Generate vector from cycle for

I have some problem with for cycle. First problem:

I have this vector Gen of 2678400 values, and its shape is shown in figure. I want to create a for cycle that calculate the mean value from the vector Gen every 86400 values. like from 1*86400 to 2*86400 ---> mean value 1 from 2*864400 to 3*86400 ---> mean value 2 etc etc until the end. And i want that the cycle generate a vector M that contain all the 30 mean values.

I tried with this

    for i=1:30

    day=Gen(i*86400:(i+1)*86400);
    M=mean(day);

but the code get me to the ws just the last vector day from 30*86400 to 31*86400 and just the mean value M that corresponds to that interval.

Second problem: is it possible to do the mean value only between the values that are different from zero for each of the thirty curves?

thanks a lot

[Stephen23]

https://www.mathworks.com/help/matlab/matlab_prog/preallocating-arrays.html

Answer 1

[Andrei Bobrov]

Variants without use loops

day1 = mean(reshape(Gen,86400,[]));

or

day1 = accumarray(ceil((1:numel(Gen))'/86400),Gen,[],@mean);