MATLAB: Why do i keep getting this error “Error using + Matrix dimensions must agree”

errorusing

Please assist:
When i try running the following i keep getting the above error:
%initial conditions
y0=[19.125;19.125;114.75;51;51;1;2;1;55;50;0.5;3.187;3.187;19.125;8.5;8.5;1;2;1;55;50;0.5;1.594;1.594;9.5625;4.25;4.25;1;2;1;55;50;0.5;1.594;1.594;9.5625;4.25;4.25;1;2;1;55;50;0.5;6.375;6.375;38.250;17;17;1;2;1;55;50;0.5];
t0=0;
tEnd=282;
h=0.0006944444;
N=ceil((tEnd-t0)/h);
%initilising solution
T=t0:h:tEnd;
Y=zeros(55,N+1);
Y(:,1)=y0;
%boundaries
for i=1:N  
    T(i+1)=T(i)+h;
    
    Y(Y<0)=0;
    y11=Y(11);
    y11(y11>8.5)=8.5;
              
   %update y
  
    yi=Y(:,i);
    a1=UASBFun(T(i), yi);
    a2=UASBFun(T(i)+0.5*h, yi+0.5*a1*h);
    a3=UASBFun(T(i)+0.5*h, yi+0.5*a2*h);
    a4=UASBFun(T(i)+    h, yi+       a3*h);
    yNew=yi+(h/6)*(a1+2*a2+2*a3+a4);
    Y(:,i+1)=yNew;
    
end
    
 %plotting
 
  yEnd=Y(:,end);
  plot(T,Y(50,:),'-r'); hold on
  plot(T,Y(51,:),'-g'); hold on
  plot(T,Y(52,:),'--r'); hold on
  plot(T,Y(53,:),'-k'); hold on
  plot(T,Y(54,:),'-b'); hold on
  plot(T,Y(55,:),'--g'); hold off
  
  xlabel('Time (days)'); ylabel('Concentration (g/m^3)');
Function:
function fval=UASBFun(~,y)
%Define  variables
Xaoba=y(1);      Xaobb=y(12);     Xaobc=y(23);   Xaobd=y(34);   Xaobe=y(45);
Xnoba=y(2);      Xnobb=y(13);     Xnobc=y(24);   Xnobd=y(35);   Xnobe=y(46);
Xamxa=y(3);      Xamxb=y(14);     Xamxc=y(25);   Xamxd=y(36);   Xamxe=y(47);
Xhana=y(4);      Xhanb=y(15);     Xhanc=y(26);   Xhand=y(37);   Xhane=y(48);
Xhaera=y(5);     Xhaerb=y(16);    Xhaerc=y(27);  Xhaerd=y(38);  Xhaere=y(49);
Xsa=y(6);          Xsb=y(17);         Xsc=y(28);       Xsd=y(39);        Xse=y(50);
Ssea=y(7);        Sseb=y(18);       Ssec=y(29);       Ssed=y(40);    Ssee=y(51);
Sno3a=y(8);      Sno3b=y(19);     Sno3c=y(30);   Sno3d=y(41);   Sno3e=y(52);
Sno2a=y(9);      Sno2b=y(20);     Sno2c=y(31);   Sno2d=y(42);   Sno2e=y(53);
Snh4a=y(10);     Snh4b=y(21);     Snh4c=y(32);   Snh4d=y(43);   Snh4e=y(54);
So2a=y(11);      So2b=y(22);      So2c=y(33);    So2d=y(44);     So2e=y(55);
The functions for each variable are long though hence i stop at this.
PLease someone assist here please…..

Best Answer

yi is initialized from one column of Y, Y is initialized to 55 rows. So yi has 55 entries.
a1 is computed by calling UASBFun, passing in something that is ignored, and passing in yi. UASBFun takes in those 55 values, and computes 11 fval from it.
The code for calculating a2 tries to do yi + 0.5 * a1 * h . 0.5 and h are scalar, a1 is length 11 as noted a moment ago. yi is length 55 as noted a moment ago. So you are trying to add something length 55 together with something length 11. That is not going to work.
I do not think that your code can be fixed; I think you will have to abandon it.
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