How to replace values at specific indexes of a python list

Question

If I have a list    to modify    5 4 3 2 1 0    And then declare two other lists   indexes    0 1 3 5  replacements    0 0 0 0    How can I take to modify s elements as index to indexes  then set corresponding elements in to modify to replacements  i e  after running  indexes should be  0 0 3 0 1 0    Apparently  I can do this through a for loop   for ind in to modify      indexes to modify ind     replacements ind    But is there other way to do this  Could I use operator itemgetter somehow

User · Answer

Why not just   map s   setitem    a  m

User · Answer

for index in a    This will cause index to take on the values of the elements of a  so using them as indices is not what you want  In Python  we iterate over a container by actually iterating over it    But wait   you say   For each of those elements of a  I need to work with the corresponding element of m  How am I supposed to do that without indices    Simple  We transform a and m into a list of pairs  element from a  element from m   and iterate over the pairs  Which is easy to do - just use the built-in library function zip  as follows   for a element  m element in zip a  m     s a element    m element   To make it work the way you were trying to do it  you would have to get a list of indices to iterate over  This is doable  we can use range len a   for example  But don t do that  That s not how we do things in Python  Actually directly iterating over what you want to iterate over is a beautiful  mind-liberating idea      what about operator itemgetter    Not really relevant here  The purpose of operator itemgetter is to turn the act of indexing into something  into a function-like thing  what we call  a callable    so that it can be used as a callback  for example  a  key  for sorting or min max operations   If we used it here  we d have to re-call it every time through the loop to create a new itemgetter  just so that we could immediately use it once and throw it away  In context  that s just busy-work

User · Answer

You can use operator setitem   from operator import setitem a    5  4  3  2  1  0  ell    0  1  3  5  m    0  0  0  0  for b  c in zip ell  m       setitem a  b  c   gt  gt  gt  a  0  0  3  0  1  0    Is it any more readable or efficient than your solution  I am not sure

User · Answer

The biggest problem with your code is that it s unreadable  Python code rule number one  if it s not readable  no one s gonna look at it for long enough to get any useful information out of it   Always use descriptive variable names   Almost didn t catch the bug in your code  let s see it again with good names  slow-motion replay style   to modify    5 4 3 2 1 0  indexes    0 1 3 5  replacements    0 0 0 0   for index in indexes      to modify indexes index     replacements index        to modify indexes index         indexes index        Yo dawg  I heard you liked indexes  so I put an index inside your indexes       so you can go out of bounds while you go out of bounds    As is obvious when you use descriptive variable names  you re indexing the list of indexes with values from itself  which doesn t make sense in this case   Also when iterating through 2 lists in parallel I like to use the zip function  or izip if you re worried about memory consumption  but I m not one of those iteration purists   So try this instead   for  index  replacement  in zip indexes  replacements       to modify index    replacement   If your problem is only working with lists of numbers then I d say that  steabert has the answer you were looking for with that numpy stuff  However you can t use sequences or other variable-sized data types as elements of numpy arrays  so if your variable to modify has anything like that in it  you re probably best off doing it with a for loop

User · Answer

A little slower  but readable I think    gt  gt  gt  s  l  m   5  4  3  2  1  0    0  1  3  5    0  0  0  0    gt  gt  gt  d   dict zip l  m    gt  gt  gt  d   dict is better then using two list i think  0  0  1  0  3  0  5  0   gt  gt  gt   d get i  j  for i  j in enumerate s    0  0  3  0  1  0

User · Answer

You can solve it using dictionary  to modify    5 4 3 2 1 0  indexes    0 1 3 5  replacements    0 0 0 0   dic      for i in range len indexes        dic indexes i   replacements i  print dic   for index  item in enumerate to modify       for i in indexes          to modify i  dic i  print to modify    The output will be    0  0  1  0  3  0  5  0   0  0  3  0  1  0

User · Answer

numpy has arrays that allow you to use other lists arrays as indices   import numpy S numpy array s  S a  m

[python] How to replace values at specific indexes of a python list?

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