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This article introduces the knowledge of "introduction to the example usage of colons in Python". Many people will encounter such a dilemma in the operation of actual cases, so let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

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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