How can i run the for loop with a stepsize smaller than one?

I have this code:

c=0:0.001:1;
volume =0.75*pi*c-pi*c.^3;
plot(c,volume)

n=4;

for i=1:4

   func=@(r) -pi*r^3+0.75*pi*r;
    if func(i+1)-func(i)<0 && func(i)-func(i+1)>0
   a=func(i);
   disp( -a )

   break
    else
        disp('no')
   end  

end

I want to find the maximum volume, it is located in x=0.5 but the problem is that when i run the code the first step goes to one, which is already a negative value. I've tried: i=0:0.001:1, since that is the part where the maximum value is, but i get a totally different answer. Help would be appreciated, thank you very much.

Answer 1

[Walter Roberson]

Your

func=@(r) -pi*r^3+0.75*pi;

needs to be

func=@(r) -pi*r^3+0.75*pi*r;

if you want to match your plot.

[Alvaro García]

i have corrected that now, but i still have the same problem. Thanks.

[per isakson]

Replace

    for i=1:4

by

    for i = 1:0.01:4

Answer 2

[Image Analyst]

You're not indexing the c ("x") properly. See my code below:

c=0:0.001:1;
volume =0.75*pi*c-pi*c.^3;
plot(c,volume, 'b-')
grid on;
xlabel('c', 'FontSize', 16);
ylabel('Volume', 'FontSize', 16);

n=4;
for k = 1 : length(c)
  ck = c(k);
  ck1 = c(k + 1);
  fprintf('c(%d)  = %f\n', k, ck);
  func=@(r) -pi*r^3+0.75*pi*r;
  if func(ck1)-func(ck)<0 && func(ck)-func(ck1)>0
    a=func(ck);
    fprintf('-a = %d\n', -a);
    fprintf('Found peak at index %d, where c = %f\n', k, ck);
    break
  else
    fprintf('no\n')
  end  
end

Just copy and paste it and try it. It works.