About symbolic MATLAB

Question

What is wrong with the following code?

clear all

syms x y a ;

u=2*x*y;
v=a^2+x^2-y^2;

diff=diff(u,'x',1)+diff(v,'y',1);
if diff==0
    disp('Liquid motion is possible.')
    disp('Stream function exists.')
else
    disp('Liquid motion is not possible.')
    disp('Stream function does not exist.')
end

diff2=diff(v,'x',1)-diff(u,'y',1);
if diff2==0
    disp('Velocity potential exists.')
else
    disp('Velocity potential does not exist.')
end

This comes in the command window when I run the above.

Liquid motion is possible.
Stream function exists.
Error using sym/subsindex (line 672)
Invalid indexing or function definition. When defining a function, ensure that the body of the function is a SYM
object. When indexing, the input must be numeric, logical or ':'.

Error in sym>privformat (line 1502)
    x = subsindex(x)+1;

Error in sym/subsref (line 694)
            [inds{k},refs{k}] = privformat(inds{k});

Error in q7 (line 17)
diff2=diff(v,'x',1)-diff(u,'y',1);

But if I rewrite(redefine) the symbolic variables after the first if construct, it runs fine. Also if I cancel the first if construct, it runs.

Don’t redefine the built-in function `diff`. And use [`isAlways`](https://www.mathworks.com/help/symbolic/isalways.html) instead of `==` for symbolic comparisons. `diff(u,’x’,1)` is the same as `diff(u,x)`. — horchler, Nov 11 '17 at 15:33
To expand on @horchler's comment, you are creating a new variable called `diff` which shadows the builtin `diff` function. In general you need to avoid naming variables with the same names as functions you want to use. — jodag, Nov 11 '17 at 15:36
@horchler Using `isAlways` instead of `==` is giving me the `else` answer in `if` construct, which is incorrect here. Any idea why? — Bernhard Listing, Nov 11 '17 at 16:34

score 0 · Accepted Answer · edited Nov 12 '17 at 14:31

0

I would avoid to overwrite a reserved name, so instead of

diff=diff(u,'x',1)+diff(v,'y',1);

I would suggest

derFcn = diff(u,'x',1)+diff(v,'y',1);

This triggers the second error;

diff2=diff(v,'x',1)-diff(u,'y',1);

at this point diff is your diff value (which, incidentally is 0) so it is equivalent to write

0(v,'x',1)

which, of course, will not compile and it is not what you meant.

So, please, make the substitution (and update your if statements accordingly).

edited Nov 12 '17 at 14:31

user229044

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answered Nov 11 '17 at 15:40

pacta_sunt_servanda

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Sorry I read the other responses only after posting mine. I think they should be posted as answers. Also, it is true that diff == 0 is not correct, as you are comparing a symbolic expression. – pacta_sunt_servanda Nov 11 '17 at 15:41
Using isAlways instead of == is giving me the else answer in if construct, which is incorrect here. Any idea why? – Bernhard Listing Nov 11 '17 at 17:33
Please don't sign your posts. Every post ends with your user card and a link to your profile already, signatures or taglines in posts are considered noise and they will be removed. – user229044 Nov 12 '17 at 14:33

About symbolic MATLAB

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