Question

I need to create a numpy array of N elements, but I want to access the
array with an offset Noff, i.e. the first element should be at Noff and
not at 0. In C this is simple to do with some simple pointer arithmetic, i.e.
I malloc the array and then define a pointer and shift it appropriately.

Furthermore, I do not want to allocate N+Noff elements, but only N elements.

Now for numpy there are many methods that come to my mind:

(1) define a wrapper function to access the array
(2) overwrite the [] operator
(3) etc

But what is the fastest method to realize this?

Thanks a lot!
Mark

Answer 1

You’ve already given (1) and (2) as both more or less sensible methods. To test speed for these kind of things try timeit magic function in ipython. Example usage:

A = array(range(10))
Noff = 2
wrapper_access = lambda i: A[i - Noff]
print wrapper_access(2)   #0
print wrapper_access(11)  #9
print wrapper_access(1)   #9 = A[-1]
timeit wrapper_access(5)

On my machine I get output from timeit 10000000 loops, best of 3: 193 ns per loop