Problems with multiplications of doubles and/or floats

Discussion in 'C++' started by J.K. Becker, Apr 14, 2004.

  1. J.K. Becker

    J.K. Becker Guest

    Hi there,

    I am trying to multiply doubles with floats (actually I tried every
    possible combination by now) and it never works (well, it does something
    but it is always wrong). I have no idea what it is and where to look for
    help, maybe some of you know?

    double=float*double; (or every possible combination of it). An example:

    0.3 * 0.7 would result in 1.7 (with lots more digits). Anyone any idea?
    If I change the types of the variables I think the result stays the same
    (but I am not 100% sure)...

    Jens
     
    J.K. Becker, Apr 14, 2004
    #1
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  2. J.K. Becker

    J.K. Becker Guest

    Sorry, I forgot:

    Linux (Debian) and gcc3.3

    Jens
     
    J.K. Becker, Apr 14, 2004
    #2
    1. Advertising

  3. J.K. Becker wrote:
    > Hi there,
    >
    > I am trying to multiply doubles with floats (actually I tried every
    > possible combination by now) and it never works (well, it does something
    > but it is always wrong). I have no idea what it is and where to look for
    > help, maybe some of you know?
    >
    > double=float*double; (or every possible combination of it). An example:
    >
    > 0.3 * 0.7 would result in 1.7 (with lots more digits). Anyone any idea?
    > If I change the types of the variables I think the result stays the same
    > (but I am not 100% sure)...
    >
    > Jens
    >


    I don't get the same answer:
    TH009MA@th009ma-shl2-01 /cygdrive/d/temp
    $ cat junk.c
    #include <stdio.h>
    #include <stdlib.h>

    int main(void)
    {
    const double three_tenths = 0.3;
    const double seven_tenths = 0.7;
    const float f_three_tenths = 0.3;
    const float f_seven_tenths = 0.7;

    printf("direct multiplication: 0.3 * 0.7 = %f\n", 0.3 * 0.7);
    printf("double * double: %f\n", three_tenths * seven_tenths);
    printf("float * float: %f\n", f_three_tenths * f_seven_tenths);
    printf("double * float: %f\n", three_tenths * f_seven_tenths);
    printf("float * double: %f\n", f_three_tenths * seven_tenths);
    return EXIT_SUCCESS;
    }

    TH009MA@th009ma-shl2-01 /cygdrive/d/temp
    $ gcc --version
    gcc (GCC) 3.3.1 (cygming special)
    Copyright (C) 2003 Free Software Foundation, Inc.
    This is free software; see the source for copying conditions. There is NO
    warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.


    TH009MA@th009ma-shl2-01 /cygdrive/d/temp
    $ ./junk
    direct multiplication: 0.3 * 0.7 = 0.210000
    double * double: 0.210000
    float * float: 0.210000
    double * float: 0.210000
    float * double: 0.210000

    ************************************************

    TH009MA@th009ma-shl2-01 /cygdrive/d/temp
    $ bcc32 junk.c
    Borland C++ 5.6 for Win32 Copyright (c) 1993, 2002 Borland
    junk.c:
    Turbo Incremental Link 5.60 Copyright (c) 1997-2002 Borland

    TH009MA@th009ma-shl2-01 /cygdrive/d/temp
    $ ./junk
    direct multiplication: 0.3 * 0.7 = 0.210000
    double * double: 0.210000
    float * float: 0.210000
    double * float: 0.210000
    float * double: 0.210000


    --
    Thomas Matthews

    C++ newsgroup welcome message:
    http://www.slack.net/~shiva/welcome.txt
    C++ Faq: http://www.parashift.com/c -faq-lite
    C Faq: http://www.eskimo.com/~scs/c-faq/top.html
    alt.comp.lang.learn.c-c++ faq:
    http://www.raos.demon.uk/acllc-c /faq.html
    Other sites:
    http://www.josuttis.com -- C++ STL Library book
     
    Thomas Matthews, Apr 14, 2004
    #3
  4. "J.K. Becker" wrote:
    >
    > Hi there,
    >
    > I am trying to multiply doubles with floats (actually I tried every
    > possible combination by now) and it never works (well, it does something
    > but it is always wrong). I have no idea what it is and where to look for
    > help, maybe some of you know?
    >
    > double=float*double; (or every possible combination of it). An example:
    >
    > 0.3 * 0.7 would result in 1.7 (with lots more digits). Anyone any idea?


