11-Garnet

Solved

How to only get one value from the answer

Forum|Forum|3 years ago
September 4, 2022
4 replies
7590 views

Hi experts,

I would like to know how to only get one value from my answer.

I mean I only need 0.847, but there are 6 answers in there. does anyone know how to do it.

Mathcad Usage

Best answer by Werner_E

OK. here are two possible solutions to the problem and personally I don't like any of the two.

The first one is based on a strange difference in behaviour between "Lookup" and "Hlookup". I have no idea why "Lookup" returns the non-real values with a positive real part as well, but "Hlookup" only returns the real values.

I don't know if its a bug or if there is a special reason for this difference. But we can take advantage of it and use "Hlookup" to get the largest real solution (and hope that its a positive value, too 😉

Another approach uses the "max" function along with a vectorized evaluation. Because the result vector has to be used twice in this approach and I don't want to call K.3 two times, the function result is stored in a local variable "_X_".

The comparison for "approximate zero" with |....|<10^-8 was necessary because of numerical inaccuracies the real solutions sometimes come along with a very tiny imaginary part.

I guess I already wrote it once or twice that I feel that a pure numeric approach without using the symbolics would be more appropriate and much cleaner and hassle free. I already suggested elsewhere something like

W

Werner_E

25-Diamond I

Its always better to attach the worksheet as well, not just a pic.

You may try the modifier "assume" to narrow down the solutions you get. I guess that you are interested in the positive solution only, so you may try "assume,x>0".
Unfortunately the symbolics quite often ignores the"assume" modifier.

You may also consider using one of the two numeric methods. Either a solve block with "find" or one of the two flavors of the "root" function.

Here you can provide an appropriate guess value, add a constraint or provide a range (e.g. from 0 to 5) where Mathcad should look for a solution.

T

terryhendicott

21-Topaz II

Hi,

Simply index the required single answer.

J

J_power

Author

11-Garnet

Hi, terry,

Thanks for your help.

According to your method.

you just need to know where the root is located and then you can use the index to get the answer.

But I would like to use Mathcad to find the positive value in these results, is any possible could do that?

T

terryhendicott

21-Topaz II

Hi,

But I would like to use Mathcad to find the positive value in these results, is any possible could do that?

Use the maximum of real values to get positive result.

1_220905-ChooseMaxr.zip

W

Werner_E

25-Diamond I

Normally an "assume, x>0" should do the job, but as already written, the symbolics in Mathcad is in no way perfect and ever so often ignores an assume modifier or interprets it not exactly the way we would like it to do.

In your case for some reason we get both real solutions

In case you use the symbolic "solve" on purpose and need the exact expression - you get them if you avoid floating point numbers

If you can't say which element in the result vector is the positive, real value (I guess real and positive are the properties you are interested in), you may let ;Mathcad find the appropriate value for you.