C=[0;cumsum(B)];
where B is your variable in column B
MATLAB uses vector operations; to append a vector to a single value (or even another vector) just once, the explicit catenation shorthand using [] and ; is fast enough to not bother with the preallocation and explicit addressing of indices within the preallocated output array.
If one were doing this inside a for...end loop of significant size, then the preallocation step would be needed to prevent rapid performance degradation by the need for dynamic allocation and reallocation.