    Yes. Post the program. You have a bug in it.

    --
    Karl Heinz Buchegger
     
    Karl Heinz Buchegger, Apr 14, 2004
    #4
  5. "J.K. Becker" <> wrote in message
    news:c5jfaq$reo$-mainz.de...
    > Hi there,
    >
    > I am trying to multiply doubles with floats (actually I tried every
    > possible combination by now) and it never works (well, it does something
    > but it is always wrong). I have no idea what it is and where to look for
    > help, maybe some of you know?
    >
    > double=float*double; (or every possible combination of it). An example:
    >
    > 0.3 * 0.7 would result in 1.7 (with lots more digits). Anyone any idea?


    No that is impossible, you have a bug somewhere else in your program. Since
    you didn't post the program its impossible to help. Post the code.

    What else did you expect, did you think someone would say 'oh yes that's the
    well known 0.7 * 0.3 bug, it's been fixed in the new compiler version'?
    Compilers do not make basic arithmetic errors. If you want help with a buggy
    program you have to post the program code, there is no other way.

    See the FAQ http://www.parashift.com/c -faq-lite/how-to-post.html#faq-5.8

    john
     
    John Harrison, Apr 14, 2004
    #5
  6. J.K. Becker

    Rolf Magnus Guest

    J.K. Becker wrote:

    > Hi there,
    >
    > I am trying to multiply doubles with floats (actually I tried every
    > possible combination by now) and it never works (well, it does
    > something but it is always wrong). I have no idea what it is and where
    > to look for help, maybe some of you know?
    >
    > double=float*double; (or every possible combination of it). An
    > example:
    >
    > 0.3 * 0.7 would result in 1.7 (with lots more digits). Anyone any
    > idea? If I change the types of the variables I think the result stays
    > the same (but I am not 100% sure)...


    Works here:

    #include <iostream>

    int main()
    {
    float a = 0.3;
    double b = 0.7;

    std::cout << a << " * " << b << " = " << a * b << '\n';
    }

    Output:

    0.3 * 0.7 = 0.21
     
    Rolf Magnus, Apr 14, 2004
    #6
  7. J.K. Becker

    Matt Guest

    J.K. Becker wrote:
    > Hi there,
    >
    > I am trying to multiply doubles with floats (actually I tried every
    > possible combination by now) and it never works (well, it does something
    > but it is always wrong). I have no idea what it is and where to look for
    > help, maybe some of you know?
    >
    > double=float*double; (or every possible combination of it). An example:
    >
    > 0.3 * 0.7 would result in 1.7 (with lots more digits). Anyone any idea?
    > If I change the types of the variables I think the result stays the same
    > (but I am not 100% sure)...
    >
    > Jens
    >


    The problem is in your print statement or output statement.

    man fprintf.
     
    Matt, Apr 15, 2004
    #7
  8. J.K. Becker

    J.K. Becker Guest

    Oh well, you got it all wrong! This stupid calculation was just an
    example, it does not matter what the numbers are, the result is always
    wrong. When I asked for help I was looking for something in the line of
    "you could have a memory leak" (of course I checked that already) or
    "this always happens on sunny days with gcc3.3, try gcc0.1prealpha" or
    something.
    The program is far to long to post here, if you really want to look at
    it have a look at the working parts first here

    https://sourceforge.net/projects/elle/

    or a description with links to all kinds of stuff there:

    http://www.microstructure.uni-tuebingen.de/phoenix/index.php

    And now that you know that I am not completely new to c++, no more rtfm,
    or man printf, I think I am past that by now...

