Plot doesn't show anything

Hello everyone, I am trying to make the following code work where I want to plot the variables fi and P together for each T value. I think I am doing something wrong regarding the "for" loop but I can't seem to solve it. I would appreciate your help.

clear,clc,clf,
R = 83.1446; 
b = 29;
i = 200:10:900
for T = i + 273.15;
  d = (-8374 + (19.437*T) - (8.148*0.001*T.^2))*10^6;
  c = (290.78 - (0.30276*T) + (1.4774*0.0001*T.^2))*10^6;
  e = (76600 - (133.9*T) + (0.1071*T.^2))*10^6;
  for i =20:1:100;
      v=i;
      y = b./(4*v);
      a = c + d./v + e./(v.^2);
      P = (R*T.*(1 + y + y.^2 - y.^3))./(v.*(1-y).^3)- a./(sqrt(T)*v.*(v+b)); 
      Z = (1+y+y.^2-y.^3)./((1-y).^3) - a./(R*T^1.5.*(v+b));
      lnf =(8*y-9*y.^2+3*y.^3)./((1-y).^3)- log(Z)- c./(R*T^1.5.*(v+b))-d./(R*T^1.5.*v.*(v+b))
       f=exp(lnf);
       fi = f.*P 
       plot(P,fi)
       drawnow
       hold on
  end
end
hold off

[Geoff Hayes]

Dimitris - when I run your code, I do see the axes being updated with "something" (though I don't know if it is correct or not). I do wonder about your loops though. You assign i to be an array

i = 200:10:900

which you iterate over (?) in the outer loop as

for T = i + 273.15;

Is this what you want? And then in your inner for loop, you re-use i as the step variable

    for i =20:1:100;
        v = i;

I recommend that you use another variable so as not to conflict with the array that you've already created.

[Dimitris Moutzouris]

Geoff, the first iteration is not needed but I use it in order to remember some things as the vectors symbolize some physical parametrs.

As for you second point,it is my fault, thank you.

Answer 1

[Cris LaPierre]

One potential issue - you have used the variable i twice. You should probably come up with a different counter variable for your second for loop.

You are also only plotting a single point at a time. This is likely why you can't see anything. Try changing your marker to something bigger.

You calculations are also returning imaginary numbers. I don't think there should be imaginary numbers in this calculation.

Here's a slightly reworked version.

R = 83.1446; 
b = 29;

for temp = 200:10:900
    T = temp + 273.15;
    d = (-8374 + (19.437*T) - (8.148*0.001*T.^2))*10^6;
    c = (290.78 - (0.30276*T) + (1.4774*0.0001*T.^2))*10^6;
    e = (76600 - (133.9*T) + (0.1071*T.^2))*10^6;
      
    for v =20:1:100
        y = b/(4*v);
        a = c + d./v + e./(v^2);
        P = (R*T.*(1 + y + y.^2 - y.^3))./(v*(1-y)^3)- a./(sqrt(T)*v*(v+b)); 
        Z = (1+y+y^2-y^3)./((1-y)^3) - a./(R*T.^1.5*(v+b));
        lnf =(8*y-9*y^2+3*y^3)/((1-y)^3)- log(Z)- c/(R*T.^1.5*(v+b))-d/(R*T.^1.5*v*(v+b));
        f=exp(lnf);
        fi = abs(f.*P); 
        plot(P,fi,'o')
        hold on
    end
end
hold off

[Cris LaPierre]

BTW, you could also achieve something similar without using for loops at all. This solution takes advantage of the elementwise operators in MATLAB.

temp = [200:10:900]';
T = temp + 273.15;
d = (-8374 + (19.437*T) - (8.148*0.001*T.^2))*10^6;
c = (290.78 - (0.30276*T) + (1.4774*0.0001*T.^2))*10^6;
e = (76600 - (133.9*T) + (0.1071*T.^2))*10^6;
  
v = 20:1:100;
y = b./(4*v);
a = c + d./v + e./(v.^2);
P = (R*T.*(1 + y + y.^2 - y.^3))./(v.*(1-y).^3)- a./(sqrt(T).*v.*(v+b)); 
Z = (1+y+y.^2-y.^3)./((1-y).^3) - a./(R*T.^1.5.*(v+b));
lnf =(8*y-9*y.^2+3*y.^3)./((1-y).^3)- log(Z)- c./(R*T.^1.5.*(v+b))-d./(R*T.^1.5.*v.*(v+b));
f=exp(lnf);
fi = abs(f.*P); 
plot(P,fi,'o')

[Dimitris Moutzouris]

Greetings Cris, your solution totally worked. At some point ,I was getting an "imaginary numbers" error but I wasn't able to understand what I was doing wrongly. Thank you very much, I still have a lot of practice to do!

[Cris LaPierre]

Welcome! I hope the answer makes sense. In the second one, I've created a 71x81 matrix. This is accomplished by defining temp as a column vector (71x1) and v as a row vector (1x81). When plotted, MATLAB treats each column as a data series (same color in the plot).

Answer 2

[Mario Malic]

clear,clc,clf,
R = 83.1446; 
b = 29;
i = 200:10:900
for T = i + 273.15;
  d = (-8374 + (19.437*T) - (8.148*0.001*T.^2))*10^6;
  c = (290.78 - (0.30276*T) + (1.4774*0.0001*T.^2))*10^6;
  e = (76600 - (133.9*T) + (0.1071*T.^2))*10^6;
  for i =20:1:100;
      v=i;
      y = b./(4*v);
      a = c + d./v + e./(v.^2);
      P = (R*T.*(1 + y + y.^2 - y.^3))./(v.*(1-y).^3)- a./(sqrt(T)*v.*(v+b)); 
      Z = (1+y+y.^2-y.^3)./((1-y).^3) - a./(R*T^1.5.*(v+b));
      lnf =(8*y-9*y.^2+3*y.^3)./((1-y).^3)- log(Z)- c./(R*T^1.5.*(v+b))-d./(R*T^1.5.*v.*(v+b))
      f=exp(lnf);
      fi = f.*P 
      y_curve(i) = fi; % Obtains values for range of i from 20 to 100 and saves it as array
      x_curve(i) = P; % same
  end
  
  hold on
  plot(x_curve,y_curve)
end

I hope this is what you wanted to get. In your code, you were ploting for each point. So if you want to have curves on your plot like this code provides, improve it (since those two lines are unnecessary) and use it. Otherwise, change your plot line like shown below.

plot(P,fi, '*')

[Dimitris Moutzouris]

Thank you for your answer Mario. This solution wasn't what I was trying to find but you gave me some food for thought with the curves option!

[Mario Malic]

plot(P,fi, '*')

This one provided what you wanted, but with valuable insights about loops from Cris, that answer is certainly better than mine.