Use for loop array value to return into for loop

clear all
clc
clear all

% y' = -3y %
% Exact solution: y = 2*exp(-3x)
% Matlab solution: Follow slope over small x interval (dx) for
% range of x. Small dx should approach exact solution.

dx      =   1;          % Step size of x
x       =   0:dx:10;    % Range of x
slope   =   -3;         % Slope of formula, f(y)
y       =   zeros(size(x)); %Create blank array of y coordinates
y(1)    =   2;              %y-value at x=0, first (1) value in array

for i = numel(y)
    y(i) = y(i-1) + dx*h*y(i-1);
end
disp('y =')
disp (y)

Hello. I just started using matlab (or any other coding program) as a student. Now I ran into a problem:

For a first ode, I have to calculate a function's slope (ax) at a certain y-coordinate and use that slope to mimic the exact solution and plot both to compare.

Starting at y=2, I follow the slope for 1 dx. Slope = -3 y since dy/dx = -3y. Then I get y = 2 - 6 = -4. Then in this point I again calculate the next y-value with the slope in y=-4 and so on. But with my code matlab doesn't do what I want it to do, I've tried many other types of codes but I give up now. :(

What am I doing wrong?

Greetings,

Martijn Jacques

Answer 1

[the cyclist]

I didn't look to see if this is the only problem with your code, but a problem is that your for loop has only one iteration. For the above code, it is equivalent to

for i = 11
    ...
end

I expect you want to do something like

for i = 2:numel(y)
    
end

[Martijn J.]

Wow that did it for me!

Thank you very much, sometimes it is the simple/smart things I look over.