    Jens

    J.K. Becker wrote:
    > Hi there,
    >
    > I am trying to multiply doubles with floats (actually I tried every
    > possible combination by now) and it never works (well, it does something
    > but it is always wrong). I have no idea what it is and where to look for
    > help, maybe some of you know?
    >
    > double=float*double; (or every possible combination of it). An example:
    >
    > 0.3 * 0.7 would result in 1.7 (with lots more digits). Anyone any idea?
    > If I change the types of the variables I think the result stays the same
    > (but I am not 100% sure)...
    >
    > Jens
    >
     
    J.K. Becker, Apr 15, 2004
    #8
  9. "J.K. Becker" <> wrote in message
    news:c5lglu$ul3$-mainz.de...
    > Oh well, you got it all wrong! This stupid calculation was just an
    > example, it does not matter what the numbers are, the result is always
    > wrong. When I asked for help I was looking for something in the line of
    > "you could have a memory leak" (of course I checked that already) or
    > "this always happens on sunny days with gcc3.3, try gcc0.1prealpha" or
    > something.
    > The program is far to long to post here, if you really want to look at
    > it have a look at the working parts first here
    >
    > https://sourceforge.net/projects/elle/
    >
    > or a description with links to all kinds of stuff there:
    >
    > http://www.microstructure.uni-tuebingen.de/phoenix/index.php
    >
    > And now that you know that I am not completely new to c++, no more rtfm,
    > or man printf, I think I am past that by now...
    >


    Well how was anyone supposed to know that? We get lots of clueless newbies
    posting here. One reason for posting source code is that it makes it easy
    for people to assess your level of expertise.

    The answer is still to post source code. Take your large program and cut out
    the irrelevant parts. When doing this one of two things will happen. You may
    remove some code you don't think is relevant and the problem goes away, this
    is a good clue as to where the bug is. The other thing that might happen is
    that you get down to a smallish program that still exhibits the behaviour
    you don't understand, then you can post that code here and someone will fix
    it.

    This is all explained in the FAQ

    http://www.parashift.com/c -faq-lite/how-to-post.html#faq-5.8

    Take the trouble, and you'll get your problem fixed.

    john
     
    John Harrison, Apr 15, 2004
    #9
  10. J.K. Becker

    J.K. Becker Guest

    I can't take the program apart, that would be more work than
    reprograming the part that is not working. There is a reason for me not
    to post the progam here, you know, believe me, if I could I would. I can
    post the function here, but that is not going to help.
    If you insist I will.



    John Harrison wrote:
    > "J.K. Becker" <> wrote in message
    > news:c5lglu$ul3$-mainz.de...
    >
    >>Oh well, you got it all wrong! This stupid calculation was just an
    >>example, it does not matter what the numbers are, the result is always
    >>wrong. When I asked for help I was looking for something in the line of
    >>"you could have a memory leak" (of course I checked that already) or
    >>"this always happens on sunny days with gcc3.3, try gcc0.1prealpha" or
    >>something.
    >>The program is far to long to post here, if you really want to look at
    >>it have a look at the working parts first here
    >>
    >>https://sourceforge.net/projects/elle/
    >>
    >>or a description with links to all kinds of stuff there:
    >>
    >>http://www.microstructure.uni-tuebingen.de/phoenix/index.php
    >>
    >>And now that you know that I am not completely new to c++, no more rtfm,
    >>or man printf, I think I am past that by now...
    >>

    >
    >
    > Well how was anyone supposed to know that? We get lots of clueless newbies
    > posting here. One reason for posting source code is that it makes it easy
    > for people to assess your level of expertise.
    >
    > The answer is still to post source code. Take your large program and cut out
    > the irrelevant parts. When doing this one of two things will happen. You may
    > remove some code you don't think is relevant and the problem goes away, this
    > is a good clue as to where the bug is. The other thing that might happen is
    > that you get down to a smallish program that still exhibits the behaviour
    > you don't understand, then you can post that code here and someone will fix
    > it.
    >
    > This is all explained in the FAQ
    >
    > http://www.parashift.com/c -faq-lite/how-to-post.html#faq-5.8
    >
    > Take the trouble, and you'll get your problem fixed.
    >
    > john
    >
    >
     
    J.K. Becker, Apr 15, 2004
    #10
  11. "J.K. Becker" <> wrote in message
    news:c5lhod$umn$-mainz.de...
    > I can't take the program apart, that would be more work than
    > reprograming the part that is not working. There is a reason for me not
    > to post the progam here, you know, believe me, if I could I would. I can
    > post the function here, but that is not going to help.
    > If you insist I will.
    >


    I'm not insisting on anything, just explaining how this group works. The
    regulars here contribute a lot of their time for no recompense. I don't
    think any have the time to take on the sort of in depth study that it sounds
    like your problem needs.

