Function PyArray_Nonzero

numpy/core/src/multiarray/item_selection.c:2769–3050

NUMPY_API * Nonzero * * TODO: In NumPy 2.0, should make the iteration order a parameter. */

 * TODO: In NumPy 2.0, should make the iteration order a parameter.
 */
NPY_NO_EXPORT PyObject *
PyArray_Nonzero(PyArrayObject *self)
{
    int i, ndim = PyArray_NDIM(self);
    PyArrayObject *ret = NULL;
    PyObject *ret_tuple;
    npy_intp ret_dims[2];

    PyArray_NonzeroFunc *nonzero;
    PyArray_Descr *dtype;

    npy_intp nonzero_count;
    npy_intp added_count = 0;
    int needs_api;
    int is_bool;

    NpyIter *iter;
    NpyIter_IterNextFunc *iternext;
    NpyIter_GetMultiIndexFunc *get_multi_index;
    char **dataptr;

    dtype = PyArray_DESCR(self);
    nonzero = dtype->f->nonzero;
    needs_api = PyDataType_FLAGCHK(dtype, NPY_NEEDS_PYAPI);

    /* Special case - nonzero(zero_d) is nonzero(atleast_1d(zero_d)) */
    if (ndim == 0) {
        char const* msg;
        if (PyArray_ISBOOL(self)) {
            msg =
                "Calling nonzero on 0d arrays is deprecated, as it behaves "
                "surprisingly. Use `atleast_1d(cond).nonzero()` if the old "
                "behavior was intended. If the context of this warning is of "
                "the form `arr[nonzero(cond)]`, just use `arr[cond]`.";
        }
        else {
            msg =
                "Calling nonzero on 0d arrays is deprecated, as it behaves "
                "surprisingly. Use `atleast_1d(arr).nonzero()` if the old "
                "behavior was intended.";
        }
        if (DEPRECATE(msg) < 0) {
            return NULL;
        }

        static npy_intp const zero_dim_shape[1] = {1};
        static npy_intp const zero_dim_strides[1] = {0};

        Py_INCREF(PyArray_DESCR(self));  /* array creation steals reference */
        PyArrayObject *self_1d = (PyArrayObject *)PyArray_NewFromDescrAndBase(
            Py_TYPE(self), PyArray_DESCR(self),
            1, zero_dim_shape, zero_dim_strides, PyArray_BYTES(self),
            PyArray_FLAGS(self), (PyObject *)self, (PyObject *)self);
        if (self_1d == NULL) {
            return NULL;
        }
        ret_tuple = PyArray_Nonzero(self_1d);
        Py_DECREF(self_1d);