Indexing on array !

Hello!

I have an array (a) which has 1M values. Array (b) should be calculated based on array (a) and last value in b.

b(20)= max(a(1:20));

for i=21:length(a)
b(i) = b(i-1) + a(i);
end

Is it possible to calculate the "b" without loop with indexing? something like:

b(21:length(a)) = b(20:length(a)-1) + a(21:length(a));

Thanks.

Answer 1

[Image Analyst]

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);
toc

% 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.

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