Subscripted Assignment Dimension Mismatch Error

Question

I'm trying to model the collisions of a ball and its x and y position over a certain period of time in a 500 by 500 box. Whenever I try to run the script for different time steps(h), I keep getting the error:

Subscripted assignment dimension mismatch.

global h
h_vector=[0.1 0.01 0.001];
t=0:h:500;

for j=1:length(h_vector)
h=h_vector(j);
[xout2,yout2]=walls_euler_method2e;
xout3(j,:)=xout2;
yout3(j,:)=yout2;
end
xout3;
yout3;

function [xout2,yout2]=walls_euler_method2e
global h
f1=5;%dx/dt
f2=-3;%dy/dt
x(1)=0;
y(1)=0;
t=0:h:500;
r=5;%radius of ball
hit_vertical_wall_left_first=0;
hit_horizontal_wall_down_first=0;
vertical_wall_left=250;
vertical_wall_right=-250;
horizontal_wall_up=250;
horizontal_wall_down=-250;
for i=2:length(t)
x(i)=x(i-1)+h.*f1;
y(i)=y(i-1)+h.*f2;
if x(i)==vertical_wall_left-r
    f1=-f1;
hit_vertical_wall_left_first=1;
elseif x(1)==vertical_wall_right+r&&hit_vertical_wall_left_first==1;
    f1=-f1;
else
    fl=f1;

 if y(i)==horizontal_wall_down+r
     f2=-f2;
     hit_horizontal_wall_down_first=1;

 elseif y(i)==horizontal_wall_up-r&&hit_horizontal_wall_down_first==1;
      f2=-f2;
 else
     f2=f2;
 end
end
end

xout2=x;
yout2=y;

On what line do you get the error? It means that you are trying to assign a number of elements to an unequal number of storage spaces, i.e. your left hand side is not equal in side to the right hand side. — Adriaan
I think the array xout2 is shorter or longer than the column width of xout3. Check this by doing size(xout2) and enlarger or shorten your arrays accordingly — Adriaan
So xout2 is longer than the column width of xout3m but I don't know how to fix this. For each iteration xout2 and xout3 should have the same column width — scoobydoo9000
I suggest moving that global h into the function's input variable. And make sure that xout3 is not already defined with an incompatible size: run clear all or at least clear xout3 before running your code. — Andras Deak

Adriaan Adriaan · Accepted Answer · 2015-11-01T20:16:52

Simple fix: use cell arrays:

xout3{j} = xout2;
yout3{j} = yout2;

You define t in steps of h, and since h becomes smaller, t increases in length, thus also your xout2. Cell arrays allow for dissimilar sized matrices, and you call them with curly braces instead of round ones.

Note that it is bad practise to use i or j as a variable. Also try and refrain from using global variables, it is better to pass your variables to the actual function, see the modified code:

function [xout2,yout2]=walls_euler_method2e(h)
f1=5;%dx/dt
f2=-3;%dy/dt
x(1)=0;
y(1)=0;
t=0:h:500;
r=5;%radius of ball
hit_vertical_wall_left_first=0;
hit_horizontal_wall_down_first=0;
vertical_wall_left=250;
vertical_wall_right=-250;
horizontal_wall_up=250;
horizontal_wall_down=-250;
for ii=2:length(t)
    x(ii)=x(ii-1)+h.*f1;
    y(ii)=y(ii-1)+h.*f2;
    if x(i)==vertical_wall_left-r
        f1=-f1;
        hit_vertical_wall_left_first=1;
    elseif x(1)==vertical_wall_right+r&&hit_vertical_wall_left_first==1;
        f1=-f1;
    else
        fl=f1;

        if y(i)==horizontal_wall_down+r
            f2=-f2;
            hit_horizontal_wall_down_first=1;

        elseif y(ii)==horizontal_wall_up-r&&hit_horizontal_wall_down_first==1;
            f2=-f2;
        else
            f2=f2;
        end
    end
end

xout2=x;
yout2=y;
end

Script:

h_vector=[0.1 0.01 0.001];
for jj=1:length(h_vector)
    t = 0:h_vector(jj):500;
    [xout2,yout2]=walls_euler_method2e(h_vector(jj));
    xout3{jj}=xout2;
    yout3{jj}=yout2;
end

Results in:

xout3 = 
    [1x5001 double]    [1x50001 double]    [1x500001 double]
yout3 = 
    [1x5001 double]    [1x50001 double]    [1x500001 double]

Subscripted Assignment Dimension Mismatch Error

1 Answers