# Using previous value to get the next in for loop

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Joel Childs on 12 May 2021
Commented: Joel Childs on 12 May 2021
Hi guys,
I have what I believe is a simple problem but I cannot manage to get to work!
I have a (6003 x 1) vector of u-values. I want to create a vector of y-values which are defined using the previous value and its corresponding u-value as such:
h=0.1;
y(1) = 0
y(2) = y(1) + h.*u(1)
y(3) = y(2) + h.*u(2) ... etc.
At the moment, my loop is defined as such:
h=0.1;
y(1) = 0;
for k=2:6003
y(k) = y(k-1) + h.*u(k-1)
end
But this makes my command window seemingly run forever. How do I correct this code? Any help is much appreciated!

Stephan on 12 May 2021
Edited: Stephan on 12 May 2021
The semicolon operator supresses outputs to the command window:
h=0.1;
y(1) = 0;
for k=2:6003
y(k) = y(k-1) + h.*u(k-1);
end
Joel Childs on 12 May 2021
Thank you both of you!

Stephen23 on 12 May 2021
Edited: Stephen23 on 12 May 2021
"But this makes my command window seemingly run forever."
So far no one has actually addressed why your code is inefficient, in particular:
1. lack of array preallocation before the loop,
2. pointless printing to the command window.
Your code will be quite efficient once you deal with both of those, for example:
h = 0.1;
n = 6003;
y = zeros(1,n);
for k = 2:n
y(k) = y(k-1) + h.*u(k-1);
end
Joel Childs on 12 May 2021
Much obliged, thank you

R2021a

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