Python - Slicing

1. Python
Slicing
Copyright © Software Carpentry 2010
This work is licensed under the Creative Commons Attribution License
See http://software-carpentry.org/license.html for more information.

2. Lists, strings, and tuples are all sequences
Python Slicing

3. Lists, strings, and tuples are all sequences
Can be indexed by integers in the range 0…len(X)-1
Python Slicing

4. Lists, strings, and tuples are all sequences
Can be indexed by integers in the range 0…len(X)-1
Can also be sliced using aa rraannggee ooff iinnddiicceess
Python Slicing

5. Lists, strings, and tuples are all sequences
Can be indexed by integers in the range 0…len(X)-1
Can also be sliced using aa rraannggee ooff iinnddiicceess
>>>>>>>>>>>> element = 'uranium'
>>>>>>>>>>>>
0 1 2 3 4 5 6 7
u r a n i u m
-7 -6 -5 -4 -3 -2 -1
Python Slicing

6. Lists, strings, and tuples are all sequences
Can be indexed by integers in the range 0…len(X)-1
Can also be sliced using aa rraannggee ooff iinnddiicceess
>>>>>>>>>>>> element = 'uranium'
>>>>>>>>>>>> print element[1:4]
ran
>>>>>>>>>>>>
0 1 2 3 4 5 6 7
u r a n i u m
-7 -6 -5 -4 -3 -2 -1
Python Slicing

7. Lists, strings, and tuples are all sequences
Can be indexed by integers in the range 0…len(X)-1
Can also be sliced using aa rraannggee ooff iinnddiicceess
>>>>>>>>>>>> element = 'uranium'
>>>>>>>>>>>> print element[1:4]
ran
>>>>>>>>>>>> print element[:4]
uran
>>>>>>>>>>>>
0 1 2 3 4 5 6 7
u r a n i u m
-7 -6 -5 -4 -3 -2 -1
Python Slicing

8. Lists, strings, and tuples are all sequences
Can be indexed by integers in the range 0…len(X)-1
Can also be sliced using aa rraannggee ooff iinnddiicceess
>>>>>>>>>>>> element = 'uranium'
>>>>>>>>>>>> print element[1:4]
ran
>>>>>>>>>>>> print element[:4]
uran
>>>>>>>>>>>> print element[4:]
0 1 2 3 4 5 6 7
u r a n i u m
-7 -6 -5 -4 -3 -2 -1
ium
>>>>>>>>>>>>
Python Slicing

9. Lists, strings, and tuples are all sequences
Can be indexed by integers in the range 0…len(X)-1
Can also be sliced using aa rraannggee ooff iinnddiicceess
>>>>>>>>>>>> element = 'uranium'
>>>>>>>>>>>> print element[1:4]
ran
>>>>>>>>>>>> print element[:4]
uran
>>>>>>>>>>>> print element[4:]
0 1 2 3 4 5 6 7
u r a n i u m
-7 -6 -5 -4 -3 -2 -1
ium
>>>>>>>>>>>> print element[-4:]
nium
>>>>>>>>>>>>
Python Slicing

10. Python checks bounds when indexing
Python Slicing

11. Python checks bounds when indexing
But truncates when slicing
Python Slicing

12. Python checks bounds when indexing
But truncates when slicing
>>>>>>>>>>>>>>>>>>>>>>>> eelleemmeenntt == ''uurraanniiuumm''
>>>>>>>>>>>>
0 1 2 3 4 5 6 7
u r a n i u m
-7 -6 -5 -4 -3 -2 -1
Python Slicing

13. Python checks bounds when indexing
But truncates when slicing
>>>>>>>>>>>>>>>>>>>>>>>> eelleemmeenntt == ''uurraanniiuumm''
>>>>>>>>>>>> print element[400]
IIIInnnnddddeeeexxxxEEEErrrrrrrroooorrrr:::: ssssttttrrrriiiinnnngggg iiiinnnnddddeeeexxxx oooouuuutttt ooooffff rrrraaaannnnggggeeee
>>>>>>>>>>>>
0 1 2 3 4 5 6 7
u r a n i u m
-7 -6 -5 -4 -3 -2 -1
Python Slicing

14. Python checks bounds when indexing
But truncates when slicing
>>>>>>>>>>>>>>>>>>>>>>>> eelleemmeenntt == ''uurraanniiuumm''
>>>>>>>>>>>> print element[400]
IIIInnnnddddeeeexxxxEEEErrrrrrrroooorrrr:::: ssssttttrrrriiiinnnngggg iiiinnnnddddeeeexxxx oooouuuutttt ooooffff rrrraaaannnnggggeeee
>>>>>>>>>>>> print element[1:400]
ranium
>>>>>>>>>>>>
0 1 2 3 4 5 6 7
u r a n i u m
-7 -6 -5 -4 -3 -2 -1
Python Slicing

15. Python checks bounds when indexing
But truncates when slicing
>>>>>>>>>>>>>>>>>>>>>>>> eelleemmeenntt == ''uurraanniiuumm''
>>>>>>>>>>>> print element[400]
IIIInnnnddddeeeexxxxEEEErrrrrrrroooorrrr:::: ssssttttrrrriiiinnnngggg iiiinnnnddddeeeexxxx oooouuuutttt ooooffff rrrraaaannnnggggeeee
>>>>>>>>>>>> print element[1:400]
ranium
>>>>>>>>>>>>
0 1 2 3 4 5 6 7
u r a n i u m
-7 -6 -5 -4 -3 -2 -1
"A foolish consistency is
the hobgoblin of little minds."
— Ralph Waldo Emerson
Python Slicing

