How to tell vpa command to consider digits after decimal point' only?

Question

Mohammad Shojaei Arani 2021-12-28

0
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https://ww2.mathworks.cn/matlabcentral/answers/1618620-how-to-tell-vpa-command-to-consider-digits-after-decimal-point-only

回答： Walter Roberson 2021-12-28

Hello friends,

I have a simple question, I think! I have a symbolic expression S with numeric coefficients. I need to round the coefficients

up to some digits 'AFTER DECIMAL POINT' only (note that I do not know, beforehand, how many digits my numbers have, so, I want to keep all digits before decimal point and a few after that). So, if I use vpa(S,4) then it only considers 4 digits in total and does not respect my what I want. Let me use a simple example as bellow:

syms x y 
f=34344.4545*x+45.6566*y;

now, vpa(S,4) gives me

34340.0*x + 45.66*y

and this is not what I want since the first coefifcient is badly rounded. Of course, in this exmple I could use vpa(S,8) to get 34344.454*x + 45.6566*y but please note that "I do not know, beforehand, how many digits my coefficients have in general".

Any idea?

Thanks in advance!

Babak