have to make a function and return an answer of how long before it passes a certain point. does not work. help needed.

i have to make a function that look like this

n(t)=n(t-1)*(1+alpha*(1-n(t-1)/K))

and know when n(t) surpasses a certain point,N.

my code looks like this:

 function tN = bacteriaGrowth(n0,alpha,K,N)
 for t=1:20
  n(t)=n(t-1)*(1+alpha*(1-n(t-1)/K));
  n(0)=n0;
  n(1)=n0*(1+alpha*(1-n0/K));
 end
 if n(t)>=N
    tN=t
 end

i just get an error in line 4, that doesn't explain what is wrong. help would be very appreciated.

[Stephen23]

"i just get an error in line 4, that doesn't explain what is wrong"

And that error is obviously so useless you want to keep it a secret. Please actually try to help us by giving editing your question and giving the complete error message. This means all of the red text.

When I tried running your code I got this error:

??? Undefined function or variable "n".
Error in ==> bacteriaGrowth at 3
  n(t)=n(t-1)*(1+alpha*(1-n(t-1)/K));

Which is obviously not the same error that you are getting because you wrote that your error "doesn't explain what is wrong", whereas the error that I get does. And an internet search of that error message gives a thousand explanations of how to fix it.

Answer 1

[Jan]

Please consider Stephen's advice and explain, what you observe and what you exactly want to calculate. I try it with a bold guess:

You have to start with n0 before you access n(t-1):

function tN = bacteriaGrowth(n0, alpha, K, N)
n = n0 * (1 + alpha * (1 - n0 / K));
t = 0;
while t < 20 && n < N
for t = 2:20  % Start at 2, because n(1) is defined already
  n = n * (1 + alpha * (1 - n / K));
  t = t + 1;
end
tN = t;

If you want to reply tN only, storing n in a vector is not useful.

Does this help you? If not, please explain the problem by editing the question, not in the sections for answers or comments. Thanks.

[jonas paludan]

thanks for the help. enden up adding some small stuff but your answer helped a lot. my final code: function tN = bacteriaGrowth(n0,alpha,K,N)

n = n0*(1+alpha*(1-n0/K)); t= 0;

while t<100000 && n < N

for t=2:100000

n=n*(1+alpha*(1-n/K)); if n>N

break

end

end

end

if n0>N

    tN=0;
else

tN=t;

end

end