[noob question] how to properly use iterators ?

[yann claveau]

Hello,

here is my basic python question :

I would like to know how to properly use ts iterator:

for ts in u.trajectory

For instance, if I want to calculate the displacement from one z-position at time t to the z-position at time t+dt. I can do this, but I guess it is not as it should be coded:

d=np.empty([u.trajectory.n_frames, u.trajectory.n_atoms])

t=0

for ts in u.trajectory:
    d[t] = u.ts.positions[:,2]
    t += 1
   
d[1:]-d[:-1]

is there a way to do a sort of u.ts[t-1].positions ?

Thanks !

[Hugo Macdermott-Opeskin]

Hi Yann, 

You can slice the `trajectory` object itself to get a subset of the frames.

See the userguide section on this for more info.

However if you are collecting multiple timepoints to compare for an MSD (which i guess is perhaps what you are doing given your previous question?) you are better off doing what you have done and collecting the positions in a big array. This is how it is done in the MSD source code. 

Hopefully that answers your question?

Cheers

Hugo MacDermott-Opeskin

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[yann claveau]

Thanks a lot again. Actually it was for another thing but, yes, also for thinking about my MSD problem. It was more a naive question to progress in Python and to better understand MDA.