How can i display symbolic expressions using decimals instead of rational fractions?

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Dennis Yeh 2015-2-27

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https://ww2.mathworks.cn/matlabcentral/answers/180657-how-can-i-display-symbolic-expressions-using-decimals-instead-of-rational-fractions

评论： Walter Roberson 2021-4-24

采纳的回答： Mischa Kim

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Hello,

When I express symbolic expressions using syms with decimal values, I often get very large fractions. Since the result is not constant, I cannot use double() to evaluate the result as a decimal. sym2poly() works in most cases, but one day I will probably need to evaluate a long expression that is not a polynomial. I have also tried sym(expression,'d') but this doesn't work for non-numeric inputs. Is there some way to display symbolic expressions using decimals?

Thanks!

Example:

syms SIG_X SIG_Y SIG_Z
S_bar = [[0.78,-0.35,-0.38];[-0.35,0.78,-0.38];[-0.38,-0.38,0.92]]; %[Pa]
alpha = [-0.018,24.3,24.3]*10^-6; %[/C]
stress = [SIG_X;SIG_Y;SIG_Z];
dT = 25;
strain = S_bar*stress+alpha'*dT';
disp(strain)

Result:

(39*SIG_X)/50 - (7*SIG_Y)/20 - (19*SIG_Z)/50 - 8500259669165361/18889465931478580854784
(39*SIG_Y)/50 - (7*SIG_X)/20 - (19*SIG_Z)/50 + 5603198512389277/9223372036854775808
(23*SIG_Z)/25 - (19*SIG_Y)/50 - (19*SIG_X)/50 + 5603198512389277/9223372036854775808

Desired Result:

78*SIG_X - 0.35*SIG_Y - 0.38*SIG_Z - 4.5e-07
78*SIG_Y - 0.35*SIG_X - 0.38*SIG_Z + 6.075e-04
92*SIG_Z - 0.38*SIG_Y - 0.38*SIG_X + 6.075e-04

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采纳的回答

Mischa Kim 2015-2-27

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Dennis, you could use something like

vpa(strain,5)

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Sambit Supriya Dash 2021-4-23

@Walter Roberson first, 'vpa' is simplifying the fraction in decimal form, secondly, 'vpa(strain,a)' is specifying the order of accuracy 'd'.

So, 'vpa(strain)', would also work, if you don't specify any decimal accuracy.

Thanks @Mischa Kim

Walter Roberson 2021-4-24

@Sambit Supriya Dash

When you vpa(strain) then that is the same as vpa(strain, digits()) -- it does the vpa at the number of default digits you have set up, rather than doing a different operation.

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