what to do in this case?

subplot (4,1,1)

fplot (@(x, y = 1) x.^3.*y-2.*x.*y.^2+y-0.2,[0, 1]);

subplot (4,1,2)

fplot (@(x, y = 0.7) x.^3.*y-2.*x.*y.^2+y-0.2,[0, 1]);

subplot (4,1,3)

fplot (@(x, y = 0.5) x.^3.*y-2.*x.*y.^2+y-0.2,[0, 1]);

subplot (4,1,4)

fplot (@(x, y == 0.1) x.^3.*y-2.*x.*y.^2+y-0.2,[0, 1]);

fplot (@(x, y = 1) x.^3.*y-2.*x.*y.^2+y-0.2,[0, 1]);

↑

Error: Unsupported use of the '=' operator. To compare values for equality, use '=='.

To specify name-value arguments, check that name is a valid identifier with no

surrounding quotes.

Answer 1

[VBBV]

fplot (@(x, y) x.^3.*y-2.*x.*y.^2+y-0.2,[0, 1, 0, 1]);

Give the values of y range just like x

[VBBV]

Or use fimplicit function

[VBBV]

subplot (4,1,1)
fimplicit(@(x, y) x.^3.*y-2.*x.*y.^2+y-0.2,[0, 1 0 1]);
subplot (4,1,2)
fimplicit(@(x, y) x.^3.*y-2.*x.*y.^2+y-0.2,[0, 1 0 0.7]);
subplot (4,1,3)
fimplicit(@(x, y) x.^3.*y-2.*x.*y.^2+y-0.2,[0, 1, 0 0.5]);
subplot (4,1,4)
fimplicit(@(x, y) x.^3.*y-2.*x.*y.^2+y-0.2,[0, 1, 0 0.1]);
 fimplicit(@(x, y) x.^3.*y-2.*x.*y.^2+y-0.2,[0, 1 0 1]);

[VBBV]

y = [1 0.7 0.5 0.1 1]

y = 1×5

1.0000 0.7000 0.5000 0.1000 1.0000

%subplot (4,1,1)
fplot (@(x) x.^3.*y-2.*x.*y.^2+y-0.2,[0, 1]);
legend('y = 1','y = 0.7','y = 0.5','y = 0.1','y = 1')

you can also plot using single fplot function since you are looking specific y value, btw, what is difference in first and last y values, if you have same function for same defined x range ?

Answer 2

[John D'Errico]

You cannot set a variable to have a value inside another statement.

For example, this line of code would not be valid in MATLAB

x = 2 + y = 3;

Would you expect that to set y to the value 3, and then add 2 to y, to get x as 5?

Similarly, this is also invalid:

fplot (@(x, y = 1) x.^3.*y-2.*x.*y.^2+y-0.2,[0, 1]);

Again, you cannot set y to a value in the middle of another statement.

If you need to give y the value of 1, then 2, then 3 and 4, so plotting that relationship, where y takes on specific values, you might do this, instead:

yvals = [1 2 3 4];
ny = numel(yvals);
for ind = 1:ny
  subplot(ny,1,ind)
  fxy = @(x,y) x.^3.*y-2.*x.*y.^2+y-0.2;
  fplot(@(x) fxy(x,yvals(ind)),[0,1])
end