double rounding error

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I am using two functions with variables a,b, and c with each being a double. the functions are as follows:

(a-32)(5/9)+273.15 = b and (b-273.15)(9/5)+32 = c these functions give the property that a = c for all values of a.

for the most part it is working fine, however if a = 0 i get c = 1.06e-14.

this is a really small number so it wouldn't be a problem however both values a and c are being converted to strings and displayed on UILabels, id do this with the following code with _Value2.text being the text shown on my UILabel:

_Value2.text = [NSString stringWithFormat:@"%.7g", c];

when a = 0, _Value2.text = @"1.06e-14" this is a problem and it only occurs when a = 0.

I am assuming this error is from a rounding error in the compiler but how can I fix/avoid this issue?

Posted 21-May-14 16:23pm

Member 10835704

Updated 21-May-14 16:24pm

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[no name] 22-May-14 1:18am

Please post your function code. Without seeing your code, no one can answer.

Member 10835704 22-May-14 2:48am

the code is in several locations i feel that it is better to summarize it, and i think i have done a good job. my question is fairly general anyways, how can i get around errors like this while still using doubles?

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Jochen Arndt · Answer 1 · 2014-05-21T21:07:00

Such rounding problems are common with floating point operations.

In your case you may just check for small numbers and assign zero before printing:

C++

if (fabs(c) <= 1e-8)
    c = 0.0;

You have this problem because you are using the '%g' format. Your formula is some kind of temperature conversion/calculation. There is no need to print more digits than the precision of your values. So you may change your format to something like '%.3f' depending on the range and accuracy of the input values. You may also use a fixed width: '%8.3f' will print aboslute values less than thousand with three digits after the decimal point.