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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. Python

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Slicing Lists, strings, and tuples are all sequences

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

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PythonSlicing Lists, strings, and tuples are all sequences Can be indexed by integers in the range 0…len(X)-1 Can also be sliced using a range of indices

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PythonSlicing Lists, strings, and tuples are all sequences Can be indexed by integers in the range 0…len(X)-1 Can also be sliced using a range of indices >>> element = 'uranium' >>> uranium 01234567 -7-6-5-4-3-2

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PythonSlicing Lists, strings, and tuples are all sequences Can be indexed by integers in the range 0…len(X)-1 Can also be sliced using a range of indices >>> element = 'uranium' >>> print element[1:4] ran >>> uranium 01234567 -7-6-5-4-3-2

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PythonSlicing Lists, strings, and tuples are all sequences Can be indexed by integers in the range 0…len(X)-1 Can also be sliced using a range of indices >>> element = 'uranium' >>> print element[1:4] ran >>> print element[:4] uran >>> uranium 01234567 -7-6-5-4-3-2

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PythonSlicing Lists, strings, and tuples are all sequences Can be indexed by integers in the range 0…len(X)-1 Can also be sliced using a range of indices >>> element = 'uranium' >>> print element[1:4] ran >>> print element[:4] uran >>> print element[4:] ium >>> uranium 01234567 -7-6-5-4-3-2

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PythonSlicing Lists, strings, and tuples are all sequences Can be indexed by integers in the range 0…len(X)-1 Can also be sliced using a range of indices >>> element = 'uranium' >>> print element[1:4] ran >>> print element[:4] uran >>> print element[4:] ium >>> print element[-4:] nium >>> uranium 01234567 -7-6-5-4-3-2

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PythonSlicing Python checks bounds when indexing

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PythonSlicing Python checks bounds when indexing But truncates when slicing

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PythonSlicing Python checks bounds when indexing But truncates when slicing >>> element = 'uranium' >>> uranium 01234567 -7-6-5-4-3-2

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PythonSlicing Python checks bounds when indexing But truncates when slicing >>> element = 'uranium' >>> print element[400] IndexError: string index out of range >>> uranium 01234567 -7-6-5-4-3-2

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PythonSlicing Python checks bounds when indexing But truncates when slicing >>> element = 'uranium' >>> print element[400] IndexError: string index out of range >>> print element[1:400] ranium >>> uranium 01234567 -7-6-5-4-3-2

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PythonSlicing Python checks bounds when indexing But truncates when slicing >>> element = 'uranium' >>> print element[400] IndexError: string index out of range >>> print element[1:400] ranium >>> uranium 01234567 -7-6-5-4-3-2 "A foolish consistency is the hobgoblin of little minds." —Ralph Waldo Emerson

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PythonSlicing Python checks bounds when indexing But truncates when slicing >>> element = 'uranium' >>> print element[400] IndexError: string index out of range >>> print element[1:400] ranium >>> uranium 01234567 -7-6-5-4-3-2 "A foolish consistency is the hobgoblin of little minds." —Ralph Waldo Emerson "Aw, you're kidding me!" —programmers

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PythonSlicing So text[1:3] is 0, 1, or 2 characters long

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PythonSlicing So text[1:3] is 0, 1, or 2 characters long '' 'a''' 'ab''b' 'abc''bc' 'abcdef''bc'

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PythonSlicing For consistency, text[1:1] is the empty string

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

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PythonSlicing 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 the empty string

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PythonSlicing 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 the empty string –Not the reverse of text[1:3]

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PythonSlicing 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 the empty string –Not the reverse of text[1:3] But text[1:-1] is everything except the first and last characters

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PythonSlicing Slicing always creates a new collection

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PythonSlicing Slicing always creates a new collection Beware of aliasing

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PythonSlicing Slicing always creates a new collection Beware of aliasing >>> points = [[10, 10], [20, 20], [30, 30], [40, 40]] >>>

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PythonSlicing Slicing always creates a new collection Beware of aliasing >>> points = [[10, 10], [20, 20], [30, 30], [40, 40]] >>> middle = points[1:-1] >>>

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PythonSlicing Slicing always creates a new collection Beware of aliasing >>> points = [[10, 10], [20, 20], [30, 30], [40, 40]] >>> middle = points[1:-1] >>> middle[0][0] = 'whoops' >>>

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PythonSlicing Slicing always creates a new collection Beware of aliasing >>> points = [[10, 10], [20, 20], [30, 30], [40, 40]] >>> middle = points[1:-1] >>> middle[0][0] = 'whoops' >>> middle[1][0] = 'aliasing' >>>

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PythonSlicing Slicing always creates a new collection Beware of aliasing >>> points = [[10, 10], [20, 20], [30, 30], [40, 40]] >>> middle = points[1:-1] >>> middle[0][0] = 'whoops' >>> middle[1][0] = 'aliasing' >>> print middle [['whoops', 20], ['aliasing', 30]] >>>

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PythonSlicing Slicing always creates a new collection Beware of aliasing >>> points = [[10, 10], [20, 20], [30, 30], [40, 40]] >>> 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]] >>>

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PythonSlicing 10 20 30 40 points

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PythonSlicing 10 20 30 40 points middle

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PythonSlicing 10 20 30 40 points middle 'whoops'

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PythonSlicing 10 20 30 40 points middle 'whoops' 'aliasing'

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

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