Numerical Output from a Symbolic Expression

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Ammar Taha
Ammar Taha 2021-4-4
评论： Star Strider 2021-4-4
I'm coding the Newton method for root finding using function handles and symbolic expression to determine the derevative of f(x), the output after four iteration is : 665857/470832, which is a correct answer but not formatted the right way. I understand that this is due to the subs function, but I really don't know what is happening in the background.
For reference I got the right format by using >>vpa(ans,5);
So why Matlab is doing that?!

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Star Strider
Star Strider 2021-4-4
‘So why Matlab is doing that?!’
Because the results of symbolic expressions and operations remain symbolilc.
Another option would be:
double(ans)
if you want to use the result in numerical calculations outside of the symbolic environment.

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Walter Roberson
Walter Roberson 2021-4-4
编辑：Walter Roberson 2021-4-4
https://www.mathworks.com/help/symbolic/sympref.html#buohjf0-pref
sympref('floatingpointoutput', true)
As to "why":
When you use a double precision value in a symbolic expression, the Symbolic Toolbox processes the double precision number with sym() to convert it to symbolic number. The default conversion is processing with the 'r' (rational) conversion, which creates rational numbers (unless it can find a simple square root or multiple of pi)
https://www.mathworks.com/help/symbolic/sym.html#bun735q-flag
You can force individual double precision numbers to be processed differently by calling sym() yourself with one of the other conversion flags.
However... what you should probably be doing instead is taking the symbolic expression of your function and its derivative and using matlabFunction() to convert them into double precision functions, and then this would no longer be relevant.

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