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Re: how to vectorize this loop
Posted:
Apr 19, 2011 9:41 AM


"Roger Stafford" <ellieandrogerxyzzy@mindspring.com.invalid> wrote in message news:ioiqil$42j$1@fred.mathworks.com... > "Mateusz Gos" <webmaster24@wp.pl> wrote in message > <ioinh4$f1g$1@fred.mathworks.com>... >> hi, >> i would like to write the following loop in form of a vector, which I am >> guessing is possible (or not?): >> >> n = 1; >> for n = 1:20 >> R = mean(measurements(1:n)); >> n = n+1; >> end >> >> thanks, >> mat >         > You should not have the "n = n+1;" line in there. The forloop with n > takes care of advancing n. Also the line "n = 1;" is totally unnecessary > since the forloop will start at n = 1 no matter how you set n in advance. > > As your code stands at present you are taking twenty different mean > operations, the first one the mean of the single number 1, then the mean > of the two number 1 and 2, then that of 1, 2, and 3, and so forth. Is > that what you had in mind? If so, all but the last one of these means > from 1 to 20 have been overwritten because you placed them in the same > variable R without indexing it with n. > > If all you want is the single mean of the numbers from 1 to 20, then do > this: > > R = mean(1:20); > > If you want the twenty different means, do this: > > R = zeros(20,1); > for n = 1:20 > R(n) = mean(1:n); > end
For this particular problem, since the mean of a vector of values is the sum of the elements in the vector divided by the number of elements in the vector, you could use CUMSUM.
% Sample data measurements = rand(1, 20); n = 20;
% Engine R = cumsum(measurements(1:n))./(1:n);
% Spot check the answer R(5)  mean(measurements(1:5))
 Steve Lord slord@mathworks.com To contact Technical Support use the Contact Us link on http://www.mathworks.com



