What do you want b(1:19) to be?
You could set up the indexes in advance, but it will take about 3 to 4 times as long to do the vectorized way (for a million elements) as the loop way.
a = rand(1, 1000000);
b = a; % Initialize b to be same length as a.
% Method 1: vectorized:
% Assign indexes to do a vectorized loop
tic
% Assign the twentieth element (ONLY) of b.
b(20)= max(a(1:20));
indexes = 21:length(a);
b = b(indexes - 1) + a(indexes);
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% Method 2 for loop:
tic
% Assign the twentieth element (ONLY) of b.
b(20)= max(a(1:20));
for i=21:length(a)
b(i) = b(i-1) + a(i);
end
toc
Results:
Elapsed time is 0.043703 seconds.
Elapsed time is 0.011023 seconds.
