reading in every Nth event from a file

[calebmiller]

I'd like to load a TTree from a rootfile but with a prescale applied. ie. Instead of loading each event into the array I only load events where event_id(mod)10=0 or event_id(mod)100=0.

I imagine this is fairly easy to do if the event_id is already in the TTree as a branch, by just requiring the event meets the mod 0 requirement. But what about if there is no explicit event number branch?

[jpivarski]

Any array in Python can use a slice syntax like this:

array[::10]

The above will take event 0, event 10, event 20, etc. (If you wanted event 9, 19, 29, etc., then you can do array[9::10]. That would agree better with eventId % 10 if the eventId started with 1 instead of 0.)

There's no way to avoid reading all the events before applying this prescale because contiguous events have to be read together (due to the columnar structure of a ROOT TBranch), but this will get you every 10th event after they've been read.

[calebmiller]

Ah that makes sense. Thanks for your help