vpa is not outputing enough digits

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Diego Mercado
Diego Mercado 2021-10-14
评论： Diego Mercado 2021-10-17
Ran in:
I'm having an error where vpa is only diplaying 5 digits when I need more. The code i have is:
syms x y
dx = 0.05
dx = 0.0500
f(x,y) = x*cos(x) + y
df = diff(f,x) + f*diff(f,y)
a = vpa(df(0,.5)/factorial(2)*dx^2)
a = 
0.001875
Obviosly here it outputs correctly to a = 0.001875, but on my end it outputs a = 0.0019. I've checked digits is set to the default 32 for vpa. The same thing occurs with
vpa(pi)
It outputs 3.1416 and not
vpa(pi)
ans = 
3.1415926535897932384626433832795

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采纳的回答

John D'Errico
John D'Errico 2021-10-14
编辑：John D'Errico 2021-10-14
I think you may have a setting messed up on your computer. For example...
X = sym(3)/32;
vpa(X)
ans =
0.09375
But now, if I try this:
sympref('floatingpointoutput',true);
vpa(X)
ans =
0.0938
Yet I have digits set as 32.
digits
Digits = 32
So you may want to reset sympref, as:
sympref('floatingpointoutput',false);
  1 个评论
Diego Mercado
Diego Mercado 2021-10-17
This was is! Thank you, don't know how that happened but gald its fixed.

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更多回答（1 个）

KSSV
KSSV 2021-10-14
编辑：KSSV 2021-10-14
It seems a is a rational number so there are no digits to display else from zeros. Where as pi is a irrational number, so you have lot of digits to display.
For more discussion refer here: https://in.mathworks.com/matlabcentral/answers/320024-how-can-i-test-if-a-number-is-irrational
  3 个评论
显示 1更早的评论
KSSV
KSSV 2021-10-14
Ran in:
You can specify the number of digits you want in vpa.
vpa(pi,12)
ans = 
3.14159265359
vpa(pi,24)
ans = 
3.14159265358979323846264
Diego Mercado
Diego Mercado 2021-10-14
编辑：Diego Mercado 2021-10-14
Ran in:
Yes, I know that you can do that, but that's not the issue. No matter what number I use, I am always getting 4 digits after the decimal as if I was typing vpa(pi,4), and I've checked with
digits
Digits = 32
to confirm that I have the default setting of 32.

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