Error in using Trapz

Please could you help me because when I use traps to get the area under the curve I had an error as follow:

Subscript indices must either be real positive integers or logicals.

Error in ipermute (line 22)

inverseorder(order) = 1:numel(order); % Inverse permutation order

Error in trapz (line 71)

if ~isempty(perm), z = ipermute(z,perm); end

Error in Untitled (line 136)

area=trapz(xx_star,hhh)

my code as follow:

sym xx_star;
for k=1:length(x2)
xx_star(k)=x2(k)
.
.
.
.

Nu(k)=1/yy1(1); %% it's constant
hhh(k)=(0.05*Nu(k))./(xx_star(k));
area=trapz(xx_star,hhh)

[Walter Roberson]

could you provide your program and data for testing ?

[Mohammad Qasem]

I sent it to you

Answer 1

[Walter Roberson]

Your code has

hhh(k)=(0.05*Nu(k))./(xx_star(k));
area=trapz(xx_star,hhh)

On the first iteration, k = 1, your xx_star is a scalar, and your hhh is a scalar. So you are calling

trapz(SCALAR, SCALAR)

it happens that when you call trapz with a scalar second parameter, instead of treating the call as being

trapz(X, Y)

it treats it as being a request to

trapz(Y, DIM)

But your hhh value does not happen to look like a valid dimension, so an error happens.

It is likely that your call to trapz() should wait until after you have completely filled in xx_star and hhh.

Note: earlier in the file you have

while (yy2<0)

but your yy2 is a vector, not a scalar. In MATLAB when you have if or while with a condition, the condition is only considered true as long as all elements in the expression are non-zero (or true). So your while would stop running as soon as any value in yy2 become >= 0 -- and likewise, your loop would never end if your yy2 did happen to turn out to be all negative.... which happens to be the case. If the trapz() had not caused an error you would have an infinite loop.

[Mohammad Qasem]

Thank you very much

What you recommend me to do ... to get the result of the area

[Walter Roberson]

I suspect you should be switching from

while (yy2<0)
    for k = ...
            
    end
end

to

for k = ...
   ...
   yy2(k) = ...
   if yy2(k) >= 0;
       break
   end
end
if k == 1
    %not enough data to trapz so respond appropriately
else
    area = trapz(xx_star, hhh);
end