16. Python checks bounds when indexing
But truncates when slicing
>>>>>>>>>>>>>>>>>>>>>>>> eelleemmeenntt == ''uurraanniiuumm''
>>>>>>>>>>>> print element[400]
IIIInnnnddddeeeexxxxEEEErrrrrrrroooorrrr:::: ssssttttrrrriiiinnnngggg iiiinnnnddddeeeexxxx oooouuuutttt ooooffff rrrraaaannnnggggeeee
>>>>>>>>>>>> print element[1:400]
ranium
>>>>>>>>>>>>
0 1 2 3 4 5 6 7
u r a n i u m
-7 -6 -5 -4 -3 -2 -1
"A foolish consistency is
the hobgoblin of little minds."
— Ralph Waldo Emerson
"Aw, you're kidding me!"
— programmers
Python Slicing

17. So text[1:3] is 0, 1, or 2 characters long
Python Slicing

18. So text[1:3] is 0, 1, or 2 characters long
'' ''
''aa'' ''''
'ab' 'b'
'abc' 'bc'
'abcdef' 'bc'
Python Slicing

19. For consistency, text[1:1] is the empty string
Python Slicing

20. For consistency, text[1:1] is the empty string
– From index 1 up to (but not including) index 1
Python Slicing

21. For consistency, text[1:1] is the empty string
– From index 1 up to (but not including) index 1
And text[3:1] is always tthhee eemmppttyy ssttrriinngg
Python Slicing

22. For consistency, text[1:1] is the empty string
– From index 1 up to (but not including) index 1
And text[3:1] is always tthhee eemmppttyy ssttrriinngg
– Not the reverse of text[1:3]
Python Slicing

23. For consistency, text[1:1] is the empty string
– From index 1 up to (but not including) index 1
And text[3:1] is always tthhee eemmppttyy ssttrriinngg
– Not the reverse of text[1:3]
But text[1:-1] is everything except the first and
last characters
Python Slicing

24. Slicing always creates a new collection
Python Slicing

25. Slicing always creates a new collection
Beware of aliasing
Python Slicing

26. Slicing always creates a new collection
Beware of aliasing
>>>>>>>>>>>>>>>>>>>>>>>> ppooiinnttss == [[[[1100,, 1100]],, [[2200,, 2200]],, [[3300,, 3300]],, [[4400,, 4400]]]]
>>>>>>>>>>>>
Python Slicing

27. Slicing always creates a new collection
Beware of aliasing
>>>>>>>>>>>>>>>>>>>>>>>> ppooiinnttss == [[[[1100,, 1100]],, [[2200,, 2200]],, [[3300,, 3300]],, [[4400,, 4400]]]]
>>>>>>>>>>>> middle = points[1:-1]
>>>>>>>>>>>>
Python Slicing

28. Slicing always creates a new collection
Beware of aliasing
>>>>>>>>>>>>>>>>>>>>>>>> ppooiinnttss == [[[[1100,, 1100]],, [[2200,, 2200]],, [[3300,, 3300]],, [[4400,, 4400]]]]
>>>>>>>>>>>> middle = points[1:-1]
>>>>>>>>>>>> middle[0][0] = 'whoops'
>>>>>>>>>>>>
Python Slicing

29. Slicing always creates a new collection
Beware of aliasing
>>>>>>>>>>>>>>>>>>>>>>>> ppooiinnttss == [[[[1100,, 1100]],, [[2200,, 2200]],, [[3300,, 3300]],, [[4400,, 4400]]]]
>>>>>>>>>>>> middle = points[1:-1]
>>>>>>>>>>>> middle[0][0] = 'whoops'
>>>>>>>>>>>> middle[1][0] = 'aliasing'
>>>>>>>>>>>>
Python Slicing

30. Slicing always creates a new collection
Beware of aliasing
>>>>>>>>>>>>>>>>>>>>>>>> ppooiinnttss == [[[[1100,, 1100]],, [[2200,, 2200]],, [[3300,, 3300]],, [[4400,, 4400]]]]
>>>>>>>>>>>> middle = points[1:-1]
>>>>>>>>>>>> middle[0][0] = 'whoops'
>>>>>>>>>>>> middle[1][0] = 'aliasing'
>>>>>>>>>>>> print middle
[['whoops', 20], ['aliasing', 30]]
>>>>>>>>>>>>
Python Slicing

31. Slicing always creates a new collection
Beware of aliasing
>>>>>>>>>>>>>>>>>>>>>>>> ppooiinnttss == [[[[1100,, 1100]],, [[2200,, 2200]],, [[3300,, 3300]],, [[4400,, 4400]]]]
>>>>>>>>>>>> middle = points[1:-1]
>>>>>>>>>>>> middle[0][0] = 'whoops'
>>>>>>>>>>>> middle[1][0] = 'aliasing'
>>>>>>>>>>>> print middle
[['whoops', 20], ['aliasing', 30]]
>>>>>>>>>>>> print points
[[10, 10], ['whoops', 20], ['aliasing', 30], [40, 40]]
>>>>>>>>>>>>
Python Slicing

32. 10
10
20
points
20
30
30
40
40
Python Slicing

33. 10
10
20
points
20
30
30
middle
40
40
Python Slicing

34. 10
10
points
'whoops'
20
30
30
middle
40
40
Python Slicing

35. 10
10
points
'whoops'
20
30
middle
'aliasing'
40
40
Python Slicing

36. created by
Greg Wilson
October 2010
Copyright © Software Carpentry 2010
This work is licensed under the Creative Commons Attribution License
See http://software-carpentry.org/license.html for more information.