Undefined function for input argument of type double

When i saved my code yesterday it was working fine but when i run it now, I am getting this error "Undefined function 'P' for input arguments of type 'double'". Why is that?

L=5100; delta_y=250; delta_z=30; Pini=3500; phi=0.2; k=10; ct=3*10^-5; B=1.25;mu=3; rw=0.25; qc=100; A=delta_z*delta_y; delta_t=10; delta_x=300;

X=150:300:4950; x=length(X);
T=1:10:100; t=length(T);

C1=158*phi*mu*ct/k;     

for m=2:1:x-1

      for n=1:1:t

           P(m,n+1)=((-C1*delta_x^2*P(m,n)/delta_t)+C2*delta_x-P(m+1,n+1)-P(m-1,n+1))/-(2+C1*delta_x^2/delta_t);

      end
  end

plot (X,P(m,1),'*',X,P(m,6),'+',X,P(m,11),'x');
xlabel ('Distance');
ylabel ('Pressure');
title ('Symmetry Test');
disp (P(m,1)),
disp (P(m,6)),
disp (P(m,11)),

Answer 1

[Star Strider]

I would preallocate ‘P’ and provide a value for ‘C2’.

This runs, I leave it to you to determine if it produces the results you want:

L=5100; delta_y=250; delta_z=30; Pini=3500; phi=0.2; k=10; ct=3*10^-5; B=1.25;mu=3; rw=0.25; qc=100; A=delta_z*delta_y; delta_t=10; delta_x=300;

X=150:300:4950; x=length(X);
T=1:10:100; t=length(T);

C1=158*phi*mu*ct/k;     
C2 = 1;                                                 % <— Insert Correct Value

P = zeros(x, t+1);                                      % <— Preallocate With Something

for m=2:1:x-1

        for n=1:1:t

             P(m,n+1)=((-C1*delta_x^2*P(m,n)/delta_t)+C2*delta_x-P(m+1,n+1)-P(m-1,n+1))/-(2+C1*delta_x^2/delta_t);

        end
    end

plot (X,P(m,1),'*',X,P(m,6),'+',X,P(m,11),'x');
xlabel ('Distance');
ylabel ('Pressure');
title ('Symmetry Test');
disp (P(m,1)),
disp (P(m,6)),
disp (P(m,11)),

[Amey]

yes that worked, but instead of using zeros or ones, i initiated P using for loop below because it was messing with my output. Oh & i removed C2 from the code mistakenly. thank you!

for n=1:1:t; for m=1:1:x-1; P(m,n)=3500; end end

[Star Strider]

My pleasure!

Another way to initialise ‘P’ to be a constant array with all elements as 3500 is:

P = ones(x-1,t)*3500;

Answer 2

[Chad Greene]

Yesterday you most likely had a variable P that you were tinkering with, so when you ran the loop it was able to access actual values in P(m,n). But running the code you posted, P does not exist before you try to access values inside P.