[Bug c/53738] New: Problem with pow() function when calculating power of 10

[Bug c/53738] New: Problem with pow() function when calculating power of 10

[chirayu.chiripal at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53738

             Bug #: 53738
           Summary: Problem with pow() function when calculating power of
                    10
    Classification: Unclassified
           Product: gcc
           Version: 4.6.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: chirayu.chiripal@gmail.com


Created attachment 27671
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=27671
Screenshot of the Bug

My program is calculating the log=log10(num) and then using pow(10,log-1) to
get the power of 10 one less than its log.


Here is the code:
// Sandbox.c
#include<stdio.h>
#include<math.h>
int main()
{    int t,n,log,po;
    printf("\nTest Cases:");
    scanf("%d",&t);
    while(t--)
    {    printf("\nNumber:");
        scanf("%d",&n);
        log=log10(n);
        printf("\nlog=%d\n",log);
        po=pow(10,log);
        printf("pow(10,log)=%d\n",po);
        po=pow(10,log-1);
        printf("pow(10,log-1)=%d\n",po);
        po=pow(10,log);
        printf("pow(10,log)=%d\n",po);
        po=pow(10,log)/10;
        printf("pow(10,log)/10=%d\n\n",po);
    }
    return 0;
}


OUTPUT (Expected):

Test Cases:2
Number:1020
log=3
pow(10,log)=1000
pow(10,log-1)=100  
pow(10,log)=1000
pow(10,log)/10=100


Number:10230
log=4
pow(10,log)=10000
pow(10,log-1)=1000
pow(10,log)=10000 
pow(10,log)/10=1000 


OUTPUT (Actual):

Test Cases:2
Number:1020
log=3
pow(10,log)=1000
pow(10,log-1)=99 // Problem Here
pow(10,log)=1000
pow(10,log)/10=100


Number:10230
log=4
pow(10,log)=10000
pow(10,log-1)=1000
pow(10,log)=9999 // Problem Here
pow(10,log)/10=999 // Problem Here


Compiler Options Used in CMD:

>gcc sandbox.c -o sand

>sand


Version shown by "gcc -v":

Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=c:/mingw/bin/../libexec/gcc/mingw32/4.6.2/lto-wrapper.exe
Target: mingw32
Configured with: ../gcc-4.6.2/configure
--enable-languages=c,c++,ada,fortran,obj
c,obj-c++ --disable-sjlj-exceptions --with-dwarf2 --enable-shared
--enable-libgo
mp --disable-win32-registry --enable-libstdcxx-debug
--enable-version-specific-r
untime-libs --build=mingw32 --prefix=/mingw
Thread model: win32
gcc version 4.6.2 (GCC)

System Specs:
Win 7 Home Premium 64 bit
Intel Core i7-2670QM @ 2.20 GHz

Works fine when run on IDEONE in C(gcc-4.3.4)
http://ideone.com/cQRrY

[Bug c/53738] Problem with pow() function when calculating power of 10

[chirayu.chiripal at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53738

--- Comment #1 from Chirayu Chiripal <chirayu.chiripal at gmail dot com> 2012-06-21 03:26:13 UTC ---
Also Tried compiling it with:

gcc -Wall -Wextra
and
-fno-strict-aliasing -fwrapv

but the result was same.

[Bug target/53738] Problem with pow() function when calculating power of 10

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53738

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Target|                            |i?86-*-mingw32
             Status|UNCONFIRMED                 |RESOLVED
          Component|c                           |target
         Resolution|                            |MOVED

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> 2012-06-21 03:33:23 UTC ---
Works for me correctly on linux:
Test Cases:2

Number:1020

log=3
pow(10,log)=1000
pow(10,log-1)=100
pow(10,log)=1000
pow(10,log)/10=100


Number:10230

log=4
pow(10,log)=10000
pow(10,log-1)=1000
pow(10,log)=10000
pow(10,log)/10=1000

>Works fine when run on IDEONE in C(gcc-4.3.4)

That is most likely linux too.

In fact there is nothing GCC can do since pow is not provided by GCC but rather
who ever provides your libc.  I would report this to the mingw folks.