ODE45 and expr function

Hi, I have this problem where I get only the last equation that is executed in the program (%N ) (the expr equations) and I want them to all run together and asign to them names like the comments C ,N,O,.

.............

tic,[t,n] = ode45(@(t,n)myequations(t,n,k1,k2,k3,k4,k5,k6....),[0 10e-3],IC)
toc
%Conservation de la masse:
%C:
expr_at_i(:)= n(:,1) + 2*n(:,9)+2*n(:,10)+3*n(:,13)+...
    n(:,22)+n(:,26)+n(:,34)+2*n(:,35)+n(:,36)...
    +n(:,37)+2*n(:,39)+4*n(:,40)+5*n(:,41)
%O:
expr_at_i(:)= n(:,4)+2*n(:,15)+2*n(:,16)+2*n(:,17)+n(:,22)...
    +n(:,24)+ 3*n(:,29)+n(:,30)+2*n(:,32)+2*n(:,34)+...
    n(:,36)+n(:,37)+3*n(:,38)+ n(:,39);
 %N 
 expr_at_i(:)= n(:,7)+2*n(:,20)+n(:,24)+n(:;26)+2*n(:,30)+n(:,32)...
     +n(:,35)+n(:,36)+n(:,37)+n(:,38)
 
 function dndt=myequations(t,n,k1,k2,k3,k4,k5,k6,k7,k8,k9,k10.....)
dndt=zeros(43,1);
%Chemin C
dndt(1)= k1*n(2)*n(42)+k15*n(5)*n(9)+k16*n(5)*n(26)+k25*n(4)*n(26)...
+k29*n(4)*n(9)+k33*n(4)*n(11)+k39*n(7)*n(26)+k40*n(7)*n(27)...

%Conservation de la masse:
%C:
expr_at_i(:)= n(:,1) + 2*n(:,9)+2*n(:,10)+3*n(:,13)+...
    n(:,22)+n(:,26)+n(:,34)+2*n(:,35)+n(:,36)...
    +n(:,37)+2*n(:,39)+4*n(:,40)+5*n(:,41)
%O:
expr_at_i(:)= n(:,4)+2*n(:,15)+2*n(:,16)+2*n(:,17)+n(:,22)...
    +n(:,24)+ 3*n(:,29)+n(:,30)+2*n(:,32)+2*n(:,34)+...
    n(:,36)+n(:,37)+3*n(:,38)+ n(:,39);
 %N 
 expr_at_i(:)= n(:,7)+2*n(:,20)+n(:,24)+n(:;26)+2*n(:,30)+n(:,32)...
     +n(:,35)+n(:,36)+n(:,37)+n(:,38)
 

[Torsten]

You overwrite the first two expr_at_i by the third one because you use the same name for three different output variables.

Instead of expr_at_i, use mass_C, mass_O and mass_N in the three places.

[Imene Yed]

@Torsten it is possible to write them in another script (the expr equation) and use the script of ode45 to import the t and n values. Thnk you so much you're helping a lot . With the methode that you mentioned earlier ?