python - 为什么我会收到数据转换警告？

Question

我是这方面的新手，非常感谢您的帮助。 我正在玩弄 mnist 数据集。我从 http://g.sweyla.com/blog/2012/mnist-numpy/ 中获取了代码但将“图像”更改为二维，以便每个图像都是一个特征向量。然后我对数据运行 PCA，然后运行 ​​SVM 并检查分数。一切似乎都工作正常，但我收到以下警告，我不确定为什么。

"DataConversionWarning: A column-vector y was passed when a 1d array was expected.\
Please change the shape of y to (n_samples, ), for example using ravel()."

我已经尝试了几件事，但似乎无法摆脱这个警告。有什么建议么？这是完整的代码(忽略缺失的缩进，看起来他们在复制这里的代码时搞砸了):

import os, struct
from array import array as pyarray
from numpy import append, array, int8, uint8, zeros, arange
from sklearn import svm, decomposition
#from pylab import *
#from matplotlib import pyplot as plt

def load_mnist(dataset="training", digits=arange(10), path="."):
"""
Loads MNIST files into 3D numpy arrays

Adapted from: http://abel.ee.ucla.edu/cvxopt/_downloads/mnist.py
"""

    if dataset == "training":
        fname_img = os.path.join(path, 'train-images.idx3-ubyte')
        fname_lbl = os.path.join(path, 'train-labels.idx1-ubyte')
    elif dataset == "testing":
        fname_img = os.path.join(path, 't10k-images.idx3-ubyte')
        fname_lbl = os.path.join(path, 't10k-labels.idx1-ubyte')
    else:
        raise ValueError("dataset must be 'testing' or 'training'")

    flbl = open(fname_lbl, 'rb')
    magic_nr, size = struct.unpack(">II", flbl.read(8))
    lbl = pyarray("b", flbl.read())
    flbl.close()

    fimg = open(fname_img, 'rb')
    magic_nr, size, rows, cols = struct.unpack(">IIII", fimg.read(16))
    img = pyarray("B", fimg.read())
    fimg.close()

    ind = [ k for k in range(size) if lbl[k] in digits ]
    N = len(ind)

    images = zeros((N, rows*cols), dtype=uint8)
    labels = zeros((N, 1), dtype=int8)
    for i in range(len(ind)):
        images[i] = array(img[ ind[i]*rows*cols : (ind[i]+1)*rows*cols ])
        labels[i] = lbl[ind[i]]

    return images, labels

if __name__ == "__main__":
    images, labels = load_mnist('training', arange(10),"path...")
    pca = decomposition.PCA()
    pca.fit(images)
    pca.n_components = 200
    images_reduced = pca.fit_transform(images)
    lin_classifier = svm.LinearSVC()
    lin_classifier.fit(images_reduced, labels)
    images2, labels2 = load_mnist('testing', arange(10),"path...")
    images2_reduced = pca.transform(images2)
    score = lin_classifier.score(images2_reduced,labels2)
    print score

感谢您的帮助!

Answer 1

我认为 scikit-learn 期望 y 是一维数组。您的 labels 变量是二维的 - labels.shape 是 (N, 1)。该警告告诉您使用 labels.ravel()，这会将 labels 转换为一维数组，形状为 (N,)。
reshape 也将起作用:labels=labels.reshape((N,))
想想看，调用挤压也是如此:labels=labels.squeeze()

我想这里的陷阱是在 numpy 中，一维数组不同于其中一个维度等于 1 的二维数组。