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

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:

0.78*SIG_X - 0.35*SIG_Y - 0.38*SIG_Z - 4.5e-07
0.78*SIG_Y - 0.35*SIG_X - 0.38*SIG_Z + 6.075e-04
0.92*SIG_Z - 0.38*SIG_Y - 0.38*SIG_X + 6.075e-04

Answer 1

[Mischa Kim]

Dennis, you could use something like

 vpa(strain,5)

[Dennis Yeh]

Thank you! This is exactly what I was looking for :D

[Marta Shelton]

Thank you, Ryan Reynolds look-alike! This helped me too!

[Marta Shelton]

Thank you, Ryan Reynolds look-alike! This helped me too!

[w.z]

Thanks for your direct answer it helps me after long search.

[Roy dela Rama]

in the code

 vpa(strain,5)

what is the specific use of 5? thanks

[Mischa Kim]

The second argument of the vpa command (in this case "5") determines the number of significant digits that are displayed.

From the documentation :

 a = vpa(1/3, 4)
 a =
 0.3333

[Roy dela Rama]

thank you! I just used it in our exam. It really helped. I was changing the number and observed the display changed but i couldn't figure out exactly what's happening...

thanks again

[Karan Gill]

Instead of using the second argument to "vpa", it's recommended to use "digits" instead so that all your calculations now use 5 digits. See: http://www.mathworks.com/help/symbolic/digits.html

[Viba Udupa]

Thank you so much for this.

[rony ben zvi]

thank you was very helpful!!

[Walter Roberson]

Karan Gill recommended changing "digits". I do not agree, not for this context. If you change "digits" then you change how the calculation is done at each step, which can result in very different intermediate answers, leading you to a different solution completely, whereas what you want is to display the current solution to fewer digits. For example, 1/(1000001 - 1000000) is clearly 1/1 and not a problem, but with digits(5), 1000001 would be rounded to 1.00000e6 and 1000000 would be rounded to the same, and subtraction would give 0, leading to 1/0 .

[Sambit Supriya Dash]

@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]

@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.

Answer 2

[Nitesh khanna]

Can we declare variables in matrix as 'real number' . if , y, then how to declare variable of matrix A as 'Real number'

A=[[sym( 'S_x', [N 3]) sym( 'S_y', [N 3]) ]

[Walter Roberson]

assume(A,'real')

[Nitesh khanna]

thank you ...so much