Cython: creating an array throws “not allowed in a constant expression”

Question

I try to rewrite a complex function form Python to Cython to speed it up massively and I encounter the following problem: while compiling my function hh_vers_vector.pyx using

Answer 1

cdef in Cython is for C definitions, as the name implies. Therefore the code

cdef float v[10000]

is translated to the following C code

float v[10000];

Meaning a static float array of size 10k, defined at compile time.

This, on the other hand

cdef float v[numSamples]

Would translate to the C code

int numSamples = <..>;
float v[numSamples];

This is an attempt to compile a static array containing a variable number of elements, which is not valid C. Hence the 'constant expression error'.

Either use a python list to store the floats, or dynamically allocate C arrays via malloc/free.