Access a Numpy Recarray via the C
If we have a Numpy recarray: x = np.array([(1.,2.)], dtype=np.dtype([('a','<f8'),('b','<f8')]))
We can access its fields in Python as:
x['a'] or x['b']
But if this array is passed to a C program as a PyArrayObject how do we access its fields?
I realize we can get the dtype in C via: PyArray_Descr *dtype = PyArray_DTYPE(arr)
PyObject *fields = dtype->fields but how can this be used to access the data at x['a'] ?
I'll try to answer my own question.
It appears that you can use the function PyObject_GetItem() to access fields in your Numpy recarray. To test this I created a simple recarray with three fields:
np.dtype([('field1', '<f8', (1,2)), ('field2', '<f8', (2,2)), ('field3', '<f8', (3,1))])
I send this array to my C++ function and exectute two loops: one loop over each field and a nested loop over the array elements in each field (eg. x['field1'], x['field2'], x['field3'] ). In the outerloop I use PyObject_GetItem() to access each field. The code is as follows:
C++ Code
#include "Python.h"
#define NPY_NO_DEPRECATED_API NPY_1_7_API_VERSION
#include "arrayobject.h"
#include <cmath>
#include <iostream>
#include <iomanip>
using namespace std;
static PyObject *readarray(PyObject *self, PyObject *args) {
PyArrayObject *arr, *x2;
PyArray_Descr *dtype;
PyObject *names, *name, *x1 = NULL;
Py_ssize_t N, i;
NpyIter *iter;
NpyIter_IterNextFunc *iternext;
double **dataptr;
npy_intp index;
if (!PyArg_ParseTuple(args, "O!", &PyArray_Type, &arr)) {
return NULL;
}
dtype = PyArray_DTYPE(arr);
names = dtype->names;
if (names != NULL) {
names = PySequence_Fast(names, NULL);
N = PySequence_Fast_GET_SIZE(names);
for (i=0; i<N; i++) {
name = PySequence_Fast_GET_ITEM(names, i);
cout << setw(7) << left << PyString_AsString(name);
x1 = PyObject_GetItem((PyObject *) arr, name);
x2 = (PyArrayObject *) x1;
dtype = PyArray_DTYPE(x2);
iter = NpyIter_New(x2, NPY_ITER_READONLY, NPY_KEEPORDER, NPY_SAME_KIND_CASTING, dtype);
if (iter == NULL) {return NULL;}
dataptr = (double **) NpyIter_GetDataPtrArray(iter);
iternext = NpyIter_GetIterNext(iter, NULL);
do {
index = NpyIter_GetIterIndex(iter);
if (index==0) {
cout << setw(6) << right << index << setw(9) << setiosflags(ios::fixed) << setprecision(4) <<**dataptr << endl;
} else {
cout << " " << setw(6) << right << index << setw(9) << setiosflags(ios::fixed) << setprecision(4) << **dataptr << endl;
}
} while (iternext(iter));
}
NpyIter_Deallocate(iter);
}
return Py_BuildValue("i", 0);
}
static PyMethodDef pyproj4methods[] = {
{"readarray", readarray, METH_VARARGS, "Documentation"},
{NULL, NULL, 0, NULL}
};
PyMODINIT_FUNC initpyproj4(void) {
Py_InitModule("pyproj4", pyproj4methods);
import_array();
}
Python Code
import numpy as np
import pyproj4 as p4
np.random.seed(22)
## Python Implementation ##
dt = np.dtype([('field1', '<f8', (1,2)), ('field2', '<f8', (2,2)), ('field3', '<f8', (3,1))])
x = np.zeros(2, dtype=dt)
for name in x.dtype.names:
m,n,p = x[name].shape
x[name] = np.random.randn(m,n,p)
it = np.nditer(x[name], ['c_index'], ['readonly'])
for num in it:
if it.index==0:
print '{0:6s} {1:6d} {2: 2.4f}'.format(name, it.index, num.item())
else:
print '{0:6s} {1:6d} {2: 2.4f}'.format(' ', it.index, num.item())
print '-----------------------'
## C-API Implementation ##
p4.readarray(x)
The output in both cases looks like:
field1 0 -0.0919
1 -1.4634
2 1.0818
3 -0.2393
field2 0 -0.4911
1 -1.0023
2 0.9188
3 -1.1036
4 0.6265
5 -0.5615
6 0.0289
7 -0.2308
field3 0 0.5878
1 0.7523
2 -1.0585
3 1.0560
4 0.7478
5 1.0647
If you know a better way to accomplish this, please don't hesitate to post your solution.
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