Why doesn't symprod recognize indexed variables?

I was expecting to get: k1*k2*k3*k4. Instead . . . .

Answer 1

[Stephan]

Hi,

try:

syms k(n)
symprod(k(n),n,1,4)

this should give you the expected result.

Best regards

Stephan

[Walter Roberson]

However, going from the symbolic function calls k(1)*k(2)*k(3)*k(4) to k1*k2*k3*k4 becomes tricky. And the k(1) and so on in the symprod result will be function calls, not indexing.

Answer 2

[Walter Roberson]

MATLAB does not permit symbolic variables to be indices because it does not know how to index a definite matrix at an unresolved location.

When you define k as a matrix and try

 symprod(k(n), n, 1, 4)

then that is equivalent to

 TTTTTT = k(n);
 symprod(TTTTTT, n, 1, 4)

because argument evaluation is always done first before the function itself is invoked. So the k(n) part has to be meaningful outside the context of the symprod() or symsum()... but it isn't.

When you want to do a symprod or symsum that would call for the dummy variable to be used as an index, then what you need to do is to replace the symprod or symsum with creation of a vector of definite values that are then prod() or sum(). For example,

 TTTTTT = k(1:4);
 prod(TTTTTT)

or more compactly,

prod(k(1:4))

or even

prod(k)

since you defined k as being only 4 long.

If you had something more complicated such as

symprod(sin(k(n)^n), n, 1, 4)

you would again proceed with definite values:

prod( sin(k(1:4)).^(1:4) )

[Sree]

Thanks for these details. Much obliged.

[Stephan]

Since this answer is meeting your requirements much better than mine, you should unaccept my answer and accept this one.

[Sree]

No need for that change! Your answer was immediately useful and solved my problem (which was a bit more involved than the example I had included in my question). The other answer was philosophically instructive, for the long term.