Using python map and other functional tools

Question

This is quite n00bish  but I m trying to learn understand functional programming in python  The following code   foos    1 0 2 0 3 0 4 0 5 0  bars    1 2 3   def maptest foo  bar       print foo  bar  map maptest  foos  bars    produces   1 0 1 2 0 2 3 0 3 4 0 None 5 0 None   Q  Is there a way to use map or any other functional tools in python to produce the following without loops etc   1 0  1 2 3  2 0  1 2 3  3 0  1 2 3  4 0  1 2 3  5 0  1 2 3    Just as a side note how would the implementation change if there is a dependency between foo and bar  e g   foos    1 0 2 0 3 0 4 0 5 0  bars    1 2 3 4 5    and print   1 0  2 3 4 5  2 0  1 3 4 5  3 0  1 2 4 5        P S  I know how to do it naively using if  loops and or generators  but I d like to learn how to achieve the same using functional tools  Is it just a case of adding an if statement to maptest or apply another filter map to bars internally within maptest

User · Answer

import itertools  foos  1 0  2 0  3 0  4 0  5 0  bars  1  2  3   print zip foos  itertools cycle  bars

User · Answer

gt  gt  gt  from itertools import repeat  gt  gt  gt  for foo  bars in zip foos  repeat bars            print foo  bars      1 0  1  2  3  2 0  1  2  3  3 0  1  2  3  4 0  1  2  3  5 0  1  2  3

User · Answer

Here s the solution you re looking for    gt  gt  gt  foos    1 0  2 0  3 0  4 0  5 0   gt  gt  gt  bars    1  2  3   gt  gt  gt    x  bars  for x in foos    1 0   1  2  3     2 0   1  2  3     3 0   1  2  3     4 0   1  2  3     5 0    1  2  3      I d recommend using a list comprehension  the   x  bars  for x in foos  part  over using map as it avoids the overhead of a function call on every iteration  which can be very significant    If you re just going to use it in a for loop  you ll get better speeds by using a generator comprehension    gt  gt  gt  y     x  bars  for x in foos   gt  gt  gt  for z in y          print z      1 0   1  2  3    2 0   1  2  3    3 0   1  2  3    4 0   1  2  3    5 0   1  2  3     The difference is that the generator comprehension is lazily loaded   UPDATE  In response to this comment      Of course you know  that you don t copy bars  all entries are the same bars list  So if you modify any one of them  including original bars   you modify all of them    I suppose this is a valid point   There are two solutions to this that I can think of   The most efficient is probably something like this   tbars   tuple bars    x  tbars  for x in foos    Since tuples are immutable  this will prevent bars from being modified through the results of this list comprehension  or generator comprehension if you go that route    If you really need to modify each and every one of the results  you can do this   from copy import copy   x  copy bars   for x in foos    However  this can be a bit expensive both in terms of memory usage and in speed  so I d recommend against it unless you really need to add to each one of them

User · Answer

Would this do it   foos    1 0 2 0 3 0 4 0 5 0  bars    1 2 3   def maptest2 bar     print bar  def maptest foo     print foo   map maptest2  bars   map maptest  foos

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Here s an overview of the parameters to the map function   sequences  function    function is the name of your function  sequences is any number of sequences  which are usually lists or tuples   map will iterate over them simultaneously and give the current values to function   That s why the number of sequences should equal the number of parameters to your function    It sounds like you re trying to iterate for some of function s parameters but keep others constant  and unfortunately map doesn t support that   I found an old proposal to add such a feature to Python  but the map construct is so clean and well-established that I doubt something like that will ever be implemented   Use a workaround like global variables or list comprehensions  as others have suggested

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The easiest way would be not to pass bars through the different functions  but to access it directly from maptest   foos    1 0 2 0 3 0 4 0 5 0  bars    1 2 3   def maptest foo       print foo  bars  map maptest  foos    With your original maptest function you could also use a lambda function in map   map  lambda foo  maptest foo  bars    foos

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Are you familiar with other functional languages  i e  are you trying to learn how python does functional programming  or are you trying to learn about functional programming and using python as the vehicle   Also  do you understand list comprehensions   map f  sequence    is directly equivalent     to    f x  for x in sequence    In fact  I think map   was once slated for removal from python 3 0 as being redundant  that didn t happen    map f  sequence1  sequence2    is mostly equivalent to    f x1  x2  for x1  x2 in zip sequence1  sequence2      there is a difference in how it handles the case where the sequences are of different length   As you saw  map   fills in None when one of the sequences runs out  whereas zip   stops when the shortest sequence stops   So  to address your specific question  you re trying to produce the result   foos 0   bars foos 1   bars foos 2   bars   etc    You could do this by writing a function that takes a single argument and prints it  followed by bars   def maptest x        print x  bars map maptest  foos    Alternatively  you could create a list that looks like this    bars  bars  bars      etc    and use your original maptest   def maptest x  y       print x  y   One way to do this would be to explicitely build the list beforehand   barses    bars    len foos  map maptest  foos  barses    Alternatively  you could pull in the itertools module   itertools contains many clever functions that help you do functional-style lazy-evaluation programming in python   In this case  we want itertools repeat  which will output its argument indefinitely as you iterate over it   This last fact means that if you do   map maptest  foos  itertools repeat bars     you will get endless output  since map   keeps going as long as one of the arguments is still producing output   However  itertools imap is just like map    but stops as soon as the shortest iterable stops   itertools imap maptest  foos  itertools repeat bars     Hope this helps  -       It s a little different in python 3 0   There  map   essentially returns a generator expression

User · Answer

How about this   foos    1 0 2 0 3 0 4 0 5 0  bars    1 2 3   def maptest foo  bar       print foo  bar  map maptest  foos   bars  len foos

User · Answer

Functional programming is about creating side-effect-free code   map is a functional list transformation abstraction   You use it to take a sequence of something and turn it into a sequence of something else   You are trying to use it as an iterator   Don t do that       Here is an example of how you might use map to build the list you want   There are shorter solutions  I d just use comprehensions   but this will help you understand what map does a bit better   def my transform function input       return  input   1  2  3    new list   map my transform  input list    Notice at this point  you ve only done a data manipulation   Now you can print it   for n l in new list      print n  ll   -- I m not sure what you mean by  without loops    fp isn t about avoiding loops  you can t examine every item in a list without visiting each one    It s about avoiding side-effects  thus writing fewer bugs

[python] Using python map and other functional tools

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