Revisions to How slicing in Python works

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edited Jul 10, 2017 at 16:59

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>>> l1
[2, 3, 4]

>>> l1[:]
[2, 3, 4]

>>> l1[::-1] # a default is -1 , b default is -(len+1)
[4, 3, 2]

>>> l1[:-4:-1] # a default is -1
[4, 3, 2]

>>> l1[:-3:-1] # a default is -1
[4, 3]

>>> l1[::] # c default is +1, so a default is 0, b default is len
[2, 3, 4]

>>> l1[::-1] # c is -1 , so a default is -1 and b default is -(len+1)
[4, 3, 2]


>>> l1[-100:-200:-1] # Interesting
[]

>>> l1[-1:-200:-1] # Interesting
[4, 3, 2]


>>> l1[-1:-1:1]
[]


>>> l1[-1:5:1] # Interesting
[4]


>>> l1[1:-7:1]
[]

>>> l1[1:-7:-1] # Interesting
[3, 2] 

>>> l1[:-2:-2] # a default is -1, stop(b) at -2 , step(c) by 2 in reverse direction
[4]

>>> l1
[2, 3, 4]

>>> l1[:]
[2, 3, 4]

>>> l1[::-1] # a default is -1 , b default is -(len+1)
[4, 3, 2]

>>> l1[:-4:-1] # a default is -1
[4, 3, 2]

>>> l1[:-3:-1] # a default is -1
[4, 3]

>>> l1[::] # c default is +1, so a default is 0, b default is len
[2, 3, 4]

>>> l1[::-1] # c is -1 , so a default is -1 and b default is -(len+1)
[4, 3, 2]


>>> l1[-100:-200:-1] # Interesting
[]

>>> l1[-1:-200:-1] # Interesting
[4, 3, 2]


>>> l1[-1:-1:1]
[]


>>> l1[-1:5:1] # Interesting
[4]


>>> l1[1:-7:1]
[]

>>> l1[1:-7:-1] # Interesting
[3, 2]

>>> l1
[2, 3, 4]

>>> l1[:]
[2, 3, 4]

>>> l1[::-1] # a default is -1 , b default is -(len+1)
[4, 3, 2]

>>> l1[:-4:-1] # a default is -1
[4, 3, 2]

>>> l1[:-3:-1] # a default is -1
[4, 3]

>>> l1[::] # c default is +1, so a default is 0, b default is len
[2, 3, 4]

>>> l1[::-1] # c is -1 , so a default is -1 and b default is -(len+1)
[4, 3, 2]


>>> l1[-100:-200:-1] # Interesting
[]

>>> l1[-1:-200:-1] # Interesting
[4, 3, 2]


>>> l1[-1:-1:1]
[]


>>> l1[-1:5:1] # Interesting
[4]


>>> l1[1:-7:1]
[]

>>> l1[1:-7:-1] # Interesting
[3, 2] 

>>> l1[:-2:-2] # a default is -1, stop(b) at -2 , step(c) by 2 in reverse direction
[4]

Active reading. [<http://en.wikipedia.org/wiki/Python_%28programming_language%29>]

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edited Feb 11, 2017 at 19:55

Peter Mortensen

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In pythonPython 2.7

Slicing in pythonPython

[a:b:c] 

len = length of string, tuple or list

c -- default is  +1. The sign of c indicates forward or backward, absolute value of c indicates steps. Default is forward with step size 1. Positive means forward, negative means backward.

a --  whenWhen c is positive or blank, default is 0. whenWhen c is negative, default is -1.

b --  whenWhen c is positive or blank, default is len. whenWhen c is negative, default is -(len+1).

whenWhen you say [a:b:c], you are saying depending on the sign of c (forward or backward), start at a and end at b ( excludingexcluding element at bth index). Use the indexing rule above and remember you will only find elements in this range:

butBut this range continues in both directions infinitely:

e.g.For example:

             0    1    2   3    4   5   6   7   8   9   10   11         
             a    s    t   r    i   n   g    
    -9  -8  -7   -6   -5  -4   -3  -2  -1

ifIf your choice of a , b, and c allows overlap with the range above as you traverse using rules for a,b,c above you will either get a list with elements (touched during traversal) or you will get an empty list.

