MATLAB Central Newsreader - average every 12th row of matrix

average every 12th row of matrix

Nix Nicholson — Thu, 05 Nov 2009 17:19:03 -0500

Hi: Beginner question - I have 7 years of some monthly data fields and I want to compute the average for each month over the 7 year period. I can do it by specifying row numbers but how could I do this so that come next year, when i have 8 years data I can just change a scaler value for year number and it will recompute a new average?
basically how do i compute a mean of every nth row, in my case every 12th.

Many thanks

Re: average every 12th row of matrix

the cyclist — Thu, 05 Nov 2009 17:30:35 -0500

"Nix Nicholson" <linznix@gmail.com> wrote in message <hcv1e7$3ua$1@fred.mathworks.com>...
> Hi: Beginner question - I have 7 years of some monthly data fields and I want to compute the average for each month over the 7 year period. I can do it by specifying row numbers but how could I do this so that come next year, when i have 8 years data I can just change a scaler value for year number and it will recompute a new average?
> basically how do i compute a mean of every nth row, in my case every 12th.
>
> Many thanks

You should be able to user the FILTER command to do this.

Re: average every 12th row of matrix

Matt — Thu, 05 Nov 2009 17:30:36 -0500

"Nix Nicholson" <linznix@gmail.com> wrote in message <hcv1e7$3ua$1@fred.mathworks.com>...
> Hi: Beginner question - I have 7 years of some monthly data fields and I want to compute the average for each month over the 7 year period. I can do it by specifying row numbers but how could I do this so that come next year, when i have 8 years data I can just change a scaler value for year number and it will recompute a new average?
> basically how do i compute a mean of every nth row, in my case every 12th.

Instead of organizing your data as a vector, you can organize it as an Nx12 matrix A where N is the number of years. Then do

MonthlyMeans=mean(A,1);

When you have N=8, just add a row and do the same thing.

Re: average every 12th row of matrix

dpb — Thu, 05 Nov 2009 17:26:55 -0500

Nix Nicholson wrote:
> Hi: Beginner question - I have 7 years of some monthly data fields
> and I want to compute the average for each month over the 7 year
> period. I can do it by specifying row numbers but how could I do this
> so that come next year, when i have 8 years data I can just change a
> scaler value for year number and it will recompute a new average?
> basically how do i compute a mean of every nth row, in my case every
> 12th.
...

idx=1:12:length(x);
avg = mean(idx,:);

Salt to suit on selecting starting month, etc., ...

--

Re: average every 12th row of matrix

dpb — Thu, 05 Nov 2009 18:32:02 -0500

dpb wrote:
> Nix Nicholson wrote:
>> Hi: Beginner question - I have 7 years of some monthly data fields
>> and I want to compute the average for each month over the 7 year
>> period. I can do it by specifying row numbers but how could I do this
>> so that come next year, when i have 8 years data I can just change a
>> scaler value for year number and it will recompute a new average?
>> basically how do i compute a mean of every nth row, in my case every
>> 12th.
> ...
>
> idx=1:12:length(x);
> avg = mean(idx,:);
>
> Salt to suit on selecting starting month, etc., ...

Should have been

avg = mean(x(idx,:));

of course...

--

Re: average every 12th row of matrix

Lindsey — Thu, 01 Sep 2011 10:29:13 -0400

Hello, thanks to you all.

I used dpb's solution written out as:

for i=1:12; monthmean(i,:)=mean(x([i:12:end],:)); end

where x is my original dataset with each row representing a month value and each column representing a variable.