MATLAB: Conditional average (need help with speed)

Question

I have a table that looks like this:

country_id   year      M       T      average_T
1            2000      10      76     NaN 
1            2001      5       39     Mean of 76 and 62
1            2002      NaN     37     Mean of 39 =39
1            2003      15      5      NaN
1            2004      10      28     Mean of 5 and 2
1            2005      10      8      Mean of 8=8
2            1999      15      1      NaN
2            2000      10      62     Mean of 1=1
2            2001      20      32     Mean of 76 and 62
2            2002      10      72     Mean of 32=32
2            2003      15      2      Mean of 5 and 2

I want to calculate the column average_T which is last year's average of the T values for the cases that have the same year and M value. (First entry for each id is NaN because we don't know past year's T for those entries)

I have written a code that can do this but it is impossible to run with my big data set:

mytable.average_T=NaN(N,1);
for k=2:N 
    if mytable{k,'country_id'} == mytable{k-1,'country_id'} 
        mytable.average_T(k,1)= mean(T(mytable.M==mytable.M(k-1)& ...
            mytable.year==mytable.year(k-1)), 'omitNaN');
     end
end

Answer 1

Grouping variables and rowfun to the rescue...

tMeans=rowfun(@(x),mean(x,'omitnan'),mytable,'InputVariables','T','GroupingVariables',{'year','M'});