Unable to convert expression into double array. Cannot use quiver.

I know the problem is I use a variable to assign the function but I don't know how to fix.

Hope anybody can help me.

Thank a lot.

Here is my code:

clc

close all

syms x y c
k = 5;
[x,y] = meshgrid(-3:1:3);
[P] = vector_inputP();
[Q] = (vector_inputQ(k));
kt = kttruongbaotoan(P,Q);
if kt == 1
    F = tinhhamthe(P,Q);
    F = expand(F);
    disp(F);
    [P] = vector_inputP();
    [Q] = (vector_inputQ(k));
    quiver(x,y,P,Q);
end
%% Question A
function [kt] = kttruongbaotoan(P,Q)
syms x y
if diff(P,y)==diff(Q,x)
    fprintf('*>Truong vecto F = < Q , P > = ');
    disp([Q,P]);
    fprintf(' la truong bao toan. Ham the F(x,y) = ');
    kt=1;
else
    fprintf('*>Truong vecto F = < Q , P > = ');
    disp([Q,P]);
    fprintf('khong la truong bao toan\n');
    kt=0;
end
end
%% Question B
function [F] = tinhhamthe(P, Q)
syms x y c
F1 = expand(int( P , x));
F2 = Q - diff(F1,y);
F2 = int(F2, y);
F = F2 + F1 + c;
end
%%
function [P] = vector_inputP(k)
syms x y
if k == 5
    P = 2 * x - 3 * y;
elseif k == 6
    P = exp(x) * cos(y);
elseif k == 7
    P = exp(x) * sin(y);
elseif k == 8
    P = 3 * x^2 - 2 * y^2;
elseif k == 9
    P = y * exp(x) + sin(y);
elseif k == 10
    P = x * y * cos(x*y) + sin(x*y);
elseif k == 11
    P = log(y)+ 2 * x * y^3;
elseif k == 12
    P = x * y * cosh(x*y) + sinh(x*y);
end
end
%%
function [Q] = vector_inputQ(k)
syms x y
if k == 5
    Q = - 3*x + 4*y - 8;
elseif k == 6
    Q = exp(x) * sin(y);
elseif k == 7
    Q = exp(x) * cos(y);
elseif k == 8
    Q = 4*x*y + 3;
elseif k == 9
    Q = exp(x) + x * cos(y);
elseif k == 10
    Q = x^2 * cos(x*y);
elseif k == 11
    Q = 3 * x^2 * y^2 + x / y;
elseif k == 12
    Q = x^2 * cosh(x*y);
end
end

Answer 1

[VBBV]

@Tai Nguyen, Use a different variable name for meshgrid outputs and use subs for symbolic expressions to replace with numeric arrays

syms x y c
k = 5;
[X,Y] = meshgrid(-3:1:3); % use a different variable name for meshgrid outputs 
[P] = vector_inputP(k);
[Q] = (vector_inputQ(k));
kt = kttruongbaotoan(P,Q);

*>Truong vecto F = < Q , P > = 

 la truong bao toan. Ham the F(x,y) = 

if kt == 1
    F = tinhhamthe(P,Q);
    F = expand(F);
    disp(F);
    [P] = vector_inputP(k);
    [Q] = (vector_inputQ(k));     
    P = double(subs(P,{x,y},{X,Y}));
    Q = double(subs(Q,{x,y},{X,Y}));
    quiver(X,Y,P,Q);
end

%% Question A
function [kt] = kttruongbaotoan(P,Q)
syms x y
if diff(P,y)==diff(Q,x)
    fprintf('*>Truong vecto F = < Q , P > = ');
    disp([Q,P]);
    fprintf(' la truong bao toan. Ham the F(x,y) = ');
    kt=1;
else
    fprintf('*>Truong vecto F = < Q , P > = ');
    disp([Q,P]);
    fprintf('khong la truong bao toan\n');
    kt=0;
end
end
%% Question B
function [F] = tinhhamthe(P, Q)
syms x y c
F1 = expand(int( P , x));
F2 = Q - diff(F1,y);
F2 = int(F2, y);
F = F2 + F1 + c;
end
%%
function [P] = vector_inputP(k)
syms x y
if k == 5
    P = 2 * x - 3 * y;
elseif k == 6
    P = exp(x) * cos(y);
elseif k == 7
    P = exp(x) * sin(y);
elseif k == 8
    P = 3 * x^2 - 2 * y^2;
elseif k == 9
    P = y * exp(x) + sin(y);
elseif k == 10
    P = x * y * cos(x*y) + sin(x*y);
elseif k == 11
    P = log(y)+ 2 * x * y^3;
elseif k == 12
    P = x * y * cosh(x*y) + sinh(x*y);
end
end
%%
function [Q] = vector_inputQ(k)
syms x y
if k == 5
    Q = - 3*x + 4*y - 8;
elseif k == 6
    Q = exp(x) * sin(y);
elseif k == 7
    Q = exp(x) * cos(y);
elseif k == 8
    Q = 4*x*y + 3;
elseif k == 9
    Q = exp(x) + x * cos(y);
elseif k == 10
    Q = x^2 * cos(x*y);
elseif k == 11
    Q = 3 * x^2 * y^2 + x / y;
elseif k == 12
    Q = x^2 * cosh(x*y);
end
end