Precision problems of real numbers in Fortran

Posted by saladoil on Stack Overflow See other posts from Stack Overflow or by saladoil
Published on 2012-04-04T17:26:06Z Indexed on 2012/04/04 17:29 UTC
Read the original article Hit count: 152

Filed under:

output

|

fortran

|

precision

|

gfortran

|

real

I've been trying to use Fortran for my research project, with the GNU Fortran compiler (gfortran), latest version, but I've been encountering some problems in the way it processes real numbers. If you have for example the code:

program test
implicit none

real :: y = 23.234, z

z = y * 100000
write(*,*) y, z

end program

You'll get as output:

23.23999    2323400.0.

I find this really strange. Can someone tell me what's exactly happening here? Looking at z I can see that y does retain its precision, so for calculations that shouldn't be a problem I suppose. But why is the output of y not exactly the same as the value that I've specified, and what can I do to make it exactly the same?

© Stack Overflow or respective owner

Related posts about output

How to change SMP affinity of an IRQ on Ubuntu domU inside Xen XCP?

as seen on Server Fault - Search for 'Server Fault'
I'd like to change IRQ SMP affinity for reasons, outlined in this question: CPU0 is swamped with eth1 interrupts But I can't — I see Input/output error when I try to write to /proc/irq/*/smp_affinity. Please point me to the HOWTO on the matter. (A formal reference on /proc/irq/*/ would be cool as… >>> More
Capturing and Transforming ASP.NET Output with Response.Filter

as seen on West-Wind - Search for 'West-Wind'
During one of my Handlers and Modules session at DevConnections this week one of the attendees asked a question that I didn’t have an immediate answer for. Basically he wanted to capture response output completely and then apply some filtering to the output – effectively injecting some additional… >>> More
How do I fix a corrupted harddrive after failed upgrade?

as seen on Ask Ubuntu - Search for 'Ask Ubuntu'
The problem originated when I was trying to fix this problem. Things went horribly, horribly wrong and I ended up with a new problem altogether. The last thing I did was run sudo apt-get install and that caused my system to freeze. I restarted my computer and it would not boot from the harddrive… >>> More
Cache Simulator in C

as seen on Stack Overflow - Search for 'Stack Overflow'
Ok this is only my second question, and it's quite a doozy. It's for a school assignment, but no one (including the TAs) seems to be able to help me. It's kind of a tall order but I'm not sure where else to turn. Essentially the assignment was to make a cache simulator. This version is direct mapping… >>> More
An issue with tessellation a model with DirectX11

as seen on Game Development - Search for 'Game Development'
I took the hardware tessellation tutorial from Rastertek and implemended texturing instead of color. This is great, so I wanted to implemended the same techique to a model inside my game editor and I noticed it doesn't draw anything. I compared the detailed tessellation from DirectX SDK sample. Inside… >>> More

Related posts about fortran

passing a fortran int array to C++ by calling C++ function in fortran

as seen on Stack Overflow - Search for 'Stack Overflow'
hi, I am trying to call a C++ function in FORTRAN subroutine. This C+ function is supposed to update an integer array. Here is a non-working code I wrote. Can someone please let me know what the issue is: ! FORTRAN function that calls a C++ function subroutine my_function() integer(4) ar(*) … >>> More
MinGW/G++/g95 link problem - libf95 undefined reference to `MAIN_'

as seen on Stack Overflow - Search for 'Stack Overflow'
Hi folks, Summing up, my problem consists on compiling g95 objects inside a C++ application. Actually, I'm constructing an interface for an old fortran program. For this task, I'm using the wxWidgets GUI library, and calling fortran subroutines when necessary. At the beginning, I was developing the… >>> More
Fortran arrays and subroutines

as seen on Stack Overflow - Search for 'Stack Overflow'
I'm going through a Fortran code, and one bit has me a little puzzled. There is a subroutine, say SUBROUTINE SSUB(X,...) REAL*8 X(0:N1,1:N2,0:N3-1),... ... RETURN END Which is called in another subroutine by: CALL SSUB(W(0,1,0,1),...) where W is a 'working array'. It appears that a specific… >>> More
fortran 90 user defined type, passing by value ?

as seen on Stack Overflow - Search for 'Stack Overflow'
I have an issue in fortran 90. I have a user defined type and when I call one of the MPI subroutines the data looks to be passed by values (not address, as I thought it should). The output arguments aren't modified. It seems to be specific to the MPI calls I tried the same thing in a simple test… >>> More
Should External Routine Be Declared Always in Fortran?

as seen on Stack Overflow - Search for 'Stack Overflow'
In my fortran code I made the following call to dnrm2 routine d = dnrm2(n, ax, 1) Just a simple call that would return me a double precision result. The problem is whether I should declare the function at the start of my script. I found that if I don't declare it, and when I compile the code in… >>> More