Find the precision of a value

Hello,

Is there a function in matlab that returns the precision of a number? for example, if I have a number 5.34 i want it to return 0.01, if I have a number 4 I want it to return 1 etc..

Thanks in advance for your answers,

[Adam]

Given that many double-precision numbers cannot be represented to an exact level of accuracy this would be problematic. e.g.

>> a = 5.34

a =

                      5.34

>> sprintf( '%0.20f', a )

ans =

    '5.33999999999999990000'

You would want this to return 0.01 which may be reasonable, but actually the precision (to use your term) of the number is way smaller.

[José-Luis]

Your question might make sense if your number is stored as a string, which your using the fprintf tag suggests.

Answer 1

[Stephen23]

No doubt there will be disagreements over what it means for floating point numbers, but here is one solution which provides the least unexpected output by converting to string, converting the last non-zero digit to 1 and the rest to 0. I do not claim that it is fast, but it does seem to be reasonably robust.

function p = getprecision(x)
f = 14-9*isa(x,'single'); % double/single = 14/5 decimal places.
s = sprintf('%.*e',f,x);
v = [f+2:-1:3,1];
s(v) = '0'+diff([0,cumsum(s(v)~='0')>0]);
p = str2double(s);
end

and tested:

>> getprecision(5.34)
ans =  0.010000
>> getprecision(5)
ans =  1
>> getprecision(53400)
ans =  100

Notes:

1. keep in mind that 5.34 and 0.01 do not really exist in binary floating point numbers. See Adam's comment.
2. '%g' is an obvious choice, but is actually more complicated because it can return fixed point values with trailing zeros.

[Jan]

+1: "the least unexpected output" is a nice definition of a "best solution".

Answer 2

[José-Luis]

Just in case your number is stored as a string:

 bla = '5.14';
 result = strsplit(bla,'.');
 result = numel(result{2})

There are better (faster) ways to do this, but coffee calls.

[Stephen23]

What about:

bla = '534000';
bla = '0.000045';
bla = '2463.00000';

[José-Luis]

Then a regex() could work.

Answer 3

[James Tursa]

See this related link:

https://www.mathworks.com/matlabcentral/answers/184255-how-to-calculate-digits-after-decimal-point