Extract elements of list at odd positions

Question

So I want to create a list which is a sublist of some existing list   For example   L    1  2  3  4  5  6  7   I want to create a sublist li such that li contains all the elements in L at odd positions   While I can do it by  L    1  2  3  4  5  6  7  li      count   0 for i in L      if count   2    1          li append i      count    1   But I want to know if there is another way to do the same efficiently and in fewer number of steps

User · Answer

You can make use of bitwise AND operator &. Let's see below:

x = [1, 2, 3, 4, 5, 6, 7]
y = [i for i in x if i&1]
>>> 
[1, 3, 5, 7]

Bitwise AND operator is used with 1, and the reason it works because, odd number when written in binary must have its first digit as 1. Let's check

23 = 1 * (2**4) + 0 * (2**3) + 1 * (2**2) + 1 * (2**1) + 1 * (2**0) = 10111
14 = 1 * (2**3) + 1 * (2**2) + 1 * (2**1) + 0 * (2**0) = 1110

AND operation with 1 will only return 1 (1 in binary will also have last digit 1), iff the value is odd.

Check the Python Bitwise Operator page for more.

P.S: You can tactically use this method if you want to select odd and even columns in a dataframe. Let's say x and y coordinates of facial key-points are given as columns x1, y1, x2, etc... To normalize the x and y coordinates with width and height values of each image you can simply perform

for i in range(df.shape[1]):
    if i&1:
        df.iloc[:, i] /= heights
    else:
        df.iloc[:, i] /= widths

This is not exactly related to the question but for data scientists and computer vision engineers this method could be useful.

Cheers!

User · Answer

list    list range 9   print list  1  2

User · Answer

I like List comprehensions because of their Math  Set  syntax  So how about this   L    1  2  3  4  5  6  7  odd numbers    y for x y in enumerate L  if x 2    0  even numbers    y for x y in enumerate L  if x 2    0    Basically  if you enumerate over a list  you ll get the index x and the value y  What I m doing here is putting the value y into the output list  even or odd  and using the index x to find out if that point is odd  x 2    0

User · Answer

For the odd positions  you probably want    gt  gt  gt  gt  list    list range 10    gt  gt  gt  gt  print list  1  2   1  3  5  7  9   gt  gt  gt  gt

User · Answer

Solution  Yes  you can   l   L 1  2    And this is all  The result will contain the elements placed on the following positions  0-based  so first element is at position 0  second at 1 etc     1  3  5   so the result  actual numbers  will be   2  4  6   Explanation  The  1  2  at the end is just a notation for list slicing  Usually it is in the following form   some list start stop step    If we omitted start  the default  0  would be used  So the first element  at position 0  because the indexes are 0-based  would be selected  In this case the second element will be selected   Because the second element is omitted  the default is being used  the end of the list   So the list is being iterated from the second element to the end   We also provided third argument  step  which is 2  Which means that one element will be selected  the next will be skipped  and so on     So  to sum up  in this case  1  2  means    take the second element  which  by the way  is an odd element  if you judge from the index   skip one element  because we have step 2  so we are skipping one  as a contrary to step 1 which is default   take the next element  Repeat steps 2 -3  until the end of the list is reached    EDIT   PreetKukreti gave a link for another explanation on Python s list slicing notation  See here  Explain Python  39 s slice notation  Extras - replacing counter with enumerate    In your code  you explicitly create and increase the counter  In Python this is not necessary  as you can enumerate through some iterable using enumerate     for count  i in enumerate L       if count   2    1          l append i    The above serves exactly the same purpose as the code you were using   count   0 for i in L      if count   2    1          l append i      count    1   More on emulating for loops with counter in Python  Accessing the index in Python   39 for  39  loops

[python] Extract elements of list at odd positions

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