# Fixing Array Index Expression

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Mark Fasano on 3 Jan 2022
Commented: Rik on 4 Jan 2022
I am trying to solve a PDE numerically. I want to save the solution every second, which with a time step of k means I want to save it every 1/k iterations. However, in my code I have:
solution = zeros(t_final+1,N+1);
for i = 1:M
for j = 0:N
% Code that updates the solution is here
end
uold = unew;
if mod(i,1/k) == 0
solution(i*k,:) = unew;
end
end
and this works for updating the second row but then fails and gives the error "Index in position 1 is invalid. Array indices must be positive integers or logical values." I don't understand this though. Doesn't the if statement above preclude any instance where i*k isn't an integer? Does anyone know what the problem is? Since k is explicitly chosen, I can simply replace 1/k with the actual number for a given k and this works. But then every time I change the value of k I have to make sure I change that line as well and I want to avoid doing that if possible.
Just for clarification of what's going on in the interior for loop: I am using two vectors uold and unew to represent the solution to the equation at time steps n and n+1. Basically, I use values from uold to solve for unew and then just keep repeating up until my final time. The variable unew is a 1xN+1 vector that is then stored inside solution when we are at a integer value in time. Each row of solution then represents the solution at a given second -- i.e. row 1 would correspond to the IC preloaded prior to the code shown above, row 2 would correspond to the solution at t=1, row 3 would correspond to the solution at t=2, ..., row t_final+1 would correspond to the solution at t=t_final.
Also, t_final is a variable I set at the top and is always set to be an integer so there aren't any issues there
##### 1 CommentShowHide None
Rik on 4 Jan 2022
I suspect a floating point error. You should try checking if i*k is integer, instead of checking if the modulo of the inverse is 0.
Mathematically those two are the same, but you're working with numbers here, not symbolic variables.

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