One last thing: if a and b are equal , then also you get an empty list:

>>> l1
[2, 3, 4]

>>> l1[:]
[2, 3, 4]

>>> l1[::-1] # a default is -1 , b default is -(len+1)
[4, 3, 2]

>>> l1[:-4:-1] # a default is -1
[4, 3, 2]

>>> l1[:-3:-1] # a default is -1 
[4, 3]

>>> l1[::] # c default is +1, so a default is 0, b default is len
[2, 3, 4]

>>> l1[::-1] # c is -1 , so a default is -1 and b default is -(len+1)
[4, 3, 2]


>>> l1[-100:-200:-1] # interestingInteresting
[]

>>> l1[-1:-200:-1] # interestingInteresting
[4, 3, 2]


>>> l1[-1:-1:1]
[]


>>> l1[-1:5:1] # interestingInteresting
[4]


>>> l1[1:-7:1]
[]

>>> l1[1:-7:-1] # interestingInteresting
[3, 2]

In python 2.7

Slicing in python

[a:b:c] 

len = length of string, tuple or list

c -- default is  +1. sign of c indicates forward or backward, absolute value of c indicates steps. Default is forward with step size 1. Positive means forward, negative means backward.

a --  when c is positive or blank, default is 0. when c is negative, default is -1.

b --  when c is positive or blank, default is len. when c is negative, default is -(len+1).

when you say [a:b:c] you are saying depending on sign of c (forward or backward), start at a and end at b ( excluding element at bth index). Use the indexing rule above and remember you will only find elements in this range

but this range continues in both directions infinitely

e.g.

             0    1    2   3    4   5   6   7   8   9   10   11         
             a    s    t   r    i   n   g    
    -9  -8  -7   -6   -5  -4   -3  -2  -1

if your choice of a , b and c allows overlap with the range above as you traverse using rules for a,b,c above you will either get a list with elements (touched during traversal) or you will get an empty list.

One last thing: if a and b are equal , then also you get an empty list

>>> l1
[2, 3, 4]

>>> l1[:]
[2, 3, 4]

>>> l1[::-1] # a default is -1 , b default is -(len+1)
[4, 3, 2]

>>> l1[:-4:-1] # a default is -1
[4, 3, 2]

>>> l1[:-3:-1] # a default is -1 
[4, 3]

>>> l1[::] # c default is +1, so a default is 0, b default is len
[2, 3, 4]

>>> l1[::-1] # c is -1 , so a default is -1 and b default is -(len+1)
[4, 3, 2]


>>> l1[-100:-200:-1] # interesting
[]

>>> l1[-1:-200:-1] # interesting
[4, 3, 2]


>>> l1[-1:-1:1]
[]


>>> l1[-1:5:1] # interesting
[4]


>>> l1[1:-7:1]
[]

>>> l1[1:-7:-1] # interesting
[3, 2]

In Python 2.7

Slicing in Python

[a:b:c]

len = length of string, tuple or list

c -- default is +1. The sign of c indicates forward or backward, absolute value of c indicates steps. Default is forward with step size 1. Positive means forward, negative means backward.

a --  When c is positive or blank, default is 0. When c is negative, default is -1.

b --  When c is positive or blank, default is len. When c is negative, default is -(len+1).

When you say [a:b:c], you are saying depending on the sign of c (forward or backward), start at a and end at b (excluding element at bth index). Use the indexing rule above and remember you will only find elements in this range:

But this range continues in both directions infinitely:

For example:

             0    1    2   3    4   5   6   7   8   9   10   11
             a    s    t   r    i   n   g
    -9  -8  -7   -6   -5  -4   -3  -2  -1

If your choice of a, b, and c allows overlap with the range above as you traverse using rules for a,b,c above you will either get a list with elements (touched during traversal) or you will get an empty list.

