rounding a float

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coolgroupsdotcom 101 Sep 06, 2005 at 08:45

I have the following code:

cout << int((log(8)/log(2))) << endl;

float somefloat = log(8)/log(2);
cout << int(somefloat) << endl;

Anyone know why it prints out two different numbers?

mike
http://www.coolgroups.com/

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anubis 101 Sep 06, 2005 at 09:40

because the log function returns a double. so in the first cast you cast a double division to an integer and in the other case you first cast down to float and then to int.

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coolgroupsdotcom 101 Sep 06, 2005 at 09:44

No, I don’t think so.

It prints

2
3

not

3
2

.

So, the second one is right, and the first one is wrong.

This works:

float somefloat = log(8)/log(2);
cout << int(somefloat) << endl;

This doesn’t:

cout << int((log(8)/log(2))) << endl;

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anubis 101 Sep 06, 2005 at 10:03

I still think the problem is within the cast. If you first cast the division to float the result is 3 and stays the same when cast to an integer. Probably due to some accuracy problems the double division yields 2.999999999…, which gets truncated to two (but due to limited floating point precision the error disappears when using float).

Try outputting the doule directly…

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anubis 101 Sep 06, 2005 at 10:33

Ok… i tried out your example and get the same strange result :)
Let me get into this for a moment.

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anubis 101 Sep 06, 2005 at 10:47

The error is happening during the cast for me and it works for log(9)/log(2) and log(7)/log(2), so I’d say this is a numerical imprecision error.

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Kippesoep 101 Sep 06, 2005 at 10:57

You’re being bitten by precision problems. Assuming Intel x86 under Windows, floating point calculations take place at high precision (53bit or 64bit), which equates to the “double” datatype. Calculations matching the “float” datatype may be carried out in lower precision (24bit). In this case, you’ve found that a small rounding error in higher precision modes is masked in lower precision mode.

To perform both calculations in lower precision mode, try the following:

//Add this to the preprocessor directives
#include <cfloat>

//Add the following before the calculation
_controlfp (_PC_24, _MCW_PC);

//Both calculations result in "3" here

//Or if you try this:
_controlfp (_PC_53, _MCW_PC);
//Or this:
_controlfp (_PC_64, _MCW_PC);

//You get 2 and 3 respectively
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anubis 101 Sep 06, 2005 at 11:17

So far i came to the same answer here. What’s strange though is that you can do this :

double x = log(8)/log(2);
printf(“%d”, (int)x);

and

printf(“%d”, (int)(log(8)/log(2)));

and still get the different results. If this error was due to some error beyond floating point precision this should expose it…

edit : Although that might be some compiler internal thing

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Kippesoep 101 Sep 06, 2005 at 11:35

In debug mode, the precision is different from release mode. In release mode, both calculations show 3.

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anubis 101 Sep 06, 2005 at 12:38

I don’t run debug mode in gcc :) Anyway… I’m 100% sure that that is a comppiler thing.

edit : The OP seems to gone though…

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coolgroupsdotcom 101 Sep 06, 2005 at 16:41

I’m not gone. It seems like it has to do with what happens when you convert a long double to a float or double (it gets rounded to 3).

If you do this:

long double somedouble = log(8)/log(2);
cout << int(somedouble) << endl;

you get 2.

@anubis

I don’t run debug mode in gcc :) Anyway… I’m 100% sure that that is a comppiler thing.

edit : The OP seems to gone though…

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Reedbeta 168 Sep 06, 2005 at 17:36

Remember that when converting to an int the result is truncated, not rounded. So if it is 2.999, it will be converted to 2. In order to avoid this, add 0.5 before converting to int.

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davepermen 101 Sep 06, 2005 at 18:56

doubles have a higher precicion IN THE CPU than anywhere else. even if you just store it into a double, it gets rounded in some way. in the cpu, its 80bit + some features, stored in a variable of type double, it gets converted to 64bit, float to 32bit..

that means only if you directly convert to int, you work with the original value with 80bit..

looks like this exactly results in one of the most insignificant bits changed, wich result in such a rounding-change