If you want to store the results of each loop in a 2 by numSteps matrix you can do this
numSteps= 10000; %k steps
c = 0.000001; %constant
a0 = [1;1] %first guess
% preallocate array to hold results
a = zeros(2,numSteps)
a(:,1) = a0 -c * M' * (M* a0 -f) %first computation
for k=2:length
a(:,k) = a(k-1) - c * M' * (M * a(k-1) - f);
end
Note, I changed your variable name from length to numSteps, length is a Matlab command that gives the maximum dimension of a matrix. Using it as a variable name will work, the variable assignment supersedes the MATLAB function, but is confusing if you think length is a function
