Clear Filters
Clear Filters

Why is this simple loop not working?

5 views (last 30 days)
Hi there,
I think I have used this procedure for the following loop many times, but now it isn't working:
h = 3
z = 4
y = 0
x = 0
for i = 1:3
z(i+1) = z(i) + (2*y(i) + 8*x(i)*(9 - x(i)))*h
y(i+1) = y(i) + z(i)*h
z(i) = z(i+1);
y(i) = y(i+1);
x(i) = x(i) + h
end
I keep getting an error saying Index must not exceed 1.
I don't know why this is happening.
Can someone help please?

Accepted Answer

Stephen23
Stephen23 on 17 Jun 2024
Edited: Stephen23 on 17 Jun 2024
"I don't know why this is happening."
The problem is very simple: you do not extend x in the same way that you extend y and z.
For both y and z you extend them using the indexing i+1 before trying to access that new index position. No problems with them!
However, with x you try and access x(i) on the next loop iteration without making any attempt to define that index position first. In other words, you never make x larger. So on the 2nd loop iteration, your code tries to access x(2) which does not exist!
"I think I have used this procedure for the following loop many times, but now it isn't working:"
Nope, it never worked trying to access an index position that does not exist. You must have changed something.
Perhaps the last line should be this?:
x(i+1) = x(i) + h;
  5 Comments
Torsten
Torsten on 17 Jun 2024
Edited: Torsten on 17 Jun 2024
When the loop index goes from i to i+1, you will automatically plug in z(i+1) for z(i). Your setting z(i) = z(i+1) only deletes z(i) and replaces it by z(i+1) which usually is unwanted.
I suggest you test what you get for the variables with your loop and make a parallel calculation with pencil and paper.
Scott Banks
Scott Banks on 17 Jun 2024
yes, sorry, you are right. I think I did that because sometimes in this situation I might assign y0, y1, z0 z1 etc rather than indexing them with 'i'. Anyways, it's good to know going foward. Thanks.

Sign in to comment.

More Answers (1)

Avni Agrawal
Avni Agrawal on 17 Jun 2024
I understand that the error message you are encountering is "Index must not exceed 1,". This suggests that MATLAB is treating `z`, `y`, and `x` as scalar variables, not arrays. This happens because you initialized `z`, `y`, and `x` as scalars (single values) rather than as vectors or arrays. When you attempt to access or assign a value to an index greater than 1 (e.g., `z(i+1)`), MATLAB throws an error because it expects `z` to have only one element.
To fix this issue, you need to initialize `z`, `y`, and `x` as arrays with predefined sizes before the loop. Additionally, ensure `h` is defined before the loop. If `h` is not defined, MATLAB will throw an error because it won't know how to increment `x` or calculate the new values for `y` and `z`.
I hope this helps!

Categories

Find more on Loops and Conditional Statements in Help Center and File Exchange

Tags

Community Treasure Hunt

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

Start Hunting!