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

Question

Aaron Ridall 2020-4-13

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评论： Ameer Hamza 2020-4-14

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

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Answer 1

Peng Li 2020-4-13

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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.

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Aaron Ridall 2020-4-13

That fixed the first issue. Thanks so much. Now it states that index exceeds the number of array elements?

Peng Li 2020-4-13

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?

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Answer 2

Ameer Hamza 2020-4-13

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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

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Dave 2020-4-13

Step through your script. What are you trying to do by C_PS(i-1)? As stated above I don't think you are wanting to reduce the index of C_PS by 1.

Ameer Hamza 2020-4-14

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.

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