# Array indices must be positive integers or logical values error in a loop.

1 view (last 30 days)
Aaron Ridall on 13 Apr 2020
Commented: Ameer Hamza on 14 Apr 2020
I'm working on writing a for loop and I expect that the value the loop will generate is negative, because it represents a decrease in particle concentration over time. However, the loop runs once and then generates the error. My code is below.
w_PS=-0.015;
K= 0.0015; %vertical turbulent diffusivity in m/s
C_PS= 29680; %concentration of particles in a fluid at the start
t= 0; %time
dz=-1;
dt=1;
z=0;
for i=1:365/dt
dC_PSdz = C_PS(i)-(C_PS(i)*w_PS*t(i));
dsink_PSdz = w_PS*C_PS(i)-K*dC_PSdz;
dC_PSdt = dsink_PSdz %dsinkdz = w*C-K*dCdz;
C_PS(i) = C_PS(i-1)+dC_PSdt*dt;
t(i) = t(i-1)+dt;
z(i) = z(i-1)+dz*dt;
end

Peng Li on 13 Apr 2020
I doubt it even could finish once as you are indexing C_PS(0), and t(0), and z(0). Didn't check very carefully but I think if you change for i=1:365/dt to for i = 2:365/dt will work.
Peng Li on 13 Apr 2020
because this time you are indexing C_PS(2) before it actually reaches that size. You'd better figure out what you really need to do. Or give an equation here. You are using the current C_PS to calculate dC_PSdz and are using the previous C_PS to update the current C_PS. is that what you intended to do?

Ameer Hamza on 13 Apr 2020
There are indexing issues in your code
w_PS=-0.015;
K= 0.0015; %vertical turbulent diffusivity in m/s
C_PS= 29680; %concentration of particles in a fluid at the start
t= 0; %time
dz=-1;
dt=1;
z=0;
for i=2:365/dt
dC_PSdz = C_PS(i-1)-(C_PS(i-1)*w_PS*t(i-1)); %<------ see the indexes
dsink_PSdz = w_PS*C_PS(i-1)-K*dC_PSdz; %<------ see the indexes
dC_PSdt = dsink_PSdz %dsinkdz = w*C-K*dCdz;
C_PS(i) = C_PS(i-1)+dC_PSdt*dt;
t(i) = t(i-1)+dt;
z(i) = z(i-1)+dz*dt;
end
##### 2 CommentsShowHide 1 older comment
Ameer Hamza on 14 Apr 2020
Dave, the for loops start from 2, so in the first iteration, you are reading the value C_PS(2-1) = C_PS(1). You cannot read value C_PS(2) at that time because it does not exist.