Very basic problem with using isAlways

I'm trying to learn to use the symbolic math functionality and I was playing around with isAlways. I ran the following commands: a=sym('a','real'); b=sym('b','real');

isAlways(a>b|a==b|a<b)

ans=1

isAlways(a>=b|a<b)

ans=1

isAlways(a>=b|a<=b)

ans=0

I'm not entirely sure what I'm doing wrong but I really did expect the last statement to be true as well. Any thoughts?

Thanks a lot!

Answer 1

[Geoff Hayes]

It could be that isAlways can't determine the validity of the inequalities. From isAlways, check the validity of this inequality. When isAlways cannot determine whether the condition is valid, it returns logical 0 by default. (See their example for this.)

Also from the same link, to change this default behavior, use Unknown. For example, specify that isAlways must return logical 1 if it cannot determine the validity of this inequality...Instead of true, you can also specify error. In this case, isAlways will throw an error if it cannot determine the validity of the condition.

Try the following

 sAlways(a>=b|a<=b,'Unknown','error')

and observe the results. If an error is thrown, then the function can't determine the validity.

[Poachumsm]

Thank you! That turned out to be right - the function can't determine the validity. I don't know enough that understand the source of this error but I do get that for whatever reason isAlways doesn't work in this case.

[Roger Stafford]

I would consider that result an actual bug. The 'isAlways' function seems to be handling the '|' symbol as if it were the exclusive OR (xor) rather than the inclusive kind (or). I would advise you to send this result to Mathworks' support personnel for a possible revision.