How to perform element-wise multiplication of two lists

Question

I want to perform an element wise multiplication  to multiply two lists together by value in Python  like we can do it in Matlab   This is how I would do it in Matlab   a    1 2 3 4  b    2 3 4 5  a    b    2  6  12  20    A list comprehension would give 16 list entries  for every combination x   y of x from a and y from b  Unsure of how to map this   If anyone is interested why  I have a dataset  and want to multiply it by Numpy linspace 1 0  0 5  num len dataset

User · Answer

you can use this for lists of the same length  def lstsum a  b       c 0     pos   0 for element in a     c   element b pos     pos  1 return c

User · Answer

Can use enumerate   a    1  2  3  4  b    2  3  4  5   ab    val   b i  for i  val in enumerate a

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Yet another answer   -1     requires import  1     is very readable  import operator a    1 2 3 4  b    10 11 12 13   list map operator mul  a  b     outputs  10  22  36  52

User · Answer

For large lists  we can do it the iter-way   product iter object   itertools imap operator mul   1 2 3 4    2 3 4 5     product iter object next   gives each of the element in the output list   The output would be the length of the shorter of the two input lists

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To maintain the list type  and do it in one line  after importing numpy as np  of course    list np array  1 2 3 4     np array  2 3 4 5      or  list np array a    np array b

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you can multiplication using lambda   foo  1 2 3 4  bar  1 2 5 55  l map lambda x y x y foo bar

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Use np multiply a b    import numpy as np a    1 2 3 4  b    2 3 4 5  np multiply a b

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Use a list comprehension mixed with zip       a b for a b in zip lista listb

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import ast sys input str   sys stdin read    input list   ast literal eval input str   list 1   input list 0   list 2   input list 1   import numpy as np  array 1   np array list 1   array 2   np array list 2   array 3   array 1 array 2   print list array 3

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Fairly intuitive way of doing this   a    1 2 3 4  b    2 3 4 5  ab                              Create empty list for i in range 0  len a         ab append a i  b i         Adds each element to the list

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The map function can be very useful here  Using map we can apply any function to each element of an iterable   Python 3 x   gt  gt  gt  def my mul x y           return x y      gt  gt  gt  a    1 2 3 4   gt  gt  gt  b    2 3 4 5   gt  gt  gt   gt  gt  gt  list map my mul a b    2  6  12  20   gt  gt  gt    Of course    map f  iterable    is equivalent to    f x  for x in iterable    So we can get our solution via    gt  gt  gt   my mul x y  for x  y in zip a b    2  6  12  20   gt  gt  gt    In Python 2 x map   means  apply a function to each element of an iterable and construct a new list  In Python 3 x  map construct iterators instead of lists    Instead of my mul we could use  mul operator  Python 2 7   gt  gt  gt from operator import mul   import mul operator  gt  gt  gt a    1 2 3 4   gt  gt  gt b    2 3 4 5   gt  gt  gt map mul a b   2  6  12  20   gt  gt  gt    Python 3 5    gt  gt  gt  from operator import mul  gt  gt  gt  a    1 2 3 4   gt  gt  gt  b    2 3 4 5   gt  gt  gt    map mul a b    2  6  12  20   gt  gt  gt    Please note that since map   constructs an iterator we use   iterable unpacking operator to get a list  The unpacking approach is a bit faster then the list constructor    gt  gt  gt  list map mul a b    2  6  12  20   gt  gt  gt

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Since you re already using numpy  it makes sense to store your data in a numpy array rather than a list  Once you do this  you get things like element-wise products for free   In  1   import numpy as np  In  2   a   np array  1 2 3 4    In  3   b   np array  2 3 4 5    In  4   a   b Out 4   array   2   6  12  20

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create an array of ones  multiply each list times the array   convert array to a list  import numpy as np  a    1 2 3 4  b    2 3 4 5   c    np ones len a   a b  tolist     2 0  6 0  12 0  20 0

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gahooa s answer is correct for the question as phrased in the heading  but if the lists are already numpy format or larger than ten it will be MUCH faster  3 orders of magnitude  as well as more readable  to do simple numpy multiplication as suggested by NPE  I get these timings   0 0049ms - gt  N   4  a    i for i in range N    c    a b for a b in zip a  b   0 0075ms - gt  N   4  a    i for i in range N    c   a   b 0 0167ms - gt  N   4  a   np arange N   c    a b for a b in zip a  b   0 0013ms - gt  N   4  a   np arange N   c   a   b 0 0171ms - gt  N   40  a    i for i in range N    c    a b for a b in zip a  b   0 0095ms - gt  N   40  a    i for i in range N    c   a   b 0 1077ms - gt  N   40  a   np arange N   c    a b for a b in zip a  b   0 0013ms - gt  N   40  a   np arange N   c   a   b 0 1485ms - gt  N   400  a    i for i in range N    c    a b for a b in zip a  b   0 0397ms - gt  N   400  a    i for i in range N    c   a   b 1 0348ms - gt  N   400  a   np arange N   c    a b for a b in zip a  b   0 0020ms - gt  N   400  a   np arange N   c   a   b   i e  from the following test program   import timeit  init        import numpy as np N      a      b   np linspace 0 0  0 5  len a       format i  j  for i in  4  40  400                     for j in    i for i in range N      np arange N      func       c    a b for a b in zip a  b          c   a   b      for i in init    for f in func      lines   i split   n       print    6 4f ms - gt              format             timeit timeit f  setup i  number 1000   lines 2   lines 3   f

User · Answer

You can try multiplying each element in a loop  The short hand for doing that is  ab    a i  b i  for i in range len a

[python] How to perform element-wise multiplication of two lists?

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