Wave averaging from specific index depending on another wave

Hi all,

I'd like to average a time-dependant wave over several timestamps (seasons, months, diurnal, etc). Back in the time, I coded a Python script that does it nicely, but would like to transpose it on Igor.
On Python, one can easily handle arrays (waves) with numpy:

"march_avg=mean(wave_to_avg[where(month==3)])" averages wave_to_avg only where month=3. Is there something similarly straight forward on Igor?

Thanks!
J-E