How to Convert symbolic variables to numeric
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Greetings!
How to convert a symbolic expression into a numerical expression?
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/1273895/image.jpeg)
best regards,
Walid.
回答(1 个)
Steven Lord
2023-1-25
If it contains no symbolic variable use double.
two = sym(2);
sqrt2 = sqrt(two)
x = double(sqrt2)
If it does contain a symbolic variable you can approximate the numeric parts with vpa. You could also use vpa even if it doesn't contain a symbolic variable.
z = vpa(sqrt2)
syms y
polynomialInY = y.^2 + sqrt2*y - 1
vpa(polynomialInY)
In this case trying to convert polynomialInY to a double array will error. You could subs a value for y into that expression (to eliminate the symbolic variable) then convert the result.
double(polynomialInY)
5 个评论
Walter Roberson
2023-1-25
It is safest to convert every number to sym() inside a symbolic expression.
In the case of a symbolic expression raised to an integer power, MATLAB can be counted on to work out itself that the power needs to be converted to symbolic... Likewise, symbolic expression divided by an integer is reliably converted. But in both cases, only if the expression is already symbolic.
sym((sym(138)/sym(490))^sym(10)) %safest
sym((sym(138)/sym(490))^10) %pretty safe
sym((sym(138)/490)^10) %pretty safe
sym((138/490)^10) %unsafe !
You have to be careful in converting scientific notation
sym(6.02139280e-23) %unsafe !
sym(6.02139280)*10^-23 %less unsafe but still inaccurate
sym(602139280)*sym(10)^-31 %safe
sym(602139280)*10^-23 %looks safe but is not really
The 10^-23 in the last example is not internally being handled by 10^-23 being exactly represented as would be the case for sym(10)^-23 . Instead, the 10^-23 is being handled by sym() examining the number being passed in and doing a continued fraction analysis to determine whether it is "sufficiently close to" a "nice number".
Steven Lord
2023-1-25
The description for the 'r' value for the flag input argument to the sym function lists the forms that it recognizes for "modest-sized integers p and q". This value for the flag input is the default which is why:
x = sym(sqrt(2))
works to give
. 2 counts as "modest-sized".
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/1274450/image.png)
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