    You might have got lucky with you post and someone have recognised the
    problem but you didn't so I guess there is no alternative but some hard work
    on your part.

    Post the function if you like, you never know it might help. One of the
    things I've learned from this group is that posters are often convinced that
    a problem is in some piece of code which they post (even though they don't
    know what the problem is) but when after much resistence that are persuaded
    to post more code, the problem turns out to have been in some part of the
    code they were holding back.

    John

    BTW top posting is frowned on in this group.
     
    John Harrison, Apr 15, 2004
    #11
  12. J.K. Becker

    J.K. Becker Guest

    John Harrison wrote:
    > "J.K. Becker" <> wrote in message
    > news:c5lhod$umn$-mainz.de...
    >
    >>I can't take the program apart, that would be more work than
    >>reprograming the part that is not working. There is a reason for me not
    >>to post the progam here, you know, believe me, if I could I would. I can
    >>post the function here, but that is not going to help.
    >>If you insist I will.
    >>

    >
    >
    > I'm not insisting on anything, just explaining how this group works. The
    > regulars here contribute a lot of their time for no recompense. I don't
    > think any have the time to take on the sort of in depth study that it sounds
    > like your problem needs.
    >
    > You might have got lucky with you post and someone have recognised the
    > problem but you didn't so I guess there is no alternative but some hard work
    > on your part.
    >
    > Post the function if you like, you never know it might help. One of the
    > things I've learned from this group is that posters are often convinced that
    > a problem is in some piece of code which they post (even though they don't
    > know what the problem is) but when after much resistence that are persuaded
    > to post more code, the problem turns out to have been in some part of the
    > code they were holding back.
    >
    > John
    >
    > BTW top posting is frowned on in this group.
    >
    >


    I know that it is not a simple error (well, it might be but I have no
    idea where). The program consits of, I don't know, 10000 lines or more,
    something like 100 and more seperate files. And to make it more fun, it
    has grown over the years so some files are c++ (OO), some are c++ (not
    OO), some c and some fortran. And it has been written by people in the
    US, Australia, UK, Germany (that's me) and France. So no way to strip
    that down to something everybody can understand quickly.
    But since we all are running out of ideas on what and where to look, I
    thought I'd try here.

    Jens
     
    J.K. Becker, Apr 15, 2004
    #12
  13. J.K. Becker wrote:
    > [...]
    >
    > J.K. Becker wrote:
    > [...]
    >> double=float*double; (or every possible combination of it). An example:
    >>
    >> 0.3 * 0.7 would result in 1.7 (with lots more digits). Anyone any
    >> idea? If I change the types of the variables I think the result stays
    >> the same (but I am not 100% sure)...
    >>
    >> Jens
    >>

    >


    If I'm understanding the problem correctly, this isn't a C++ problem, but an
    implementation artifact.

    Let's say that in your implementation, float has 6 digits of precision & double
    has 12, so 0.3 is 0.300000, and 0.7 is 0.700000000000. These aren't integers
    though, they are finite precision representations of real numbers, so 0.3 is
    actually any number between 0.299999500000 and 0.300000499999. When the
    generated code, or, more likely, the computer's Floating Point Hardware (FPU),
    converts the 0.3 from float to double, it is perfectly entitled to convert it to
    anything within its valid range.

    Most people thing in base ten, so we intuitively expect that the 0.300000 will
    be seamlessly converted into 0.300000000000. The FPU is probably constructed as
    a binary device, and so it will still zero fill, but in binary. When converted
    back to decimal, this gives some value that is close to, but not, exactly
    0.300000000000. The value it gives is still correct though.

