How do I multiply each element in a list by a number

Question

I have a list   my list    1  2  3  4  5    How can I multiply each element in my list by 5  The output should be    5  10  15  20  25

User · Answer

I found it interesting to use list comprehension or map with just one object name x  Note that whenever x is reassigned  its id x  changes  i e  points to a different object  x    1  2  3  id x  2707834975552 x    1 5   x for x in x  id x  2707834976576 x  1 5  3 0  4 5  list map lambda x   2   x   3  x    1 0  2 0  3 0  id x    not reassigned 2707834976576 x   list map lambda x   2   x   3  x   x  1 0  2 0  3 0  id x  2707834980928

User · Answer

You can just use a list comprehension   my list    1  2  3  4  5  my new list    i   5 for i in my list    gt  gt  gt  print my new list   5  10  15  20  25    Note that a list comprehension is generally a more efficient way to do a for loop   my new list      for i in my list      my new list append i   5    gt  gt  gt  print my new list   5  10  15  20  25    As an alternative  here is a solution using the popular Pandas package   import pandas as pd  s   pd Series my list    gt  gt  gt  s   5 0     5 1    10 2    15 3    20 4    25 dtype  int64   Or  if you just want the list    gt  gt  gt   s   5  tolist    5  10  15  20  25

User · Answer

from functools import partial as p from operator import mul map p mul 5  my list    is one way you could do it     your teacher probably knows a much less complicated way that was probably covered in class

User · Answer

Best way is to use list comprehension    def map to list my list  n     multiply every value in my list by n   Use list comprehension      my new list    i   n for i in my list      return my new list   To test  print map to list  1 2 3   -1     Returns   -1  -2  -3

User · Answer

You can do it in-place like so    l    1  2  3  4  5   l       x   5 for x in l    This requires no additional imports and is very pythonic

User · Answer

Since I think you are new with Python  lets do the long way  iterate thru your list using for loop and multiply and append each element to a new list    using for loop  lst    5  20  15  product      for i in lst      product append i 5  print product   using list comprehension  this is also same as using for-loop but more  pythonic   lst    5  20  15   prod    i   5 for i in lst  print prod

User · Answer

With map  not as good  but another approach to the problem   list map lambda x  x 5  5  10  15  20  25     also  if you happen to be using numpy or numpy arrays  you could use this  import numpy as np list np array x    5

User · Answer

Multiplying each element in my list by k   k   5 my list    1 2 3 4  result   list map lambda x  x   k  my list     resulting in   5  10  15  20

User · Answer

A blazingly faster approach is to do the multiplication in a vectorized manner instead of looping over the list  Numpy has already provided a very simply and handy way for this that you can use    gt  gt  gt  import numpy as np  gt  gt  gt    gt  gt  gt  my list   np array  1  2  3  4  5    gt  gt  gt    gt  gt  gt  my list   5 array   5  10  15  20  25     Note that this doesn t work with Python s native lists  If you multiply a number with a list it will repeat the items of the as the size of that number   In  15   my list    1000  In  16   len my list  Out 16   5000   If you want a pure Python-based approach using a list comprehension is basically the most Pythonic way to go   In  6   my list    1  2  3  4  5   In  7    5   i for i in my list  Out 7    5  10  15  20  25    Beside list comprehension  as a pure functional approach  you can also use built-in map   function as following   In  10   list map  5    mul    my list   Out 10    5  10  15  20  25    This code passes all the items within the my list to 5 s   mul   method and returns an iterator-like object  in python-3 x   You can then convert the iterator to list using list   built in function  in Python-2 x you don t need that because map return a list by default    benchmarks   In  18    timeit  5   i for i in my list  463 ns    10 6 ns per loop  mean    std  dev  of 7 runs  1000000 loops each   In  19    timeit list map  5    mul    my list   784 ns    10 7 ns per loop  mean    std  dev  of 7 runs  1000000 loops each   In  20    timeit  5   i for i in my list   100000  20 8 ms    115   s per loop  mean    std  dev  of 7 runs  10 loops each   In  21    timeit list map  5    mul    my list   100000   30 6 ms    169   s per loop  mean    std  dev  of 7 runs  10 loops each    In  24   arr   np array my list   100000   In  25    timeit arr   5 899   s    4 98   s per loop  mean    std  dev  of 7 runs  1000 loops each

[python] How do I multiply each element in a list by a number?

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