如何使用fromiter构造一个np.array [英] How to construct an np.array with fromiter

问题描述

我正在尝试通过从python生成器采样来构造np.array，每次调用next都会产生数组的一行.这是一些示例代码:

I'm trying to construct an np.array by sampling from a python generator, that yields one row of the array per invocation of next. Here is some sample code:

import numpy as np
data = np.eye(9)
labels = np.array([0,0,0,1,1,1,2,2,2])

def extract_one_class(X,labels,y):
""" Take an array of data X, a column vector array of labels, and one particular label y.  Return an array of all instances in X that have label y """

    return X[np.nonzero(labels[:] == y)[0],:]

def generate_points(data, labels, size):
""" Generate and return 'size' pairs of points drawn from different classes """

     label_alphabet = np.unique(labels)
     assert(label_alphabet.size > 1)

     for useless in xrange(size):
         shuffle(label_alphabet)
         first_class = extract_one_class(data,labels,label_alphabet[0])
         second_class = extract_one_class(data,labels,label_alphabet[1])
         pair = np.hstack((first_class[randint(0,first_class.shape[0]),:],second_class[randint(0,second_class.shape[0]),:]))
         yield pair

points = np.fromiter(generate_points(data,labels,5),dtype = np.dtype('f8',(2*data.shape[1],1)))

extract_one_class函数返回数据的子集:属于一个类标签的所有数据点.我想将点设为np.array和shape = (size,data.shape[1]).目前，上面的代码段返回错误:

The extract_one_class function returns a subset of data: all data points belonging to one class label. I would like to have points be an np.array with shape = (size,data.shape[1]). Currently the code snippet above returns an error:

ValueError: setting an array element with a sequence.

fromiter的文档声称返回一维数组.还有一些人以前使用fromiter在numpy中构造记录数组(例如 http://iam .al/post/21116450281/numpy-is-my-homeboy ).

The documentation of fromiter claims to return a one-dimensional array. Yet others have used fromiter to construct record arrays in numpy before (e.g http://iam.al/post/21116450281/numpy-is-my-homeboy).

假设我可以以这种方式生成数组时，我是否超出预期?还是我的numpy不太正确?

Am I off the mark in assuming I can generate an array in this fashion? Or is my numpy just not quite right?

推荐答案

您可以修改generate_points以产生单个浮点数而不是np.arrays，使用np.fromiter形成一维数组，然后使用.reshape(size, -1)使其成为2D数组.

You could modify generate_points to yield single floats instead of np.arrays, use np.fromiter to form a 1D array, and then use .reshape(size, -1) to make it a 2D array.

points = np.fromiter(
    generate_points(data,labels,5)).reshape(size, -1)

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