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Problem with 'substitute' in Mathcad15

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Regular Member

Problem with 'substitute' in Mathcad15

I tried to follow advice given in another thread to do this (See file):

Mathcad complains about the units of 's', but that's not the problem. I want to substitute i*omega for s.  Under Symbolics, Variable > substitute is greyed out. Pressing CTRL-SHFT-. gives a place holder, which disappears, followed by the arrow shown. I tried re-starting Mathcad, but I still get the same problem. Of course, Mathcad says s is undefined, but I suppose it wouldn't need to be defined for the substitution operation.

1 ACCEPTED SOLUTION

Accepted Solutions

Re: Problem with 'substitute' in Mathcad15

Not sure what you are looking for at the end of the day, but wouldn't Terry's approach be all you need? Without any symbolic "substitute".

Or is your goal to apply some further symbolic manipulations on F after the substitution?

P.S.: I would suggest to use a literal index for your constants omega.1, etc. instead of a vector index. The way you defined those constants you had defined a 5 element vector omega (first element being 0 as Mathcads first index is by default 0, not 1). I guess you did not intend to define a vector omega, right?

B.png

20 REPLIES 20

Re: Problem with 'substitute' in Mathcad15

Hi,

fdt.jpg

Re: Problem with 'substitute' in Mathcad15

Hi,

 

Capture.PNG

The above is a problem as s is undefined so MathCad is taking the value of s as 1 second by default.

In the first function F (which has a green highlight) uses s as a parameter to the function so s is defined in the right hand side as it gets passed into the function. s's scope is limited to the function F.  It is not defined as s in the scope of the document.

The green highlight shows that F is also a predefined unit in mathcad.  Good habit is to use more than one symbol to define functions

Capture-2.PNG 

In the above when you call F(the first one) by use of F(gamma).  Gamma is defined on the worksheet so the message "undefined variable" message disappears.

Note the second function F(now with a green highlight) is also a redefinition of the first F.

Capture-4.PNG

F2 is now a function that does not overwrite first function F.

F2 has as a parameter omega.  F2 is simply F with a passed in parameter i*omega that is substituted for s in F

 

Cheers

Terry

 

Highlighted

Re: Problem with 'substitute' in Mathcad15

Yes, F is a predefined unit, for Farad.

But in this case F gets a green underline because F() is a function (too), the F-distribution...

 

Luc

Re: Problem with 'substitute' in Mathcad15

I think I understand so far. But it's the next step that fails. Neither Symbolics > Variable > Substitute (which is greyed out) or CTRL-SHFT-. give me the keyword 'substitiute' and its placeholders.

By the way, that's 'lambda', not 'gamma'.

Re: Problem with 'substitute' in Mathcad15

I adopted your syntax, but I still can't get 'substitute' to appear.

Re: Problem with 'substitute' in Mathcad15

You should define omega as an independent variable and define the constant values of omega with the dot and not omega as a vector.. Furthermore I think that you will find difficulties when plotting the function with omega defined as a vector.

About the substitution you should follow the following procedure:

fdt1.jpg

Re: Problem with 'substitute' in Mathcad15

Hi,

 

Enclosed is file.  Note the k:=k etc at the top of the file

Re: Problem with 'substitute' in Mathcad15

In short, you should select the expression of which you want to substitute one variable and use the 'substitute' from the symbolics toolbar, not from the menu.

(The menu will allow you to do symbolics, but they are not 'live' symbolics: if you change the expression later, the substitution will not follow suit automatically; you'd have to redo the substitution...)

To get the toolbar, select (from the menu): View=>Toolbar=>Symbolic.

 

Success!
Luc

Re: Problem with 'substitute' in Mathcad15

Thank you. The Help is totally silent on the curious need to 'assert' the symbols k, lambda and omega. I will need to wrie a new worksheet to see if  the issue is resolved.