approximating decimal number to 2 places

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Hi,
i want to know is it possible to approximate a decimal number to 2 places?
when i save a variable with a value 0.3 it saves it as 0.30000000001
anyway to prevent this or save the variable as 0.30 only?
thanks.

Posted 17-Sep-13 7:04am

Yasser El-Sayyed

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Richard MacCutchan 17-Sep-13 13:16pm

Floating point numbers are approximations of their real values. You need to understand the implications of this in computing; the main one being that you should not use floating point in financial applications.

6 solutions

Solution 5

Floating point Arithmetic may not be useful when doing a lot of operations with rational numbers. One possibility is to include a rational library.

When using C++, boost contains a good rational library:

boost/rational.hpp[^]

With that library you can save a variable as 3.0 (and continue using that variable as a number, not a string).

Posted 17-Sep-13 17:21pm

ErnestoNet

Solution 6

Thanks a lot guys, i really appreciate it.

Posted 17-Sep-13 19:14pm

Yasser El-Sayyed

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Matt T Heffron · Accepted Answer · 2013-09-17T07:20:00

This is due to the nature of floating-point representations, e.g. double or float.
The values are stored as a binary value (of limited precision) and a binary scale factor.
The value 0.3 cannot be represented exactly in this way, so the closest possible value is stored, in this case 0.30000000001.
You can control the way it is displayed with the formatting string of the sprintf/printf/...
This is also the reason that comparisons involving floating point values should usually include a little wiggle-room.
E.g., you shouldn't compare some value as exactly equal to 0.3 but that the difference between them is less than some acceptable tolerance.

CPallini · Accepted Answer · 2013-09-17T10:02:00

Solution 4

What Every Computer Scientist Should Know About Floating-Point Arithmetic[^].

Posted 17-Sep-13 10:02am

CPallini

Comments

H.Brydon 17-Sep-13 23:29pm

+5 - Exactly. The same page I searched for and was going to put in my own solution.

CPallini 18-Sep-13 1:41am

Thank you.

nv3 · Accepted Answer · 2013-09-17T07:15:00

The value of 0.3 can internally not be represented exactly as a floating point number. That is the reason you see 0.3000...0001. But when printing that value in an output you perform a rounding operation, for example:

C++

double x = 0.3;
printf ("x = %.2f\n", x);

will print

C++

x = 0.30

just as expected. So printf does the rounding for you.

When dealing internally with floating point numbers I would suggest that you do not try to round them. Do all your calculation and do the rounding at the very end, when outputting the results.

abbaspirmoradi · Accepted Answer · 2013-09-17T07:11:00

Look at this links:

http://www.cplusplus.com/forum/beginner/3600/[^]

http://stackoverflow.com/questions/11123213/rounding-off-double-two-decimal-points[^]

http://stackoverflow.com/questions/1343890/rounding-number-to-2-decimal-places-in-c[^]

http://answers.yahoo.com/question/index?qid=20100928145027AATpb86[^]