Set precision an element in Matlab matrix

问题

I want to set precision for all element of a matrix. Below is what I did:

>>A

A =

         0    1.0000         0         0         0         0
 -137.0830         0         0         0         0         0
         0         0         0    1.0000         0         0
  365.5546         0         0         0         0         0
         0         0         0         0         0    1.0000
  365.5546         0         0         0         0         0

>> vpa(A,2)

ans =

[      0, 1.0, 0,   0, 0,   0]
[ -144.0,   0, 0,   0, 0,   0]
[      0,   0, 0, 1.0, 0,   0]
[  377.0,   0, 0,   0, 0,   0]
[      0,   0, 0,   0, 0, 1.0]
[  377.0,   0, 0,   0, 0,   0]

The result is not my desire, it should be: -137.08, 365.55, 365.55 in the first column. Please help to suggest me how to get it. Thank you so much!

回答1:

You are not using vpa correctly. From the docs:

vpa(x,d) uses at least d significant digits

Nota that it says at least. That is why you still get 377 when you only asked for 2 significant digits.

It seems that you don't know what significant digits are. From Wikipedia:

The significant figures of a number are digits that carry meaning contributing to its measurement resolution. This includes all digits except:

All leading zeros;

Trailing zeros when they are merely placeholders to indicate the scale of the number (exact rules are explained at identifying significant figures); and

Spurious digits introduced, for example, by calculations carried out to greater precision than that of the original data, or measurements reported to a greater precision than the equipment supports.

So you want this

>> vpa(365.5546, 5)
ans =
365.55

Now, to be consistent, you need to find out what is the maximum of your matrix and compute the desired number of significant digits from there.

max_number = floor(log10(max(abs(A(:))+1)) + 1);
decimals = 2;
vpa(A, max_number + decimals)

Here, max_number is the maximum number of integer digits that your matrix has, and decimals is the number of decimal places that you want to have.

回答2:

The second input to vpa is the number of significant digits which is not the same as the number of values after the radix point. The number -137.08 actually has five signficant digits so you'll want to use a second input of 5

vpa(A, 5)

% [       0, 1.0, 0,   0, 0,   0]
% [ -137.08,   0, 0,   0, 0,   0]
% [       0,   0, 0, 1.0, 0,   0]
% [  365.55,   0, 0,   0, 0,   0]
% [       0,   0, 0,   0, 0, 1.0]
% [  365.55,   0, 0,   0, 0,   0]

回答3:

vpa(x,d) : d is significant digits, not decimal places.

pvalue = vpa(-137.0830);
twopvalue = vpa(-137.0830,2);

% pvalue = -137.08299999999999840838427189738
% twopvalue = -144.0

If you want to get -137.08, 365.55, 365.55 in the first column. You can use roundn(A,-2)

roundn(A,-2)

ans =

         0    1.0000         0         0         0         0
 -137.0800         0         0         0         0         0
         0         0         0    1.0000         0         0
  365.5500         0         0         0         0         0
         0         0         0         0         0    1.0000
  365.5500         0         0         0         0         0

来源：https://stackoverflow.com/questions/42380463/set-precision-an-element-in-matlab-matrix

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matlab

precision