Index exceeds the number of array elements?

Question

vishnuvardhan naidu tanga 2020-5-27

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评论： Ameer Hamza 2020-5-27

Hello,

I am trying to extract data from Xsteam.I am trying to retrive data with respect to temperature. the code is as follows.

p = 10;
a = 1000;
b = 0.1;
rho_s = zeros(1,((a-1)/b));
L = length(rho_s);
for i = 1:L
    T(i)= 1+b*i;
    rho_s(i) =XSteam('rho_pT', p, T(i));
    
    drho_s_dT = (rho_s(i+1)-rho_s(i))/(T(i+1)-T(i));
   
end

when i run this code I am getting an error as "Index exceeds the number of array elements". Can some one check and help me how can I do this.

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John Marriott 2020-5-27

Ok - the columns of which variable outputs NaN?- it doesnt return NaN when I run it - are you using the XSteam function posted by Rik?

vishnuvardhan naidu tanga 2020-5-27

编辑：vishnuvardhan naidu tanga 2020-5-27

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yes and I have changed the code a little. I have consider only temperature instead of pressure.

a = 1000;
b = 0.1;
rho_s = zeros(1,((a-1)/b));
L = length(rho_s);
for i = 1:L
    T(i)= 1+b*i;
    rho_s(i) =XSteam('rhoV_T', T(i));
    
    if i >=2
    drho_s_dT(i) = (rho_s(i)-rho_s(i-1))/(T(i)-T(i-1));
    end
end

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Answer 1

Ameer Hamza 2020-5-27

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在 MATLAB Online 中打开

It seems that you are trying to find a derivative of rho_s w.r.t. T, you can use gradient() function

p = 10;
a = 1000;
b = 0.1;
rho_s = zeros(1,((a-1)/b));
L = length(rho_s);
T = zeros(1, L);
for i = 1:L
    T(i)= 1+b*i;
    rho_s(i) =XSteam('rho_pT', p, T(i));
end
drho_s_dT = gradient(rho_s, T);

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vishnuvardhan naidu tanga 2020-5-27

编辑：vishnuvardhan naidu tanga 2020-5-27

在 MATLAB Online 中打开

Hello Ameer Hamza

Thank you for the suggestion it works. But when i consider interms of temperature only again from 3729 the column turns to NaN. is there any suggestion for that. the code is

a = 1000;
b = 0.1;
rho_s = zeros(1,((a-1)/b));
L = length(rho_s);
for i = 1:L
    T(i)= 1+b*i;
    rho_s(i) = XSteam('rhoV_T', T(i));
 
   
end
drho_s_dT = gradient(rho_s, T); 

Ameer Hamza 2020-5-27

This is probably caused by the thermodynamical equation implemented in XSteam() and not a MATLAB specific issue. You will need to understand the equations and the implementation of this function to see what might be going wrong. Also, you aren't passing variable 'p' in your code, which might be causing the issue.

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