Why to use vpa instead of double in dealing with symbolic variables?

Question

Amit Kumar 2014-1-29

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/114197-why-to-use-vpa-instead-of-double-in-dealing-with-symbolic-variables

回答： Sean de Wolski 2014-1-29

Hello, I have a matrix in symbolic form and I want to convert it or display it as float. How to do this? Here is my simple code:

syms X Y
F=[(50*Y)/3 + 127/120, (50*X)/3 + 31/1000;54*Y - 319/600,    54*X + 917/1000];
double(F)

However matlab gives error for last line and I have to use vpa. I don't want to display F as having fractions, so I want to convert it to float. Anyone having any comments?

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Answer 1

Sean de Wolski 2014-1-29

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https://ww2.mathworks.cn/matlabcentral/answers/114197-why-to-use-vpa-instead-of-double-in-dealing-with-symbolic-variables#answer_122647

在 MATLAB Online 中打开

For display

vpa(F,5)

You can't convert it to double because X and Y are symbolic. To convert it to double, you would first have to use subs to substitute values for X and Y.