Introduction to the example usage of colons in Python
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Examples
A = [1, 2, 3, 4], [2, 3, 4, 5], [5, 6, 7, 8]]
B = np.array ([1, 2, 3, 4), [2, 3, 4, 5], [5, 6, 7, 8])
1. About [: J] or [: I]: this is a slicing operation, where elements before subscript I or j are retained, applicable to list (that is, arrays) in Python, as well as to numpy scientific structures (array, etc.).
The following is represented by list
# an is a list of Python (called array in data structure)
In [64]: a
Out [64]: [[1, 2, 3, 4], [2, 3, 4, 5], [5, 6, 7, 8]]
# example 1
In [65]: a [: 2]
Out [65]: [[1, 2, 3, 4], [2, 3, 4, 5]]
# example 2
In [66]: a [1] [: 3]
Out [66]: [2, 3, 4]
two。 About [:, j] or [:, I]: this is also a slicing operation, except that all the elements of the first dimension are retained and the second dimension elements are retained to j; only the scientific data structure of numpy is applicable.
Here is an example of using the data structure of numpy
# b is a numpy scientific data structure in array format
In [76]: b
Out [76]: array ([1,2,3,4]
[2, 3, 4, 5]
[5, 6, 7, 8])
# example 1
In [77]: b [:, 2]
Out [77]: array ([3,4,7])
# example 2
In [78]: b [:, 3]
Out [78]: array ([4,5,8])
# look at the negative example, if it is a list structure
In [6]: x
Out [6]: [[1, 2, 3, 4], [2, 3, 4, 5], [5, 6, 7, 8]]
In [5]: x [:,: 2]
TypeError Traceback (most recent call last)
In ()
-> 1 x [:,: 2]
TypeError: list indices must be integers or slices, not tuple
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