Convert from int to double and back in Cython

edit

The statement

result += (fact - j*(<int>t))

will cast t to an int (dropping the fractional part like floor does) without calling floor. In the annotated cython code this change reduces the "yellowness" of that line. I'm not sure why it's yellow at all though, it seems like this statement should translate easily to pure C code.

If you want to use the C math library version of floor I think you need an appropriate include statement.

Edit:

If you use the @cython.cdivision(True) decorator, cython will not add exception checking for division by zero, this allows the division statement to be converted to one line of pure C code. In your code j is never zero, so this is okay. Also, you can skip the double t entirely if you just write fact/j. Since both are integers, the division occurs as integers and the result is the floor of the rational number fact/j.

Here is new code with the changes:

%cython
import cython

@cython.cdivision(True)    # turn division by zero checking off
def primeish(int n):
    cdef int fact
    cdef int result
    cdef int j
    cdef int t
    if n==1:
        return 0
    elif n==2:
        return 1
    elif n==3:
        return 2
    else:
        result = -1
        fact = 1
        for j in range(3,n+1):
            fact = fact*(j-2)
            t = fact / j    # integer division
            result += fact - t*j
        return result