Hello, I want to make function Runge-Kutta orde 4 with equation depend on time, but in MATLAB i get some error

I want to use this equation with runge-kutta orde 4

function M1 =fRK4M1(M1)
 %M1 =M1(j)+1./(1+exp(-T(j)));
 m1 = (50*M1(j+1)*(1-(M1(j+1)/75))-2*(10^-4)*M1(j+1)*M1(j+1)-M1(j+1)*((5*10^-4).*M2(j+1))-((10-3)*(10^-4)*M1(j+1)*M3(j+1))-((100-10)*0.1*M1(j+1)*O(j+1))-((10^-3)*M1(j+1)));
 M1 = dt *m1;
end

Answer 1

[Cris LaPierre]

You have not defined j, so in MATLAB, by default that letter is used to create complex numbers. As the error message states, your index must be positive integers.

j+1

ans = 1.0000 + 1.0000i

a=1:5;
a(j+1)

Array indices must be positive integers or logical values.

[cindyawati cindyawati]

Thank you for your response @Cris LaPierre. So, I must using looping like this? Because j is depend on time

t(1)=0;
dt=0.01; %time interval
t=0:dt:100; %time span

for j = 1:length((t)-1) 
    T(j+1)=T(j)+dt;
    M1 =M1(j)+1./(1+exp(-T(j)));
end

Unrecognized function or variable 'T'.

[Cris LaPierre]

Keep in mind that MATLAB is case sensitive. T and t are different variables.