Python - replace boolean value of a list with values ​​from two different lists

Question

I have one list with boolean values ​​like

lyst = [True,True,False,True,False] 

and two different lists, for example:

 car = ['BMW','VW','Volvo'] a = ['b','c'] 

I just want to replace True with values ​​from car and False with values ​​from a . or create a new list with the sequence lyst and the values ​​from car and a . The result should be

 [BMW,VW,b,Volvo,c]. 

my code is:

 for elem1,elem2,elem3 in zip(lyst,car,a): subelem2=elem2 subelem3=elem3 if elem1 != True: result_list.append(subelem2) else: result_list.append(subelem3) 

but this creates a list match of more than 5.

How can i do this?

Answer 1

 car = iter(car) a = iter(a) [next(car) if item else next(a) for item in lyst] 

---

Well, I could not help myself:

 car = iter(car) a = iter(a) [(flag and next(car)) or next(a) for flag in lyst] 

This uses some logical expression functions:

• boolean and will NOT evaluate the second operand if the first operand is False
• boolean and will return the second operand (the actual object) if both operands evaluate to True
• expression boolean or will return the first object that evaluates to True

A potential problem would be that any elements in car or a are evaluated as False. Also, it is not as readable as the triple in the first solution.

But it was fun.

---

I think I will add that after looking at this, I probably shouldn’t reassign the original list names - I should have chosen something else for the iterator names. After exhaustion, iterators cannot be reset, and since the names of the lists have been reassigned, the original information of the list is lost.

Answer 2

Assuming lyst has the correct number of True and False entries, you can do this efficiently by creating iterators of two other lists. This is more efficient than pop ing from the front lists.

 lyst = [True, True, False, True, False] car = ['BMW', 'VW', 'Volvo'] a = ['b', 'c'] selector = [iter(a), iter(car)] result = [next(selector[b]) for b in lyst] print(result) 

Output

 ['BMW', 'VW', 'b', 'Volvo', 'c'] 

The selector[b] construct works because False has a numeric value of 0 and True has a numeric value of 1, so we can use the logical values ​​of lyst to index into a selector list.

---

Delays

I decided to write a timeit script to compare the speeds of the various algorithms posted here, as well as the vaultah code from the target error question .

To test these algorithms, I included them in functions. To obtain accurate timeit timing timeit , the function is executed several times. Algorithms that use .pop consume the lists from which they appear, so I had to add code to these functions to create copies of these lists. Obviously, this copying process slows down these functions, and in some applications it may not matter that the input data lists are destroyed, but I believe that copying time should be included in order to sufficiently compare the speeds of these functions for more general use.

timeit measures the time of the wall clock, not the processor time, so other processes running on the system will affect the timings. Therefore, when running the timeit code, timeit recommended to reduce the load on the system as much as possible. Ideally, you should disable network access, or at least avoid using your browser while testing. And definitely don’t listen to music or watch the video. :)

The code below runs on Python 2.6+, as well as Python 3. I tested it on Python 2.6.6 and Python 3.6.0.

