Floating point computations differ in different runs of same program

[Rahul]

Hi,

I have a C program which does lots of computations in floats and
doubles,
It is processing the same file (same data) but in different runs of
the program (on the same machine) the results vary sometimes by
numeric value 1 (which is after rounding the resulting float number)
Is this expected behavior with floats and doulbe (with lots of
intermediate conversion in between) ?

Since There is only one thread so I guess the result should be same
always.

Any reason/pointers for the above mentioned behavior.

Thanks in advance
Rahul

[Lionel B]

Re: Floating point computations differ in different runs of sameprogram

On Thu, 14 Feb 2008 01:17:12 -0800, Rahul wrote:

Hi,
>
I have a C program

Um... so why are you posting to a C++ newsgroup?

which does lots of computations in floats and doubles,
It is processing the same file (same data) but in different runs of the
program (on the same machine) the results vary sometimes by numeric
value 1 (which is after rounding the resulting float number) Is this
expected behavior with floats and doulbe (with lots of intermediate
conversion in between) ?
>
Since There is only one thread so I guess the result should be same
always.
>
Any reason/pointers for the above mentioned behavior.

Sure there's no (pseudo)-random number generation going on?

You'll probably have to post some (minimal) code that demonstrates the
problem to sort this out... but comp.lang.c might be a better bet.

--
Lionel B

[Fred Zwarts]

Re: Floating point computations differ in different runs of same program

"Rahul" <rahulsharma@lu cent.comwrote in message news:1209312c-0031-4a44-9ccb-bf02e3cfe680@s1 9g2000prg.googl egroups.com...

Hi,

I have a C program which does lots of computations in floats and
doubles,
It is processing the same file (same data) but in different runs of
the program (on the same machine) the results vary sometimes by
numeric value 1 (which is after rounding the resulting float number)
Is this expected behavior with floats and doulbe (with lots of
intermediate conversion in between) ?

Since There is only one thread so I guess the result should be same
always.

Any reason/pointers for the above mentioned behavior.

Possible causes are:
+ Use of uninitialized variables.
+ Use of a random generator.
+ Use of the changing environment, e.g. the current time.
+ ...

[dohboy]

Re: Floating point computations differ in different runs of same program

On Thu, 14 Feb 2008 16:59:27 +0100, "Fred Zwarts" <F.Zwarts@KVI.n lwrote:

>"Rahul" <rahulsharma@lu cent.comwrote in message news:1209312c-0031-4a44-9ccb-bf02e3cfe680@s1 9g2000prg.googl egroups.com...

>Hi,
>>
>I have a C program which does lots of computations in floats and
>doubles,
>It is processing the same file (same data) but in different runs of
>the program (on the same machine) the results vary sometimes by
>numeric value 1 (which is after rounding the resulting float number)
>Is this expected behavior with floats and doulbe (with lots of
>intermediate conversion in between) ?
>>
>Since There is only one thread so I guess the result should be same
>always.
>>
>Any reason/pointers for the above mentioned behavior.

>
>Possible causes are:
+ Use of uninitialized variables.
+ Use of a random generator.
+ Use of the changing environment, e.g. the current time.
+ ...

Maybe a vortex in the space-time continuum. It happens to my programs all the time. ;-)
Possibly explains what happens to my socks in the dryer, too.