如何从可重复的元组创建多维numpy数组? [英] How do you create a multidimensional numpy array from an iterable of tuples?

问题描述

我想从一个可迭代对象创建一个numpy数组，该数组会生成值的元组，例如数据库查询.

I would like to create a numpy array from an iterable, which yields tuples of values, such as a database query.

像这样:

data = db.execute('SELECT col1, col2, col3, col4 FROM data')
A = np.array(list(data))

有没有一种更快的方法，而无需先将可迭代对象转换为列表?

Is there a way faster way of doing so, without converting the iterable to a list first?

推荐答案

我不是numpy的有经验的用户，但是以下是一般问题的可能解决方案:

I am not an experienced user of numpy, but here is a possible solution for the general question:

>>> i = iter([(1, 11), (2, 22)])
>>> i
<listiterator at 0x5b2de30>                    # a sample iterable of tuples
>>> rec_array = np.fromiter(i, dtype='i4,i4')  # mind the dtype
>>> rec_array                                  # rec_array is a record array
array([(1, 11), (2, 22)], 
    dtype=[('f0', '<i4'), ('f1', '<i4')])
>>> rec_array['f0'], rec_array[0]              # each field has a default name
(array([1, 2]), (1, 11))
>>> a = rec_array.view(np.int32).reshape(-1,2) # let's create a view
>>> a
array([[ 1, 11],
       [ 2, 22]])
>>> rec_array[0][1] = 23
>>> a                                          # a is a view, not a copy!
array([[ 1, 23],
       [ 2, 22]])

我假设所有列都属于同一类型，否则rec_array已经是您想要的.

I assume that all columns are of the same type, otherwise rec_array is already what you want.

关于您的特殊情况，我不完全理解您的示例中的db是什么.如果它是一个游标对象，则可以调用其fetchall方法并获取元组列表.在大多数情况下，数据库库不想保留部分读取的查询结果，而是等待您的代码处理每一行，也就是说，当execute方法返回时，所有数据已经​​存储在列表中，并且在那里使用fetchall而不是迭代cursor实例几乎不是问题.

Concerning your particular case, I do not completely understand what is db in your example. If it is a cursor object, then you can just call its fetchall method and get a list of tuples. In most cases, the database library does not want to keep a partially read query result, waiting for your code processing each line, that is by the moment when the execute method returns, all data is already stored in a list, and there is hardly a problem of using fetchall instead of iterating cursor instance.

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