 #!/usr/bin/env python3 ''' Merge two lists according to a list of booleans Given data lists `alist` and `blist`, iterate over boolean list `chooser`. If the current item in `chooser` is False, append the next item from `alist` to the output list. If the current item in `chooser` is True, append the next item from `blist` to the output list. See https://stackoverflow.com/q/42028606/4014959 Speed tests of implementations by various authors Timing code written by PM 2Ring 2016.12.28 ''' # Python 2 / 3 compatible from __future__ import print_function, division from timeit import Timer from random import seed, shuffle seed(1234) # The functions to test def merge_wwii_ifelse(alist, blist, chooser): alist = iter(alist) blist = iter(blist) return [next(blist) if item else next(alist) for item in chooser] def merge_wwii_andor(alist, blist, chooser): alist = iter(alist) blist = iter(blist) return [(flag and next(blist)) or next(alist) for flag in chooser] def merge_PM2R_it(alist, blist, chooser): selector = [iter(alist), iter(blist)] return [next(selector[b]) for b in chooser] def merge_PM2R_pop(alist, blist, chooser): alist = list(alist) blist = list(blist) return [blist.pop() if item else alist.pop() for item in reversed(chooser)][::-1] def merge_AlexL(alist, blist, chooser): alist = list(alist) blist = list(blist) return [blist.pop(0) if item else alist.pop(0) for item in chooser] def merge_Laurent(alist, blist, chooser): selector = {True: iter(blist), False: iter(alist)} return [next(selector[flag]) for flag in chooser] def merge_Shihab(alist, blist, chooser): i = j = 0 ans = [] for v in chooser: if v: ans.append(blist[i]) i += 1 else: ans.append(alist[j]) j += 1 return ans def merge_EvKounis(alist, blist, chooser): mapping = {True: list(blist), False: list(alist)} return [mapping[c].pop(0) for c in chooser] def merge_vaultah(alist, blist, chooser): i1 = iter(alist) i2 = iter(blist) return [next(i2 if x else i1) for x in chooser] funcs = ( merge_wwii_ifelse, merge_wwii_andor, merge_PM2R_it, merge_PM2R_pop, merge_AlexL, merge_Laurent, merge_Shihab, merge_EvKounis, merge_vaultah, ) # - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - def qdata_test(): print('Testing all functions with the question data') alist, blist, chooser = ( ['b', 'c'], ['BMW', 'VW', 'Volvo'], [True, True, False, True, False], ) expected = ['BMW', 'VW', 'b', 'Volvo', 'c'] print('Expected:', expected) for func in funcs: print(func.__name__, func(alist, blist, chooser) == expected) def make_data(size): ''' Create two equal-sized data lists and a random chooser list ''' a = list(range(1, size + 1)) b = [-u for u in a] c = [False] * size + [True] * size shuffle(c) return a, b, c def verify(): results = [func(alist, blist, chooser) for func in funcs] first = results[0] return all(first == u for u in results[1:]) def time_test(loops, reps): ''' Print timing stats for all the functions ''' timings = [] for func in funcs: fname = func.__name__ setup = 'from __main__ import alist, blist, chooser, ' + fname cmd = '{0}(alist, blist, chooser)'.format(fname) t = Timer(cmd, setup) result = t.repeat(reps, loops) result.sort() timings.append((result, fname)) timings.sort() for result, fname in timings: print('{0:18} {1}'.format(fname, result)) qdata_test() maxloops = 5 ** 7 for i in range(1, 7): size = 5 ** i loops, reps = maxloops // size, 3 print('\nList size = {0}, loops = {1}'.format(size, loops)) alist, blist, chooser = make_data(size) print('All functions verified', verify()) time_test(loops, reps) 

