Why is my plot blank ?

>> syms y
>> syms U
>> A=(-U-U*exp(-y))./(exp(-y)-1)
 
A =
 
-(U + U*exp(-y))/(exp(-y) - 1)
 
>> B=(2*U)./(exp(-y)-1)
 
B =
 
(2*U)/(exp(-y) - 1)
 
>> f(U,y)= A+B*exp(-y)
 
f(U, y) =
 
(2*U*exp(-y))/(exp(-y) - 1) - (U + U*exp(-y))/(exp(-y) - 1)
 
>> plot(f(U,0.01),f(U,100))
Error using plot
Data must be numeric, datetime, duration or an array convertible to double.

((Trying to plot two versions of the function. One with y=0.01 and y=100.))

[David Hill]

What is U? You will need to vary U to be able to plot it.

[Rik]

I'm on mobile so I can't test it, but you probably need to divide this into 2 calls to plot. And is there a reason why you're using symbolic variables instead of anonymous functions (and fplot)?

[Blake Walkowiak]

U represents a velocity, when I define U,

>> U=1

I still do not have a plot. When I use the fplot command I get a straight y=x line. I'm using the varbiables as they relate to the solution of an ODE with boundary conditions.

Answer 1

[Walter Roberson]

U is a symbolic variable, so f(U,0.01) is a symbolic expression involving the symbolic variable U

plot() can never plot symbolic expressions that contain unresolved symbolic variables.

Try fplot()

[Walter Roberson]

Though now that I look at it again, you should probably be using fplot3(), like

fplot3( f(U, 0.01), f(U, 100), 0)

Provided, that is, that the idea is that you want to plot a parametric curve as the U values change.