Problem using a counter in a for loop

The concentration of a drug in the body CP can be modeled by the equation:

where DG is the dose administered (mg), Vd is the volume of distribution (L), ka is the absorption rate constant (h-1), ke is the elimination rate constant (h-1), and t is the time (h) since the drug was administered. For a certain drug, the following quantities are given: DG = 150 mg, Vd = 50 L, ka = 1.6 h-1, ke = 0.4 h-1.

Assume a first dose is administered at t = 0, and subsequently four more doses are administered at intervals of 4 hours (i.e. at t = 4, 8, 12, 16). Write a MATLAB program to calculate and plot Cp versus t for 24 hours in increments of 0.1 hours. Use a for loop to calculate Cp for each time increment and add drug dosages at the appropriate times. Consider using a loop counter (e.g. k=k+1) to address the Cp array at each time increment (i.e. Cp(k)). To plot Cp versus t, you may need to create an array for the x-axis values (or time values).

Dg = 150;%mg
Vd = 50;%L
ka = 1.6;%h^-1
ke = 0.4;%h-1

This is what I've done so far

t=0;
for t=1:0.1:24 % loop for time = 1 to 24 hours every 0.1 hour
if mod(t,6)==0 
Cp = (Dg/Vd)*(ka/(ka-ke))*(exp(-ke * t)-exp(-ka * t)) 
t=t+1;
end
end


plot(t,Cp)
title('Concentration over Time')
xlabel('Time (hr)')
ylabel('Concentration')

I have to use a for loop and I have to use a conditional. The error message I'm getting says "Variable Cp must be of size [1 241]. It is currentnly of size [1 1]. Check where the variable is assigned a value."

Answer 1

[madhan ravi]

Dg = 150;%mg
Vd = 50;%L
ka = 1.6;%h^-1
ke = 0.4;%h-1
t=1:0.1:24;
C=zeros(size(t));
for k=1:numel(t) % loop for time = 1 to 24 hours every 0.1 hour
if ~mod(t(k),6)
Cp(k) = (Dg/Vd)*(ka/(ka-ke))*(exp(-ke * t(k))-exp(-ka * t(k)));
end
end
plot(t,Cp)
title('Concentration over Time')
xlabel('Time (hr)')
ylabel('Concentration')

[madhan ravi]

I suggest you to use logical indexing. I assume that your supposed to use loop because it’s an assignment.

[EP]

This worked somewhat. Now the error message says "Variable Cp must be of size [1 241]. It is currently of size [1 231]. Check where the variable is assigned a value." I'm not sure what to alter.

[EP]

Yes, it's an assignment. The for loop is required.

[madhan ravi]

I didn’t get any error message running the code. Use clear all at the beginning and try again.

[EP]

Same result.

[madhan ravi]

Try changing the value of step size in t such that t has 241 values instead of 231.

[EP]

I'm sorry, I keep getting error messages. Now it just says Cp has an incorrect value. But I did upload the rest of the information given to me in the original problem. Maybe there's something in there that can help?

I really appreciate all of your assistance!!

[madhan ravi]

Use linspace() to specify number of interval for t such that it has 241 elements. I am not really sure how your teacher has set an algorithm for this problem.

[EP]

I reuploaded the question with all of the original problem. I tried using the linspace for t, and still getting an error message that variable Cp has incorrect values.

[Walter Roberson]

Time should start from 0 in the for loop, not from 1. You are missing entries for 0.1, 0.2, .. 0.9

Why are you taking mod 6 when the drugs are to be administered every 4 hours?

Where are you adding the dosage to what is currently in the body?

[EP]

Walter, I understand the changes in time you're suggesting. But what do you mean about adding the dosage? Do you mean a counter?

[Walter Roberson]

"four more doses are administered at intervals of 4 hours".

When a dose is given, it does not replace the current amount of drug in the body: it adds to the current amount of drug in the body. If, after 4 hours, (say) 3/4 of the first dose of drug has been metabolized, then upon the second dose, the body is not starting with 0 again: it would be starting from (e.g.) 1/4 dose, leading to (say) 5/4 of the dose in the body at the end of the injection. 4 hours later, 3/4 of that gets used, 5/16 of one dose left in body, add a full dose. 1+5/16 now in body. And so on.

Therefore every 4 hours, you need to add D_G to what is currently in the body.

[EP]

Are you suggesting a line inside the for loop Cp=Cp+Dg? That would add another dose after each loop. But it doesn't give the correct values.