    You can see how your compiler / FPU handle these conversions by single stepping
    through this fragment.

    float x = 0.3;

    int main( int, char ** )
    {
    some_external_fn( &x ); // stop the optimizer making assumptions about x
    double y = x;
    std::cout << y << std::eol;
    return 0;
    }

    Regards

    Bruce
     
    Bruce Clement, Apr 15, 2004
    #13
  14. J.K. Becker

    J.K. Becker Guest

    This is the function where the error occurs. As you can see there are a
    lot of calculations in it, they all work fine except the one mentioned
    (which seems to be the easiest one).

    Fx = un.x * dEdp;
    Fy = un.y * dEdp;

    This always gives the wrong results... Everything before it works and
    everything after it works too (except that everything is calculated with
    the wrong values for Fx and Fy).


    But I guess it is too complex for a quick fix...


    Thanks anyway

    Jens


    int GBE_MoveNode( int index, Coords * movedir )
    {
    int i, l, k, moveflag = 0;
    double E1, E2, E3, E4, vangle[3],lenL[3] , lenK[3];
    double truetimestep = 3.1536e10;
    float Fx,Fy;
    double lenP, lenV, lenF, mobility1, mobility2, mobility3 , switchd;
    Coords p1, p2, gvector, un, sigma1, sigma2, sigma3, L[3], F;
    Coords newxy, xynb, xy, prev, V;
    float dEdp;
    int nb[3];
    char cc[255];

    mobility1 = 1e-12;
    mobility2 = 1e-12;
    mobility3 = 1e-12;

    switchd = ElleSwitchdistance() / 10;
    //Get position of first node
    ElleNodePosition( index, & p1 );
    ElleNodePrevPosition( index, & prev );

    //this is the first energy we need: E((x+dx),y)
    p2.x = p1.x + switchd;
    p2.y = p1.y;
    ElleSetPosition( index, & p2 );
    GetNodeEnergy( index, & p2, & E1 );

    //this is the second energy we need:E((x-dx),y)
    p2.x = p1.x - switchd;
    p2.y = p1.y;
    ElleSetPosition( index, & p2 );
    GetNodeEnergy( index, & p2, & E2 );

    //this is the third energy we need: E(x,(y+dy))
    p2.x = p1.x;
    p2.y = p1.y + switchd;
    ElleSetPosition( index, & p2 );
    GetNodeEnergy( index, & p2, & E3 );

    //this is the fourth energy we need: E(x,(y-dy))
    p2.x = p1.x;
    p2.y = p1.y - switchd;
    ElleSetPosition( index, & p2 );
    GetNodeEnergy( index, & p2, & E4 );

    //Reset node position to starting values
    ElleSetPosition( index, & p1 );
    ElleSetPrevPosition( index, & prev );

    //So now we can calculate the gradient vector (P)
    gvector.x = ( E1 - E2 ) / ( 2 * switchd );
    gvector.y = ( E3 - E4 ) / ( 2 * switchd );
    if ( gvector.x == 0.0 && gvector.y == 0.0 )
    {
    movedir->x = 0.0;
    movedir->y = 0.0;
    return ( 0 );
    }
    else
    {
    //reverse gradient
    gvector.x *= -1;
    gvector.y *= -1;
    //length of gradient
    lenP = sqrt( ( gvector.x * gvector.x ) + ( gvector.y * gvector.y ) );
    //unit vector
    un.x = gvector.x / lenP;
    un.y = gvector.y / lenP;
    dEdp = ( ( gvector.x / gvector.y ) + ( gvector.y / gvector.x ) ) /
    lenP;

