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A
AS_97819678-Gravel
8-Gravel
July 18, 2022
Question

Loop program

  • July 18, 2022
  • 2 replies
  • 3252 views

The problem is the R loop solve 2 instead of 1 and the Vx loop solve 2 instead of 3. What’s the problem?

 

2 replies

P
ppal_25568718-Opal
18-Opal
July 18, 2022

 

Don't know why the units cause problems.

 

See if this helps https://community.ptc.com/t5/Mathcad/Using-Units-in-a-for-loop/m-p/84351

 

ppal_0-1658184186869.png

 

W
Werner_E25-Diamond I
25-Diamond I
July 19, 2022

When you learn programming than one of the first things you learn is that you should never compare float values for equality but rather ask if the absolut value of their difference is smaller than a certain small value like 10^-10.

The use of the units results in very small floating point rounding/conversion errors and so (l-b) is not exactly equal to x.

Werner_E_0-1658193125530.png

You may do it that way:

Werner_E_1-1658193371023.png

 

Or define a function

Werner_E_2-1658193589971.png

 

BTW, your programs don't contain any loops (no "if" and no "while").

    A
    AS_97819678-GravelAuthor
    8-Gravel
    July 20, 2022

    I didnt quite get why Matcad solve 2 if x = (l-b) = 60?  Why should we have to add e = 10^10?

    W
    Werner_E25-Diamond I
    25-Diamond I
    July 20, 2022

    Because the value stored in x and the result of l-b are not exactly equal. Because of internal round off or conversion errors they are off by about 10^-18 as I had shown. Thats the reason why if x=l-b ... does not trigger - Prime realizes/thinks the the expressions are NOT equal.

    B.png

    If you have two real floating point values  (and using unit mm seems to turn the integers into floats) a and b you should not ask if a=b but rather if "a" is approximately equal to "b" within a certain accuracy. So you don't ask "if a=b ..." but rather "if |a-b|< eps ..." with eps being a small value like 10^-12 or the like.

     

    But there is an even simpler solution to your problem: Chose menu ribbon "Calculation", then open "Calculation Options" and turn on "Approximate Equality". This should make your if statement trigger the way it should be.

     

     

     

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