Need help using a loop to find maximum

Here is my code so far:

%Finding the max N given z
clc; clear;
c = [2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3]; %example data set
N=length(c);
eta=4;
D=100;

for i = 1:N
    q(i)=(D*(N-(1/eta)))^eta * ((1-(N-1)*eta)*c(i) + eta*(sum(c)-c(i)))/(sum(c)^(1+eta));      
end

j=1;

for i = 1:N
    if q(i) > 0
        c_new(j)=c(i);
        j=j+1;
    end
end

N=length(c_new);
for i = 1:N
    q_new(i)=(D*(N-(1/eta)))^eta * ((1-(N-1)*eta)*c_new(i) + eta*(sum(c_new)-c_new(i)))/(sum(c_new)^(1+eta));
end

So basically I am wanting to take a given data set, [c], run each variable in [c] through a function to produce a second dataset, [q]. If any value of q is negative, I want to get rid of those corresponding entries in [c], run [c] again through the function and get a new set of values in [q]. I want to keep doing this iteration and check until there are no negative values in [q]. In the end I should have some value for N, the number of entries in [c].

I am a bit inexperienced with programming and not sure how to capture this concept and put it in Matlab.

Your help is much appreciated.

Answer 1

[Fangjun Jiang]

You are close. What you need is a while-loop. I've modified your code to make it work for this particular example data. It still has many potential problems, such as what if q never gets all positive? Also, there are many places that can be improved. Hope you can start from here and improve the code gradually.

%Finding the max N given z
clc; clear;
c = [2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3]; %example data set
eta=4;
D=100;

q_is_all_positive=false;
%
while ~q_is_all_positive
N=length(c);
for i = 1:N
    q(i)=(D*(N-(1/eta)))^eta * ((1-(N-1)*eta)*c(i) + eta*(sum(c)-c(i)))/(sum(c)^(1+eta));      
end

j=1;
q_is_all_positive=true;
for i = 1:N
    if q(i) > 0
        c_new(j)=c(i);
        j=j+1;
    else
        q_is_all_positive=false;
    end
end

c=c_new;
clear q c_new;
end

N=length(c)

[Jamie]

Thanks!