Plomblem with double precision

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double var = 79999.95;
cout << var << endl;

output I m getting is 79999.9, But I want the op to be 79999.95. I want the accurate precesion without rounding off.I know that setprecision(2) will print the output as 79999.95.But since I want to use that value (79999.95)for further computation, So how can I get the val as 79999.95 ,since I think that setprecesion is only printing the value.

What I have tried:

I have tried of <double = floor((val * 1000) +0.5)/ 1000; >

Posted 14-Jun-18 22:34pm

Vaishudeepak

Updated 15-Jun-18 0:04am

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Jochen Arndt · Answer 1 · 2018-06-14T23:09:00

There are two things to observe with floating point values:

They can not represent most values exactly
Printed values are rounded to a specified or default resolution which might be affected by the above

79999.95 is one of the values where the first point applies. It can't be represented exactly with a double. The stored value is 79999,949999999997 which can be verified by setting a corresponding output precision (17 significant digits). For a single precision float, the stored value would be 79999,95313.

The limited resolution and inaccuracy is not a problem for simple mathematic operations. But it might become a problem with complex operations where rounding errors can accumulate. Then the algorithm implementation has to take care of that.

The problem of showing a probably wrong rounded value is no problem when knowing about the precision of the value itself. And you should know that for your input values.

Related reads:
The Floating-Point Guide[^]
What Every Computer Scientist Should Know About Floating-Point Arithmetic[^]

KarstenK · Answer 2 · 2018-06-15T00:05:00

Solution 2

Print it into a string buffer and you are done.

C++

char s[20];
sprint(s,"%0.2f",var);
cout << s << endl;

Posted 15-Jun-18 0:05am

KarstenK