How to get every first element in 2 dimensional list

Question

I have a list like this  a     4 0  4  4 0    3 0  3  3 6    3 5  6  4 8    I want an outcome like this  EVERY first element in the list   4 0  3 0  3 5  I tried a   1  0   but it doesn t work

User · Accepted Answer

You can get the index  0  from each element in a list comprehension   gt  gt  gt   i 0  for i in a   4 0  3 0  3 5    Also just to be pedantic  you don t have a list of list  you have a tuple of tuple

User · Answer

Try using   for i in a     print i 0     i represents individual row in a So i 0  represnts the 1st element of each row

User · Answer

You could use this   a     4 0  4  4 0    3 0  3  3 6    3 5  6  4 8   a   np array a  a   0  returns  gt  gt  gt  array  4    3    3 5

User · Answer

use zip columns   zip  rows   transpose rows to columns print columns 0   print the first column  you can also do more with the columns print columns 1    or print the second column columns append  7 7 7    add a new column to the end backToRows   zip  columns    now we are back to rows with a new column print backToRows  you can also use numpy a   numpy array a  print a   0   Edit  zip object is not subscriptable  It need to be converted to list to access as list  column   list zip  row

User · Answer

If you have access to numpy  import numpy as np a transposed   a T   Get first row print a transposed 0    The benefit of this method is that if you want the  quot second quot  element in a 2d list  all you have to do now is a transposed 1   The a transposed object is already computed  so you do not need to recalculate  Description Finding the first element in a 2-D list can be rephrased as find the first column in the 2d list  Because your data structure is a list of rows  an easy way of sampling the value at the first index in every row is just by transposing the matrix and sampling the first list

User · Answer

Compared the 3 methods   2D list  5 323603868484497 seconds Numpy library   0 3201274871826172 seconds Zip  Thanks to Joran Beasley    0 12395167350769043 seconds   D2 list  list range 100    100 t1 time time   for i in range 10  5       for j in range 10           b  k j  for k in D2 list  D2 list time time time  -t1  array np array D2 list  t1 time time           for i in range 10  5       for j in range 10           b array   j          Numpy time time time  -t1  D2 trans   list zip  D2 list    t1 time time           for i in range 10  5       for j in range 10           b D2 trans j  Zip time time time  -t1  print   2D List   D2 list time  print   Numpy   Numpy time  print   Zip   Zip time    The Zip method works best  It was quite useful when I had to do some column wise processes for mapreduce jobs in the cluster servers where numpy was not installed

User · Answer

You can get it like    x 0  for x in a    which will return a list of the first element of each list in a

[python] How to get every first element in 2 dimensional list

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