Clear Filters
Clear Filters

have array entries filled depending on their indices?

1 view (last 30 days)
I want to fill an array
A=zeros(1000,1);
based on another array:
B=linspace(0,100,1000);
Now I want to fill every A(i) with the product of all B(k) with k=1:i-1, I do that alike this:
for i=1:length(A)
A(i)=prod(B(1:i-1));
end
For large sizes of A this becomes very time intensive. Would there be a way to do it faster? I couldnt find a way to use arrayfun for that.
And suggest I want to fill A(i), but do some more stuff inside the for loop?

Answers (1)

Jan
Jan on 23 May 2022
Edited: Jan on 23 May 2022
The code is strange, because all elements of A are 0 except for the first one. B(1) is zero, so prod(B(1:n)) is zeros also. Maybe you mean:
n = 10000;
B = linspace(1,100,n); % Not starting at 0
tic
A = zeros(n, 1);
for i = 1:n
A(i) = prod(B(1:i-1));
end
toc
Elapsed time is 0.106487 seconds.
% Alternative: A naive loop avoiding repeated work:
tic
D = zeros(n, 1);
c = 1;
for i = 1:n
D(i) = c;
c = c * B(i);
end
toc
Elapsed time is 0.006453 seconds.
% Faster:
tic
C = [1, cumprod(B(1:n - 1))].';
toc
Elapsed time is 0.002222 seconds.
Even the loop is much faster than calculating the product from scratch in each iteration again. cumprod is much faster again.

Products


Release

R2020b

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!