What is double colon in Python when subscripting sequences

Question

I know that I can use something like string 3 4  to get a substring in Python  but what does the 3 mean in somesequence   3

User · Answer

The third parameter is the step   So    3  would return every 3rd element of the list string

User · Answer

Python sequence slice addresses can be written as a start end step  and any of start  stop or end can be dropped   a   3  is every third element of the sequence

User · Answer

TL DR This visual example will show you how to a neatly select elements in a NumPy Matrix  2 dimensional array  in a pretty entertaining way  I promise   Step 2 below illustrate the usage of that  quot double colons quot     in question   Caution  this is a NumPy array specific example with the aim of illustrating the a use case of  quot double colons quot     for jumping of elements in multiple axes  This example does not cover native Python data structures like List   One concrete example to rule them all    Say we have a NumPy matrix that looks like this  In  1   import numpy as np  In  2   X   np arange 100  reshape 10 10   In  3   X Out 3   array    0   1   2   3   4   5   6   7   8   9           10  11  12  13  14  15  16  17  18  19           20  21  22  23  24  25  26  27  28  29           30  31  32  33  34  35  36  37  38  39           40  41  42  43  44  45  46  47  48  49           50  51  52  53  54  55  56  57  58  59           60  61  62  63  64  65  66  67  68  69           70  71  72  73  74  75  76  77  78  79           80  81  82  83  84  85  86  87  88  89           90  91  92  93  94  95  96  97  98  99     Say for some reason  your boss wants you to select the following elements    quot But How    quot     Read on   We can do this in a 2-step approach  Step 1 - Obtain subset Specify the  quot start index quot  and  quot end index quot  in both row-wise and column-wise directions   In code  In  5   X2   X 2 9 3 8   In  6   X2 Out 6   array   23  24  25  26  27           33  34  35  36  37           43  44  45  46  47           53  54  55  56  57           63  64  65  66  67           73  74  75  76  77           83  84  85  86  87     Notice now we ve just obtained our subset  with the use of simple start and end indexing technique  Next up  how to do that  quot jumping quot      read on   Step 2 - Select elements  with the  quot jump step quot  argument  We can now specify the  quot jump steps quot  in both row-wise and column-wise directions  to select elements in a  quot jumping quot  way  like this   In code  note the double colons   In  7   X3   X2   3    2   In  8   X3 Out 8   array   23  25  27           53  55  57           83  85  87     We have just selected all the elements as required       Consolidate Step 1  start and end  and Step 2   quot jumping quot   Now we know the concept  we can easily combine step 1 and step 2 into one consolidated step - for compactness  In  9   X4   X 2 9 3 8    3   2       In  10   X4     Out 10       array   23  25  27               53  55  57               83  85  87     Done

User · Answer

Did I miss or nobody mentioned reversing with    -1  here    Operating System List systems     Windows    macOS    Linux   print  Original List    systems     Reversing a list    Syntax  reversed list   systems start stop step   reversed list   systems   -1     updated list print  Updated List    reversed list   source  https   www programiz com python-programming methods list reverse

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it means  nothing for the first argument  nothing for the second  and jump by three   It gets every third item of the sequence sliced  Extended slices is what you want  New in Python 2 3

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Explanation  s i j k  is  according to the documentation   slice of s from i to j with step k    When i and j are absent  the whole sequence is assumed and thus s   k  means  every k-th item    Examples  First  let s initialize a list    gt  gt  gt  s   range 20   gt  gt  gt  s  0  1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19    Let s take every 3rd item from s    gt  gt  gt  s   3   0  3  6  9  12  15  18    Let s take every 3rd item from s 2      gt  gt  gt  s 2    2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19   gt  gt  gt  s 2  3   2  5  8  11  14  17    Let s take every 3rd item from s 5 12     gt  gt  gt  s 5 12   5  6  7  8  9  10  11   gt  gt  gt  s 5 12 3   5  8  11    Let s take every 3rd item from s  10     gt  gt  gt  s  10   0  1  2  3  4  5  6  7  8  9   gt  gt  gt  s  10 3   0  3  6  9

User · Answer

You can also use this notation in your own custom classes to make it do whatever you want  class C object       def   getitem   self  k           return k    Single argument is passed directly  assert C   0     0    Multiple indices generate a tuple  assert C   0  1      0  1     Slice notation generates a slice object  assert C   1 2 3     slice 1  2  3     If you omit any part of the slice notation  it becomes None  assert C         slice None  None  None  assert C          slice None  None  None  assert C   1       slice 1  None  None  assert C    2      slice None  2  None  assert C     3     slice None  None  3     Tuple with a slice object  assert C      1      slice None  None  None   1     Ellipsis class object  assert C           Ellipsis   We can then open up slice objects as   s   slice 1  2  3  assert s start    1 assert s stop    2 assert s step    3   This is notably used in Numpy to slice multi-dimensional arrays in any direction   Of course  any sane API should use   3 with the usual  every 3  semantic   The related Ellipsis is covered further at  What does the Ellipsis object do

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Python uses the    to separate the End  the Start  and the Step value

User · Answer

seq   n  is a sequence of each n-th item in the entire sequence  Example   gt  gt  gt  range 10    2   0  2  4  6  8   The syntax is  seq start end step   So you can do  in Python 2    gt  gt  gt  range 100  5 18 2   5  7  9  11  13  15  17

User · Answer

When slicing in Python the third parameter is the step  As others mentioned  see Extended Slices for a nice overview   With this knowledge     3  just means that you have not specified any start or end indices for your slice  Since you have specified a step  3  this will take every third entry of something starting at the first index  For example    gt  gt  gt   123123123    3   111

[python] What is :: (double colon) in Python when subscripting sequences?

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