[gmx-users] Length of .xtc file in frames for C-Code

[Barnett, James W.]

> I have a question to all the people who are familiar with C-code. How can I
> read in the total number of frames in my *.xtc file? Especially when I do
> the analysis somewhere in the middle?

Are you using the XDR library or something else? In the XDR library there is a 
way to get the "status" each time you read in a frame. At the end of the file 
you get a different status, signaling there is nothing else to read in (or 
maybe signaling an error).

Typically I read in a frame, do my calcs, etc, and then when I'm at the end of 
the file the loop is exited.  To start somewhere in the middle of the file I 
set it up to read in X frames before I do my calcs (just cycle through the 
loop but skip the calcs).  I don't use C in particular, but the principle 
remains.

Alternatively you can use the trjconv tool to get rid of unneeded frames and 
then use your analysis program.

James Barnett

> 
> E.g: I have a file with 100.000 frames. When I start the programm with
> 
> ./program -f traj.xtc 
> 
> then I want somewhere in my code something like:
> 
> int totalnumerbofframesyouareanalyzing = 100.000
> 
> 
> When I do the same with:
> 
> ./program -f traj.xtc -b 20.000
> 
> then I want in my code:
> 
> int totalnumerbofframesyouareanalyzing = 80.000
> 
> Thanks for your help!
> GtrAngus
>