tel Out[22]: True In [23]: 'jck' not in tel Out[23]: Flse 8

9 It is lso possible to delete key:vlue pir with del. If you store using key tht is lredy in use, the old vlue ssocited with tht key is forgotten. It is n error to extrct vlue using non-existent key. In [25]: del tel['spe'] tel KeyError Trcebck (most recent cll lst) <ipython-input-25-23bb8c95821> in <module>() ----> 1 del tel['spe'] 2 tel KeyError: 'spe' dict comprehensions cn be used to crete dictionries from rbitrry key nd vlue expressions: In [26]: {x: x**2 for x in (2, 4, 6)} Out[26]: {2: 4, 4: 16, 6: 36} 5 Sets A set is n unordered collection with no duplicte elements. Bsic uses include membership testing nd eliminting duplicte entries. Set objects lso support mthemticl opertions like union, intersection, difference, nd symmetric difference. Curly brces {} or the set() function cn be used to crete sets. Note: to crete n empty set you hve to use set(), not {}; the ltter cretes n empty dictionry. In [46]: bsket = {'pple', 'ornge', 'pple', 'per', 'ornge', 'bnn'} print(bsket) # show tht duplictes hve been removed {'pple', 'per', 'ornge', 'bnn'} In [47]: 'ornge' in bsket Out[47]: True In [48]: = set('brcdbr') b = set('lczm') print() print(b) 9

10 {'c', 'b', '', 'd', 'r'} {'c', 'm', 'l', '', 'z'} In [49]: - b Out[49]: {'b', 'd', 'r'} In [50]: b Out[50]: {'', 'b', 'c', 'd', 'l', 'm', 'r', 'z'} In [51]: & b Out[51]: {'', 'c'} In [52]: ^ b # XOR: exclusive OR Out[52]: {'b', 'd', 'l', 'm', 'r', 'z'} In [53]: = {x for x in 'brcdbr' if x not in 'bc'} # set comprehensions In [54]: Out[54]: {'d', 'r'} 6 Looping When looping through dictionries, the key nd corresponding vlue cn be retrieved t the sme time using the items() method. In [55]: knights = {'gllhd': 'the pure', 'robin': 'the brve'} for k, v in knights.items(): print(k, v) robin the brve gllhd the pure When looping through sequence, the position index nd corresponding vlue cn be retrieved t the sme time using the enumerte() function. In [56]: for i, v in enumerte(['tic', 'tc', 'toe']): print(i, v) 0 tic 1 tc 2 toe 10

11 To loop over two or more sequences t the sme time, the entries cn be pired with the zip() function. In [57]: questions = ['nme', 'quest', 'fvorite color'] nswers = ['lncelot', 'the holy gril', 'blue'] for q, in zip(questions, nswers): print('wht is your {0}? It is {1}.'.formt(q, )) Wht is your nme? It is lncelot. Wht is your quest? It is the holy gril. Wht is your fvorite color? It is blue. To loop over sequence in reverse, first specify the sequence in forwrd direction nd then cll the reversed() function. In [58]: for i in reversed(rnge(1, 10, 2)): print(i) To loop over sequence in sorted order, use the sorted() function which returns new sorted list while leving the source unltered. In [59]: bsket = ['pple', 'ornge', 'pple', 'per', 'ornge', 'bnn'] for f in sorted(set(bsket)): print(f) pple bnn ornge per To chnge sequence you re iterting over while inside the loop (for exmple to duplicte certin items), it is recommended tht you first mke copy. Looping over sequence does not implicitly mke copy. The slice nottion mkes this especilly convenient: In [60]: words = ['ct', 'window', 'defenestrte'] for w in words[:]: # Loop over slice copy of the entire list. if len(w) > 6: words.insert(0, w) words 11

12 Out[60]: ['defenestrte', 'ct', 'window', 'defenestrte'] In [ ]: words2 = ['ct', 'window', 'defenestrte'] for w in words2: # No copies. if len(w) > 6: words2.insert(0, w) words2 7 Conditions The conditions used in while nd if sttements cn contin ny opertors, not just comprisons. The comprison opertors in nd not in check whether vlue occurs (does not occur) in sequence. The opertors is nd is not compre whether two objects re relly the sme object; this only mtters for mutble objects like lists. All comprison opertors hve the sme priority, which is lower thn tht of ll numericl opertors. Comprisons cn be chined. For exmple, < b == c tests whether is less thn b nd moreover b equls c. Comprisons my be combined using the Boolen opertors, nd nd or, nd the outcome of comprison (or of ny other Boolen expression) my be negted with not. These hve lower priorities thn comprison opertors; between them, not hs the highest priority nd or the lowest, so tht A nd not B or C is equivlent to ((A nd (not B)) or C). As lwys, prentheses cn be used to express the desired composition. The Boolen opertors nd nd or re so-clled short-circuit opertors: their rguments re evluted from left to right, nd evlution stops s soon s the outcome is determined. For exmple, if A nd C re true but B is flse, A nd B nd C does not evlute the expression C. When used s generl vlue nd not s Boolen, the return vlue of short-circuit opertor is the lst evluted rgument. Keyword (Sclr) Function Bitwise True if nd logicl_nd() & Both True or logicl_or() Either or Both True not logicl_not() ~ Not True logicl_xor() ^ One True nd One Flse 7.1 Logicl Opertors The core logicl opertors re Greter thn: >, greter() Greter thn or equl to: >=, greter_equl() Less thn: <, less() Less thn or equl to: <= less_equl() Equl to: == equl() Not equl to:!= not_equl All comprisons re done element-by-element nd return either True or Flse. Note tht in Python, unlike C, ssignment cnnot occur inside expressions. it voids common clss of problems encountered in C progrms: typing = in n expression when == ws intended. 12

13 7.2 Multiple tests The functions ll nd ny tke logicl input nd re self-descriptive. ll returns True if ll logicl elements in n rry re 1. If ll is clled without ny dditionl rguments on n rry, it returns True if ll elements of the rry re logicl true nd 0 otherwise. ny returns logicl(true) if ny element of n rry is True. 7.3 is* is A number of specil purpose logicl tests re provided to determine if n rry hs specil chrcteristics. Some operte element-by-element nd produce n rry of the sme dimension s the input while other produce only sclrs. These functions ll begin with is. isnn isinf isfinite isposfin, isnegfin isrel iscomplex isrel is_string_like is_numlike issclr isvector 13