[R] average at specific hour "endpoints" of the day

[Massimo Bressan]

hello 

given my reproducible example 

#--- 
date<-seq(ISOdate(2017,1, 1, 0), by="hour", length.out = 48) 
v1<-1:48 
df<-data.frame(date,v1) 

#-- 

I need to calculate the average of variable v1 at specific hour "endpoints" of the day: i.e. at hours 6.00 and 22.00 respectively 

the desired result is 

date v1 
01/01/17 22:00 15.5 
02/01/17 06:00 27.5 
02/01/17 22:00 39.5 

at hour 06:00 of each day the average is calculated by considering the 8 previous records (hours from 23:00 to 6:00) 
at hour 22:00 of each day the average is calculated by considering the 16 previous records (hours from 7:00 to 22:00) 

any hint please? 

I've been trying with some functions within the "xts" package but withouth much result... 

thanks for the help 



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