Python 3 exit

 Testing all functions with the question data Expected: ['BMW', 'VW', 'b', 'Volvo', 'c'] merge_wwii_ifelse True merge_wwii_andor True merge_PM2R_it True merge_PM2R_pop True merge_AlexL True merge_Laurent True merge_Shihab True merge_EvKounis True merge_vaultah True List size = 5, loops = 15625 All functions verified True merge_Shihab [0.1553873940010817, 0.15573736099941016, 0.1570748160011135] merge_vaultah [0.19238512099946092, 0.19444048600053065, 0.19622620799964352] merge_wwii_ifelse [0.1928826420007681, 0.20096588600063114, 0.2038269639997452] merge_wwii_andor [0.19839437199880194, 0.19887267900048755, 0.1997791949997918] merge_PM2R_it [0.21258554000087315, 0.21627756400084763, 0.21734632500010775] merge_Laurent [0.22584542599906854, 0.22584707799978787, 0.2275525179993565] merge_PM2R_pop [0.252919409000242, 0.2563238849998015, 0.28485306599941396] merge_AlexL [0.295069166999383, 0.2970452879999357, 0.3192335510011617] merge_EvKounis [0.3210714779997943, 0.32545770300021104, 0.3283817559986346] List size = 25, loops = 3125 All functions verified True merge_wwii_andor [0.13853849199949764, 0.13861457399980281, 0.16665312800068932] merge_vaultah [0.1385455899999215, 0.13872790400091617, 0.13888424499964458] merge_wwii_ifelse [0.14001116700092098, 0.1411627879988373, 0.14938874099971144] merge_PM2R_pop [0.1415618489991175, 0.14180967100037378, 0.14234910300001502] merge_Shihab [0.1417505749996053, 0.14575945399883494, 0.1469801869989169] merge_PM2R_it [0.15280632599933597, 0.1578172520003136, 0.20611876800103346] merge_Laurent [0.15372244299942395, 0.15373632599948905, 0.15608775699911348] merge_AlexL [0.19783953799924348, 0.19835066099949472, 0.19853855400106113] merge_EvKounis [0.21501469599934353, 0.21525065300011192, 0.21861338300004718] List size = 125, loops = 625 All functions verified True merge_PM2R_pop [0.10481183099909686, 0.10530150200065691, 0.1054428099996585] merge_vaultah [0.12480986199989275, 0.125469589000204, 0.12560611799926846] merge_wwii_andor [0.1248406070008059, 0.1251086979991669, 0.12896737399933045] merge_wwii_ifelse [0.1248461369996221, 0.12487849500030279, 0.1357243729999027] merge_Shihab [0.13901417600027344, 0.14433371599989187, 0.14757765399917844] merge_Laurent [0.14081091899970488, 0.14126963899980183, 0.21554046200071753] merge_PM2R_it [0.14273404000050505, 0.1436745359987981, 0.14957189699998708] merge_AlexL [0.1876233860002685, 0.18891249499938567, 0.2016287040005409] merge_EvKounis [0.20533967399933317, 0.2060009060005541, 0.2169854090006993] List size = 625, loops = 125 All functions verified True merge_PM2R_pop [0.10215267800049332, 0.10283978299958108, 0.10370849799983262] merge_vaultah [0.12245723900014127, 0.1226432970015594, 0.12315383100030886] merge_wwii_ifelse [0.12513774199942418, 0.12616848099969502, 0.12886606600113737] merge_wwii_andor [0.12801914100055, 0.12805822200061812, 0.12933381000038935] merge_PM2R_it [0.137118164999265, 0.13727735000065877, 0.1388498169999366] merge_Laurent [0.138064671000393, 0.1399875509996491, 0.1408491909987788] merge_Shihab [0.14056324699959077, 0.14599118399928557, 0.190011668999432] merge_AlexL [0.2494661869986885, 0.24949409599867067, 0.25023536899971077] merge_EvKounis [0.2604512350008008, 0.2616422920000332, 0.26348238600076] List size = 3125, loops = 25 All functions verified True merge_PM2R_pop [0.10108045999913884, 0.1956245539986412, 0.19625802900009148] merge_vaultah [0.1205880960005743, 0.12112638399958087, 0.12208285199994862] merge_wwii_andor [0.12255161999928532, 0.12333094499990693, 0.12521908300004725] merge_wwii_ifelse [0.12545375700028671, 0.12909694299924013, 0.1335049829995114] merge_PM2R_it [0.13350266700035718, 0.1339889190003305, 0.13768064900068566] merge_Laurent [0.13691045599989593, 0.1544653910004854, 0.15945969300082652] merge_Shihab [0.13996293700074602, 0.14073836100033077, 0.15528064499994798] merge_AlexL [0.5525499420000415, 0.562283696999657, 0.954857885999445] merge_EvKounis [0.5699720739994518, 0.5713485720007156, 0.57778780100125] List size = 15625, loops = 5 All functions verified True merge_PM2R_pop [0.10786392400041223, 0.10926139099865395, 0.10992666599850054] merge_vaultah [0.12552327299999888, 0.1260173209993809, 0.126270750999538] merge_wwii_ifelse [0.13077028499901644, 0.13549192800019227, 0.13690466499974718] merge_wwii_andor [0.13939281700004358, 0.14145590299995092, 0.1771287490009854] merge_Laurent [0.14111154199963494, 0.141571592001128, 0.14596649399936723] merge_PM2R_it [0.14352190899990092, 0.1444555029993353, 0.1446186849989317] merge_Shihab [0.1475988830006827, 0.14831254499949864, 0.15240716699918266] merge_AlexL [2.040518506999433, 2.0523803409996617, 2.1246015890010312] merge_EvKounis [2.0611778700003924, 2.0809960999995383, 2.0933345120010927] 

