Plot doesn't show in figure

Hello,

For my study I need to learn to work with MatLab and we just started using for loop. Now I tried making a diagram with volume at the x-axis and pressure at the y-axis, using XSteam.

T_iso=[400, 600, 800]        %vector with isotherms
for T=T_iso                        
  for i = (1:1:15)           %assignment told to use i for pressure (pressure in MPa)          
  v=XSteam('v_pT', i, T)     %calculating v, using XSteam      
  end
  plot(v,i)                  %trying to plot
end

Everything goes well, until I try to plot this. I do get a figure window with the right values on both the axes, but it does not show any lines or something. Does anyone know how to solve this? Thanks a lot!

Sabine

Answer 1

[Stephen23]

The plot does not show anything because you are not actually plotting anything. You need to use indexing in the loop to store the values that are being calculated, otherwise they will simply get overwritten, and when you go to plot there is nothing to plot. Try this:

I_vec = 1:15; 
T_iso = [400,600,800]; % isotherms
% preallocate output matrix:
out = NaN(numel(I_vec),numel(T_iso));
% loop over input vectors:
for ix = 1:numel(I_vec)
    for tx = 1:numel(T_iso)
        out(ix,tx) = XSteam('v_pT', I_vec(ix), T_iso(tx));
    end
end
%
plot(out)

Sadly the author of XSteam did not vectorize their code, or otherwise allow array inputs. If they had it would be possible to get rid of the loops alltogether.