Rms calculation of specific rows

I'd like to calculate rms of 3rd and 4th columns of a 21600x4 matrix (filedata{i}) but I couldn't do it. If you have any idea about how to do I'll be appreciate it. Thanks in advance.

M_Rms(:,:,i)= rms(filedata{i},3);

Answer 1

[Scott MacKenzie]

The rms of the data in column 3 in the matrix filedata is

rms(filedata{:,3})

[mehtap agirsoy]

Thanks Scott.I've tried:

A=rms(filedata{i}(:,3:4,i),3);

M_Rms(:,:,i)=[filedata{i}(:,1:2) A];

but after 7th iteration I get an error :Index in position 3 exceeds array bounds (must not exceed 4). I'm really confused.