Python 2 exit

 Testing all functions with the question data Expected: ['BMW', 'VW', 'b', 'Volvo', 'c'] merge_wwii_ifelse True merge_wwii_andor True merge_PM2R_it True merge_PM2R_pop True merge_AlexL True merge_Laurent True merge_Shihab True merge_EvKounis True merge_vaultah True List size = 5, loops = 15625 All functions verified True merge_wwii_ifelse [0.13266801834106445, 0.13341188430786133, 0.14590716361999512] merge_vaultah [0.1372370719909668, 0.20317387580871582, 0.28184700012207031] merge_wwii_andor [0.1373291015625, 0.14022302627563477, 0.14162182807922363] merge_Shihab [0.14127588272094727, 0.14717292785644531, 0.15103507041931152] merge_PM2R_it [0.16237378120422363, 0.16309309005737305, 0.1668241024017334] merge_Laurent [0.18718504905700684, 0.21074390411376953, 0.24513697624206543] merge_PM2R_pop [0.28445720672607422, 0.29493808746337891, 0.32616496086120605] merge_EvKounis [0.29379916191101074, 0.33054614067077637, 0.34055399894714355] merge_AlexL [0.29791903495788574, 0.29801201820373535, 0.31824994087219238] List size = 25, loops = 3125 All functions verified True merge_vaultah [0.10121989250183105, 0.10190892219543457, 0.10230588912963867] merge_wwii_andor [0.10298919677734375, 0.10324811935424805, 0.1042180061340332] merge_wwii_ifelse [0.10428714752197266, 0.1065058708190918, 0.13893890380859375] merge_PM2R_it [0.11803603172302246, 0.11927890777587891, 0.12329411506652832] merge_Shihab [0.12048506736755371, 0.1209111213684082, 0.14031100273132324] merge_Laurent [0.12093997001647949, 0.12250900268554688, 0.12366700172424316] merge_PM2R_pop [0.14476203918457031, 0.14788198471069336, 0.15009903907775879] merge_AlexL [0.2096550464630127, 0.2101140022277832, 0.21013402938842773] merge_EvKounis [0.22748994827270508, 0.24114799499511719, 0.27338886260986328] List size = 125, loops = 625 All functions verified True merge_wwii_ifelse [0.093290090560913086, 0.095035076141357422, 0.10503697395324707] merge_wwii_andor [0.095438957214355469, 0.096124887466430664, 0.096685171127319336] merge_vaultah [0.096237897872924805, 0.096857070922851562, 0.10109710693359375] merge_Laurent [0.11233901977539062, 0.11253595352172852, 0.11261391639709473] merge_PM2R_it [0.11286282539367676, 0.11314296722412109, 0.11331295967102051] merge_PM2R_pop [0.11631202697753906, 0.11657595634460449, 0.11874699592590332] merge_Shihab [0.12019991874694824, 0.12040495872497559, 0.12069797515869141] merge_AlexL [0.21399688720703125, 0.21718001365661621, 0.23727011680603027] merge_EvKounis [0.22643208503723145, 0.22728395462036133, 0.22952795028686523] List size = 625, loops = 125 All functions verified True merge_wwii_ifelse [0.091418981552124023, 0.091669797897338867, 0.097893953323364258] merge_vaultah [0.092507839202880859, 0.09267878532409668, 0.09857487678527832] merge_wwii_andor [0.094152212142944336, 0.094262123107910156, 0.095048904418945312] merge_PM2R_pop [0.10612797737121582, 0.10874819755554199, 0.15479302406311035] merge_Laurent [0.10751605033874512, 0.10817885398864746, 0.1108400821685791] merge_PM2R_it [0.10856819152832031, 0.10870695114135742, 0.10909485816955566] merge_Shihab [0.12109684944152832, 0.12231802940368652, 0.12318301200866699] merge_AlexL [0.26717281341552734, 0.27173185348510742, 0.27331304550170898] merge_EvKounis [0.27798080444335938, 0.27915000915527344, 0.28347897529602051] List size = 3125, loops = 25 All functions verified True merge_wwii_ifelse [0.089553117752075195, 0.090708017349243164, 0.096336126327514648] merge_wwii_andor [0.092846870422363281, 0.092998027801513672, 0.09307098388671875] merge_vaultah [0.093343973159790039, 0.096785068511962891, 0.11030793190002441] merge_Laurent [0.10524392127990723, 0.10576510429382324, 0.10692310333251953] merge_PM2R_pop [0.10529899597167969, 0.10684394836425781, 0.1188349723815918] merge_PM2R_it [0.1075129508972168, 0.1087181568145752, 0.11004185676574707] merge_Shihab [0.12116694450378418, 0.1221461296081543, 0.12650799751281738] merge_AlexL [0.57863092422485352, 0.58119797706604004, 0.62129878997802734] merge_EvKounis [0.59500694274902344, 0.59542298316955566, 0.59793877601623535] List size = 15625, loops = 5 All functions verified True merge_wwii_ifelse [0.093538999557495117, 0.094748973846435547, 0.099496126174926758] merge_wwii_andor [0.097050189971923828, 0.097167015075683594, 0.14334392547607422] merge_vaultah [0.097479820251464844, 0.097939968109130859, 0.10231399536132812] merge_Laurent [0.10894298553466797, 0.10906291007995605, 0.11107587814331055] merge_PM2R_pop [0.11378717422485352, 0.11389708518981934, 0.11422491073608398] merge_PM2R_it [0.11379504203796387, 0.1164860725402832, 0.11690497398376465] merge_Shihab [0.12729287147521973, 0.12743711471557617, 0.12863397598266602] merge_AlexL [2.062777042388916, 2.0632898807525635, 2.104421854019165] merge_EvKounis [2.0776879787445068, 2.1075038909912109, 2.1273050308227539] 

