loss of precision in doing file I/O

[giff]

Hi all,

I have a doubt, I'll try to expose it to you as clearly as I can, maybe
it is not completely in topic, sorry about that.

I have a few vectors of doubles (output of some calculations) and I
check the norm and the dot product between couples of them before
storing them in a txt file, obtaining the expected results.

The vectors are orthonormal so the norm=1 and the dot product gives
something like 1.0e-18 that I suppose is quite close to zero...

When I read the vectors again from the txt file, I check the same things
and I find that the dot product is now around 1.0e-8.

Is this normal? Is there a way to avoid this loss of precision?

I am programming in c++ using visual studio 6 on win2000.

Thanks for your precious help.

--
there are no numbers in my email address

[osmium]

"giff" writes:

I have a doubt, I'll try to expose it to you as clearly as I can, maybe it
is not completely in topic, sorry about that.

I have a few vectors of doubles (output of some calculations) and I check
the norm and the dot product between couples of them before storing them
in a txt file, obtaining the expected results.

The vectors are orthonormal so the norm=1 and the dot product gives
something like 1.0e-18 that I suppose is quite close to zero...

When I read the vectors again from the txt file, I check the same things
and I find that the dot product is now around 1.0e-8.

Is this normal? Is there a way to avoid this loss of precision?

I am programming in c++ using visual studio 6 on win2000.

If you read and write to the file using >> and << the data will be converted
to a character representation and the default precision will be used.
1.0e-8 sounds about like what I would expect. You could change the
precision [see ios::precision( ) ] or save the file in the binary form [see
read() and write()].

[mlimber]

giff wrote:

Hi all,

I have a doubt, I'll try to expose it to you as clearly as I can, maybe
it is not completely in topic, sorry about that.

I have a few vectors of doubles (output of some calculations) and I
check the norm and the dot product between couples of them before
storing them in a txt file, obtaining the expected results.

The vectors are orthonormal so the norm=1 and the dot product gives
something like 1.0e-18 that I suppose is quite close to zero...

When I read the vectors again from the txt file, I check the same things
and I find that the dot product is now around 1.0e-8.

Is this normal? Is there a way to avoid this loss of precision?
See this FAQ and those following:

http://www.parashift.com/c++-faq-lit...html#faq-29.16

Can you show us a *complete* but *minimal* code snippet that we can cut
and paste into our editors to try it? It may be user error.
I am programming in c++ using visual studio 6 on win2000.

Irrelevant. (If it is relevant, you're in the wrong newsgroup.)

Cheers! --M

[Victor Bazarov]

giff wrote:

I have a few vectors of doubles (output of some calculations) and I
check the norm and the dot product between couples of them before
storing them in a txt file, obtaining the expected results.

The vectors are orthonormal so the norm=1 and the dot product gives
something like 1.0e-18 that I suppose is quite close to zero...

When I read the vectors again from the txt file, I check the same
things and I find that the dot product is now around 1.0e-8.

Is this normal? Is there a way to avoid this loss of precision?
Write more digits to the output.
I am programming in c++ using visual studio 6 on win2000.

That shoulnd't matter.

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

[giff]

osmium ha scritto:

If you read and write to the file using >> and << the data will be converted
to a character representation and the default precision will be used.
that's probably what happens
1.0e-8 sounds about like what I would expect. You could change the
precision [see ios::precision( ) ] or save the file in the binary form [see
read() and write()].

thanks for the hint
--
there are no numbers in my email address

[giff]

mlimber ha scritto:

Is this normal? Is there a way to avoid this loss of precision?
See this FAQ and those following:

http://www.parashift.com/c++-faq-lit...html#faq-29.16

thanks

Can you show us a *complete* but *minimal* code snippet that we can cut
and paste into our editors to try it? It may be user error.

std::ofstream os("filename.tx t" , ios_base::trunc |ios_base::out );

for(int n=0; n<nP; n++){

for(int i=0; i<nV; i++){

os << ((double*)(shap eV2d[n]->data.ptr))[i * 2] << " "
<< ((double*)(shap eV2d[n]->data.ptr))[i*2+1] << endl;

}
}

I write nP vectors stored as CvMat arrays (I am using the opencv libraries)

here is the reading routine:

std::ifstream is( filename.txt );

for(int n=0; n<nP; n++ ) {

for(int v=0; v<nV; v++){

is >> x >> y;
((double*)(shap eV2d[n]->data.ptr))[2 * v] = x;
((double*)(shap eV2d[n]->data.ptr))[2*v+1] = y;

}
}
Anyway I don't think this could be a huge problem for my project, I mean
1.0e-8 is still quite small and comparable to zero...

thanks

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there are no numbers in my email address

[giff]

Victor Bazarov ha scritto:

Write more digits to the output.

I am using <<, do you mean I should use a printf instead?
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[Marcus Kwok]

giff <gi*********@44 4gmail.com> wrote:

I have a few vectors of doubles (output of some calculations) and I
check the norm and the dot product between couples of them before
storing them in a txt file, obtaining the expected results.

The vectors are orthonormal so the norm=1 and the dot product gives
something like 1.0e-18 that I suppose is quite close to zero...

When I read the vectors again from the txt file, I check the same things
and I find that the dot product is now around 1.0e-8.
I ran into something similar before where I would take a dot product of
a couple different (mathematical) vectors and then normalize them. When
the dot product was 1e-18 or whatever, the algorithm normalized this
which obviously threw all my calculations out the window. I had to
explicitly set the result to 0 in order for things to work correctly.

For example,

const double Epsilon = 1e-10; // or whatever is appropriate

// ...

double result = /* compute dot product */;

// need #include <cmath> for std::abs
if (std::abs(resul t) < Epsilon) {
result = 0;
}
Is this normal? Is there a way to avoid this loss of precision?

The above are symptoms of the limitations of floating-point type numbers
in general. You can search the FAQ for more info. In particular, the
FAQ has a link to David Goldberg's paper "What Every Computer-Scientist
Should Know About Floating Point Arithmetic".

--
Marcus Kwok
Replace 'invalid' with 'net' to reply

[Victor Bazarov]

giff wrote:

Victor Bazarov ha scritto:

Write more digits to the output.

I am using <<, do you mean I should use a printf instead?

No, I don't mean that. RTFM about stream manipulators and
specifically about 'setprecision'.

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

[Jeff Flinn]

Victor Bazarov wrote:

giff wrote:

I have a few vectors of doubles (output of some calculations) and I
check the norm and the dot product between couples of them before
storing them in a txt file, obtaining the expected results.

The vectors are orthonormal so the norm=1 and the dot product gives
something like 1.0e-18 that I suppose is quite close to zero...

When I read the vectors again from the txt file, I check the same
things and I find that the dot product is now around 1.0e-8.

Is this normal? Is there a way to avoid this loss of precision?

Write more digits to the output.

I am programming in c++ using visual studio 6 on win2000.

That shoulnd't matter.

Aah, but it does. There have been recent discussions on the boost mailing
list concerning roundtripping doubles/floats to/from text files with many VC
versions losing 1-2 bits precision for certain numeric values. These have
been posted on MS website as bugs, but have apparently been refuted by MS as
a design feature.

Jeff Flinn