MATLAB: Comparing 2 arrays with different lengths

Question

I have two arrays with different lengths (due to different sampling rates) that I need to compare. I'd like to downsample the larger array to match the smaller one in length, however the factor is not an integer but a decimal.

For an example:

a =
     1     1.375     1.75     2.125     2.5     2.875     3.25

b =
     1     2     3

Is there any way to manipulate these arrays to match lengths?

Answer 1

That's easy to do with clever use of interp1. The trick is that the keypoints used for interpolation is an array going from 1 up to as many values as you have in a which we will call N, and the interpolated keypoints would be a linearly increasing array where the first point is 1, the last point is N and you evenly divide up this range to have as many points as there are in b.

Simply put:

anew = interp1(1:numel(a), a, linspace(1, N, numel(b)));

linspace generates a linearly increasing array from 1 to N = numel(a) for as many points as you want, which we determine as the total number of elements in b. This exactly specifies the right keypoints you want to give you a downsampled version of a that matches the length of b, though there will be some interpolation required. The default interpolation method is linear.

Using the sample input from a you provided, we get:

>> anew

anew =

    1.0000    2.1250    3.2500