One last thing: if a and b are equal, then also you get an empty list:

>>> l1
[2, 3, 4]

>>> l1[:]
[2, 3, 4]

>>> l1[::-1] # a default is -1 , b default is -(len+1)
[4, 3, 2]

>>> l1[:-4:-1] # a default is -1
[4, 3, 2]

>>> l1[:-3:-1] # a default is -1
[4, 3]

>>> l1[::] # c default is +1, so a default is 0, b default is len
[2, 3, 4]

>>> l1[::-1] # c is -1 , so a default is -1 and b default is -(len+1)
[4, 3, 2]


>>> l1[-100:-200:-1] # Interesting
[]

>>> l1[-1:-200:-1] # Interesting
[4, 3, 2]


>>> l1[-1:-1:1]
[]


>>> l1[-1:5:1] # Interesting
[4]


>>> l1[1:-7:1]
[]

>>> l1[1:-7:-1] # Interesting
[3, 2]

indentation

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edit approved Jul 11, 2015 at 15:52

The6thSense

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[a:b:c]

len = length of string, tuple or list

c -- default is +1. sign of c indicates forward or backward, absolute value of c indicates steps. Default is forward with step size 1. Positive means forward, negative means backward.

a -- when c is positive or blank, default is 0. when c is negative, default is -1.

b -- when c is positive or blank, default is len. when c is negative, default is -(len+1).

[a:b:c] 

len = length of string, tuple or list

c -- default is  +1. sign of c indicates forward or backward, absolute value of c indicates steps. Default is forward with step size 1. Positive means forward, negative means backward.

a --  when c is positive or blank, default is 0. when c is negative, default is -1.

b --  when c is positive or blank, default is len. when c is negative, default is -(len+1).

Understanding index assignment is very important.

In forward direction, starts at 0 and ends at len-1

In backward direction, starts at -1 and ends at -len

In forward direction, starts at 0 and ends at len-1

In backward direction, starts at -1 and ends at -len

-len, -len+1, -len+2, ..., 0, 1, 2,3,4 , len -1

-len, -len+1, -len+2, ..., 0, 1, 2,3,4 , len -1

...,-len -2 ,-len-1,-len, -len+1, -len+2, ..., 0, 1, 2,3,4 , len -1, len, len +1, len+2 , ....

...,-len -2 ,-len-1,-len, -len+1, -len+2, ..., 0, 1, 2,3,4 , len -1, len, len +1, len+2 , ....

[a:b:c]

len = length of string, tuple or list

c -- default is +1. sign of c indicates forward or backward, absolute value of c indicates steps. Default is forward with step size 1. Positive means forward, negative means backward.

a -- when c is positive or blank, default is 0. when c is negative, default is -1.

b -- when c is positive or blank, default is len. when c is negative, default is -(len+1).

Understanding index assignment is very important.

In forward direction, starts at 0 and ends at len-1

In backward direction, starts at -1 and ends at -len

-len, -len+1, -len+2, ..., 0, 1, 2,3,4 , len -1

...,-len -2 ,-len-1,-len, -len+1, -len+2, ..., 0, 1, 2,3,4 , len -1, len, len +1, len+2 , ....

[a:b:c] 

len = length of string, tuple or list

c -- default is  +1. sign of c indicates forward or backward, absolute value of c indicates steps. Default is forward with step size 1. Positive means forward, negative means backward.

a --  when c is positive or blank, default is 0. when c is negative, default is -1.

b --  when c is positive or blank, default is len. when c is negative, default is -(len+1).

Understanding index assignment is very important.

In forward direction, starts at 0 and ends at len-1

In backward direction, starts at -1 and ends at -len

-len, -len+1, -len+2, ..., 0, 1, 2,3,4 , len -1

...,-len -2 ,-len-1,-len, -len+1, -len+2, ..., 0, 1, 2,3,4 , len -1, len, len +1, len+2 , ....

remove repeated instance of l1[-1:5:1] example

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edited Apr 18, 2015 at 15:01

MattBagg

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edited Oct 22, 2012 at 5:40

Ankur Agarwal

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answered Oct 22, 2012 at 5:33

Ankur Agarwal

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