    ////////
    //////// wrong results in this calculation
    ////////
    Fx = un.x * dEdp;
    Fy = un.y * dEdp;
    ////////
    ////////
    ////////

    lenF = sqrt( ( Fx * Fx ) + ( Fy * Fy ) );
    ElleNeighbourNodes( index, nb );
    for ( i = 0, k = 0; i < 3; i++ )
    {
    if ( nb != NO_NB && ElleNodeIsActive( index ) )
    {
    ElleNodePosition( nb, & xynb );
    L[k].x = p1.x - xynb.x;
    L[k].y = p1.y - xynb.y;
    lenL[k] = sqrt( ( L[k].x * L[k].x ) + ( L[k].y * L[k].y ) );
    vangle[k] = fabs(90- ( acos( ( Fx * L[k].x + Fy * L[k].y ) / (
    lenL[k] * lenF ) ) ));
    lenK[k] = lenL[k] * fabs( sin( vangle[k] * ( 180 / 3.1415926535
    ) ) );
    k++;
    }
    }
    sigma1.x = Fx / ( lenK[0] + ( mobility1 / mobility2 ) * lenK[1] );
    sigma1.y = Fy / ( lenK[0] + ( mobility1 / mobility2 ) * lenK[1] );
    V.x = sigma1.x * (mobility1/switchd/switchd);
    V.y = sigma1.y * (mobility1/switchd/switchd);
    lenV = sqrt( ( V.x * V.x ) + ( V.y * V.y ) );
    movedir->x = p1.x - V.x;
    movedir->y = p1.y - V.y;
    moveflag = 1;
    return ( moveflag );
    }
    }
     
    J.K. Becker, Apr 15, 2004
    #14
  15. J.K. Becker

    J.K. Becker Guest

    Bruce Clement wrote:

    > If I'm understanding the problem correctly, this isn't a C++ problem,
    > but an implementation artifact.
    >
    > Let's say that in your implementation, float has 6 digits of precision &
    > double has 12, so 0.3 is 0.300000, and 0.7 is 0.700000000000. These
    > aren't integers though, they are finite precision representations of
    > real numbers, so 0.3 is actually any number between 0.299999500000 and
    > 0.300000499999. When the generated code, or, more likely, the computer's
    > Floating Point Hardware (FPU), converts the 0.3 from float to double, it
    > is perfectly entitled to convert it to anything within its valid range.
    >
    > Most people thing in base ten, so we intuitively expect that the
    > 0.300000 will be seamlessly converted into 0.300000000000. The FPU is
    > probably constructed as a binary device, and so it will still zero fill,
    > but in binary. When converted back to decimal, this gives some value
    > that is close to, but not, exactly 0.300000000000. The value it gives is
    > still correct though.
    >
    > You can see how your compiler / FPU handle these conversions by single
    > stepping through this fragment.
    >
    > float x = 0.3;
    >
    > int main( int, char ** )
    > {
    > some_external_fn( &x ); // stop the optimizer making assumptions
    > about x
    > double y = x;
    > std::cout << y << std::eol;
    > return 0;
    > }
    >
    > Regards
    >
    > Bruce
    >


    I thought about that too, but the error is to big to be a rounding
    problem (well I think). 0.3 *0.7 can give something 0.210001238389741291
    (just an example) which would be fine with me, but not 1.72094230948.
    The error is just too big. It is interesting to note though that it
    always calculates the same so it does not multiply arbitrary numbers
    that it gets from memory somewhere, but always the same numbers....

    Jens
     
    J.K. Becker, Apr 15, 2004
    #15
  16. J.K. Becker

    J.K. Becker Guest

    Is there the slightest chance that the debugger shows wrong values but
    the calculation acutally uses the right ones? (gdb/ddd)

    See how desperate I am?!

    Jens
     
    J.K. Becker, Apr 15, 2004
    #16
  17. J.K. Becker wrote:

    [...]

    >
    > I thought about that too, but the error is to big to be a rounding
    > problem (well I think). 0.3 *0.7 can give something 0.210001238389741291
    > (just an example) which would be fine with me, but not 1.72094230948.
    > The error is just too big. It is interesting to note though that it
    > always calculates the same so it does not multiply arbitrary numbers
    > that it gets from memory somewhere, but always the same numbers....
    >


    OK,

    Jens, please don't take what follows as dismissive or demeaning. I've been
    making my living from developing code for over 25 years, and when I don't
    understand the problem I find it makes sense to go back to basics & attempt to
    double check that I've eliminated all the obvious problems.

    So let's look at it in context.

