将numpy整数数组传递给C代码 [英] Passing numpy integer array to c code

问题描述

我正在尝试编写Cython代码以转储密集的特征矩阵，将目标向量对转换为libsvm格式，比sklearn的内置代码更快.我收到一个编译错误，抱怨将目标向量(整数的numpy数组)传递给相关的c函数时发生类型问题.

I'm trying to write Cython code to dump a dense feature matrix, target vector pair to libsvm format faster than sklearn's built in code. I get a compilation error complaining about a type issue with passing the target vector (a numpy array of ints) to the relevant c function.

代码如下:

import numpy as np
cimport numpy as np
cimport cython

cdef extern from "cdump.h":
    int filedump( double features[], int numexemplars, int numfeats, int target[], char* outfname)

@cython.boundscheck(False)
@cython.wraparound(False)
def fastdumpdense_libsvmformat(np.ndarray[np.double_t,ndim=2] X, y, outfname):
    if X.shape[0] != len(y):
        raise ValueError("X and y need to have the same number of points")

    cdef int numexemplars = X.shape[0]
    cdef int numfeats = X.shape[1]

    cdef bytes py_bytes = outfname.encode()
    cdef char* outfnamestr = py_bytes

    cdef np.ndarray[np.double_t, ndim=2, mode="c"] X_c
    cdef np.ndarray[np.int_t, ndim=1, mode="c"] y_c
    X_c = np.ascontiguousarray(X, dtype=np.double)
    y_c = np.ascontiguousarray(y, dtype=np.int)
    retval = filedump( &X_c[0,0], numexemplars, numfeats, &y_c[0], outfnamestr)

    return retval

当我尝试使用distutils编译此代码时，出现错误

When I attempt to compile this code using distutils, I get the error

cythoning fastdump_svm.pyx to fastdump_svm.cpp

Error compiling Cython file:
------------------------------------------------------------ ...

    cdef np.ndarray[np.double_t, ndim=2, mode="c"] X_c
    cdef np.ndarray[np.int_t, ndim=1, mode="c"] y_c
    X_c = np.ascontiguousarray(X, dtype=np.double)
    y_c = np.ascontiguousarray(y, dtype=np.int)
    retval = filedump( &X_c[0,0], numexemplars, numfeats, &y_c[0], outfnamestr)
                                                         ^
------------------------------------------------------------

fastdump_svm.pyx:24:58: Cannot assign type 'int_t *' to 'int *'

任何想法如何解决此错误?我最初遵循的是传递y_c.data的范例，该范例有效，但这显然不是推荐的方法.

Any idea how to fix this error? I originally was following the paradigm of passing y_c.data, which works, but this is apparently not the recommended way.

推荐答案

在启动numpy数组以匹配计算机上的C int时，也可​​以使用dtype=np.dtype("i").

You can also use dtype=np.dtype("i") when initiating a numpy array to match the C int on your machine.

cdef int [:] y_c
c_array = np.ascontiguousarray(y, dtype=np.dtype("i"))

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