[英](Numpy C API) Iterating over a single array: NpyIter vs for loop (with PyArray_DATA)
I am writing some C extension code for a python module.我正在为 python 模块编写一些 C 扩展代码。 The function I want to write is (in python)我想写的函数是(在python中)
output = 1./(1. + input)
where input
is a numpy array of any shape.其中input
是任何形状的 numpy 数组。
Originally I was using NpyIter_MultiNew
:最初我使用的是NpyIter_MultiNew
:
static PyObject *
helper_calc1(PyObject *self, PyObject *args){
PyObject * input;
PyObject * output = NULL;
if (!PyArg_ParseTuple(args, "O", &input)){
return NULL;
}
// -- input -----------------------------------------------
PyArrayObject * in_arr;
in_arr = (PyArrayObject *) PyArray_FROM_OTF(input, NPY_DOUBLE, NPY_ARRAY_IN_ARRAY);
if (in_arr == NULL){
goto fail;
}
// -- set up iterator -------------------------------------
PyArrayObject * op[2];
npy_uint32 op_flags[2];
npy_uint32 flags;
op[0] = in_arr;
op_flags[0] = NPY_ITER_READONLY;
op[1] = NULL;
op_flags[1] = NPY_ITER_WRITEONLY | NPY_ITER_ALLOCATE;
flags = NPY_ITER_EXTERNAL_LOOP | NPY_ITER_BUFFERED | NPY_ITER_GROWINNER;
NpyIter * iter = NpyIter_MultiNew(2, op,
flags, NPY_KEEPORDER, NPY_NO_CASTING,
op_flags, NULL);
if (iter == NULL){
goto fail;
};
NpyIter_IterNextFunc * iternext = NpyIter_GetIterNext(iter, NULL);
if (iternext == NULL){
NpyIter_Deallocate(iter);
goto fail;
};
// -- iterate ---------------------------------------------
npy_intp count;
char ** dataptr = NpyIter_GetDataPtrArray(iter);
npy_intp * strideptr = NpyIter_GetInnerStrideArray(iter);
npy_intp * innersizeptr = NpyIter_GetInnerLoopSizePtr(iter);
do {
count = *innersizeptr;
while (count--){
*(double *) dataptr[1] = 1. / (1. + *(double *)dataptr[0]);
dataptr[0] += strideptr[0];
dataptr[1] += strideptr[1];
}
} while (iternext(iter));
output = NpyIter_GetOperandArray(iter)[1];
if (NpyIter_Deallocate(iter) != NPY_SUCCEED){
goto fail;
}
Py_DECREF(in_arr);
return output;
fail:
Py_XDECREF(in_arr);
Py_XDECREF(output);
return NULL;
}
However, since this is just a single array (ie I don't need to be concerned about broadcasting multiple arrays), Is there any reason I can't iterate over the data myself using, PyArray_DATA
, a for
loop and the array size?但是,由于这只是一个数组(即我不需要担心广播多个数组),有什么理由我PyArray_DATA
使用PyArray_DATA
、 for
循环和数组大小迭代数据?
static PyObject *
helper_calc2(PyObject *self, PyObject *args){
PyObject * input;
PyObject * output = NULL;
if (!PyArg_ParseTuple(args, "O", & in)){
return NULL;
}
// -- input -----------------------------------------------
PyArrayObject * in_arr;
in_arr = (PyArrayObject *) PyArray_FROM_OTF(input, NPY_DOUBLE, NPY_ARRAY_IN_ARRAY);
if (in_arr == NULL){
Py_XDECREF(in_arr);
return NULL;
}
int ndim = PyArray_NDIM(in_arr);
npy_intp * shape = PyArray_DIMS(in_arr);
int size = (int) PyArray_SIZE(in_arr);
double * in_data = (double *) PyArray_DATA(in_arr);
output = PyArray_SimpleNew(ndim, shape, NPY_DOUBLE);
double * out_data = (double *) PyArray_DATA((PyArrayObject *) output);
for (int i = 0; i < size; i++){
out_data[i] = 1. / (1. + in_data[i]);
}
Py_DECREF(in_arr);
return output;
fail:
Py_XDECREF(in_arr);
Py_XDECREF(output);
return NULL;
}
This second version runs more quickly and the code is shorter.第二个版本运行速度更快,代码更短。
Are there any dangers I need to look out for when using, PyArray_DATA
with a for
loop instead of NpyIter_MultiNew
?在使用PyArray_DATA
和for
循环而不是NpyIter_MultiNew
时,我是否需要注意任何危险?
From the PyArray_DATA
documentation:从PyArray_DATA
文档:
If you have not guaranteed a contiguous and/or aligned array then be sure you understand how to access the data in the array to avoid memory and/or alignment problems.如果您不能保证连续和/或对齐的数组,那么请确保您了解如何访问数组中的数据以避免内存和/或对齐问题。
But I believe that this is taken care of by PyArray_FROM_OTF
with the NPY_ARRAY_IN_ARRAY
flag.但我相信这是由带有NPY_ARRAY_IN_ARRAY
标志的PyArray_FROM_OTF
处理的。
You should be ok in this case.在这种情况下你应该没问题。 Like you mentioned NPY_ARRAY_IN_ARRAY
with PyArray_FROM_OTF
takes care of that issue and since you are casting the data pointer to the right type, you should be fine.就像你提到的NPY_ARRAY_IN_ARRAY
和PyArray_FROM_OTF
了这个问题,因为你将数据指针转换为正确的类型,你应该没问题。 However, in general, as you seem to be aware of, you can't use this approach if you are directly accepting a NumPy array from Python code.但是,一般来说,正如您似乎知道的那样,如果您直接从 Python 代码接受 NumPy 数组,则不能使用这种方法。
Happy C extension writing.快乐的 C 扩展写作。
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