    ////////
    //////// wrong results in this calculation
    ////////
    Fx = un.x * dEdp;
    Fy = un.y * dEdp;
    ////////
    ////////
    ////////

    Presumably this means that when un.x = 0.3 and dEdp = 0.7 that Fx becomes
    1.7209423...

    If that's the case, there's only a very few possibilities:
    1. The prerequisites are not met (either un.x != 0.3 or dEdp != 0.7)
    2. The result isn't what you think it is (Fx does = 0.21) and something else is
    wrong
    3. The debugger is confused (I've seen this happen with optimised code)
    4. The compiler is generating bad code
    5. You've uncovered a hardware bug in the FPU.
    ...

    1 & 2 can be eliminated by breaking the program at this step & examining
    variables before & after the assignments

    If 3 is a possibility, try switching into machine code & tracing the statement
    through at the instruction level. You may find that the variables being loadedd
    / stored aren't what you expected.

    4 & 5 Are very unlikely, but can be eliminated as possibilities by debugging at
    machine instruction level as per 3.


    Good Luck

    Bruce
     
    Bruce Clement, Apr 15, 2004
    #17
  18. J.K. Becker wrote:
    > Is there the slightest chance that the debugger shows wrong values but
    > the calculation acutally uses the right ones? (gdb/ddd)
    >
    > See how desperate I am?!
    >
    > Jens
    >

    Yes,

    I've seen this in debuggers especially when dealing with optimised code. The
    windows VC6 compiler was especially bad (thought 'this' was in a different
    register to the one it was actually in).

    You need to make sure you've compiled up for debugging.

    I've also seen it in GDB when the compile failed (I'd missed that it had a
    compile error :( )

    B.
     
    Bruce Clement, Apr 15, 2004
    #18
  19. J.K. Becker

    J.K. Becker Guest

    Bruce Clement wrote:

    [snip]
    >
    > Presumably this means that when un.x = 0.3 and dEdp = 0.7 that Fx
    > becomes 1.7209423...
    >
    > If that's the case, there's only a very few possibilities:
    > 1. The prerequisites are not met (either un.x != 0.3 or dEdp != 0.7)
    > 2. The result isn't what you think it is (Fx does = 0.21) and
    > something else is wrong
    > 3. The debugger is confused (I've seen this happen with optimised code)
    > 4. The compiler is generating bad code
    > 5. You've uncovered a hardware bug in the FPU.
    > ...
    >
    > 1 & 2 can be eliminated by breaking the program at this step & examining
    > variables before & after the assignments
    >
    > If 3 is a possibility, try switching into machine code & tracing the
    > statement through at the instruction level. You may find that the
    > variables being loadedd / stored aren't what you expected.
    >
    > 4 & 5 Are very unlikely, but can be eliminated as possibilities by
    > debugging at machine instruction level as per 3.
    >
    >
    > Good Luck
    >
    > Bruce
    >

    Ah, don't worry, these are the kind of tips I was looking for! I had a
    look with a debugger and I am using makeg, so I guess #1 is unlikely. I
    agree that I have not found a hardware error (oh how I wish that would
    be the case: "See, it is not my fault, there is nothing I can do, it is
    IBM and AMD's fault...." :) ).
    I will try to debug the whole thing with machine code, although I really
    have no idea yet if that is going to help me (my assembler-skills are,
    well, rudimentary would be a compliment). But I think that is the best
    way to go for now

    If you can think of anything else....

    Thanks

    Jens
     
    J.K. Becker, Apr 15, 2004
    #19
  20. J.K. Becker

    J.K. Becker Guest

    John Harrison wrote:
    > "J.K. Becker" <> wrote in message
    > news:c5lkcb$uqc$-mainz.de...
    >
    >>Is there the slightest chance that the debugger shows wrong values but
    >>the calculation acutally uses the right ones? (gdb/ddd)
    >>

    >
    >
    > That's entirely possible. Have you tried dumping the values out to a file?
    >
    > john
    >
    >


    I did a printf in the function, and it showed the same vaules as the
    debugger does so I guess it is not that....

    Jens
     
    J.K. Becker, Apr 15, 2004
    #20
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