These results were obtained from my rather ancient 32-bit single-core 2GHZ machine with 2 GB of RAM working with the Debian manufacturer of Linux.

Answer 3

 >>> lyst = [True,True,False,True,False] >>> car = ['BMW','VW','Volvo'] >>> a = ['b', 'c'] >>> [car.pop(0) if item else a.pop(0) for item in lyst] ['BMW', 'VW', 'b', 'Volvo', 'c'] >>> 

Answer 4

My first idea is to use dict to select True (car) and False (a) values.

Since the values ​​are in order, you can do an iteration.

Here is a solution that uses the efficient functions iter() and next() :

 lyst = [True, True, False, True, False] car = ['BMW', 'VW', 'Volvo'] a = ['b', 'c'] selector = {True: iter(car), False: iter(a)} result = [next(selector[flag]) for flag in lyst] print(result) # -> ['BMW', 'VW', 'b', 'Volvo', 'c'] 

You can generalize this solution if you have more complex options than True / False.

Answer 5

The code by @Alex is great, but the complexity is a bit bad due to this pop (0) operation. To avoid this - `

 >>> lyst = [True,True,False,True,False] >>> car = ['BMW','VW','Volvo'] >>> a = ['b', 'c'] >>> i=0 >>> j=0 >>> ans=[] >>> for v in lyst: if v: ans.append(car[i]) i+=1 else: ans.append(a[j]) j+=1 >>> ans ['BMW', 'VW', 'b', 'Volvo', 'c'] 

As for the other answers, I'm a little surprised to see the use of all these complex syntaxes and data structures for such an easy task.

Answer 6

Another option, similar to one from @AlexL, but with if replaced by dictionary lookups.

 car = ["BMW", "VW", "Volvo"] a = ["b", "c"] my_list = [True, True, False, True, False] mapping = {True: car, False: a} new_list = [mapping[c].pop(0) for c in my_list]