How to plot this matrix?

I am trying to plot two different lines on the same graph and am having some trouble. This is the code i have written so far:

     n = 16.7185;
     for i = 1:1:40

         n = n+0.1;
         T2 = T1*n^l;
         T4 = T3/n^l;
         efficiency = 1-1/n;

         m(i,2)= W/(T3-T4-T2+T1)/Cp;
         q(i,2) = W/efficiency;
         plot(i,m(i,2));
         plot(i,q(i,2));
         hold all

     end
     hold off
     grid on

The values for l,W,T1 and T3 are predefined and I am getting no erroes. Where exactly am I going wrong? When i try to see the plot it comes out as blank and when I go into the detailed view to modify the axes I see that it is trying to plot a line for each data point but I don't understand why it would do that.

[Vidhan Malik]

Changed code to this but still getting blank graph

 n = 16.7185;

 for i = 1:1:40    
    n = n+0.1;
    T2 = T1*n^l;
    T4 = T3/n^l;
    efficiency = 1-1/n;
    m(i,1) = n;
    q(i,1) = n;
    m(i,2)= W/(T3-T4-T2+T1)/Cp;
    q(i,2) = W/efficiency;
    plot(n,m(i,2));
    plot(n,q(i,2));
    hold all

end

Answer 1

[Geoff Hayes]

Vidhan - what are the values for your predefined variables? If I randomly assign some numbers (to them) then I see two lines. Once the loop has finished, what does the data in the arrays m and q look like?

Note that each time that you call plot, the code is creating a graphics object on the parent figure. Since you are collecting all of your data into two arrays, then you may as well just plot the data once the arrays have been populated. For example,

m = zeros(40,2);
q = zeros(40,2);
n = 16.7185;
for k = 1:1:40    
  n = n+0.1;
  T2 = T1*n^l;
  T4 = T3/n^l;
  efficiency = 1-1/n;
  m(k,1) = n;
  q(k,1) = n;
  m(k,2)= W/(T3-T4-T2+T1)/Cp;
  q(k,2) = W/efficiency;
end
figure;
hold on;
plot(m(:,1),m(:,2),'b.');
plot(q(:,1),q(:,2),'r.');

(Note that I replaced i with k since MATLAB uses i and j to represent the imaginary number. It is good practice to avoid using indexing variables with the same name.)

[Vidhan Malik]

These are my predefined values

 y=1.4;
 l=(y-1)/y;
  T1=300;
 T3=1500;
 W=500;
 Cp=1.004;

I seem to only be able to see one plot though, how do you get the two lines on the same graph?

[Geoff Hayes]

Vidhan - even with your pre-defined values, I still see two lines drawn (using the code from above).

Answer 2

[Roger Stafford]

In http://www.mathworks.com/help/matlab/ref/hold.html Mathworks states that for 'hold all' "This syntax will be removed in a future release. Use hold on instead". Also you are applying it too late to catch the first 'm' point.

In any case it would be better to first accumulate 'm' and 'q' in arrays and then plot them afterwards all at once. You wouldn't need to do a 'hold' at all then.

 for i = 1:40
  ....
 end
 plot(1:40,m,'r-',1:40,q,'g-') % Plot in red and green