If inside of double loop - r

Question

Inside of double loop I create subset of data for each combination of Tv stations with months which is done by loops. For example I have monthsnumbers 7,8,9 and stations A,B,C. It happens that for Month 9 there is no station C.
Then subset is empty and function throws and error of no possible aggregation.
So as you can see I tried to use if statement that if there are 0 rows don't continue with the code but go on to the next loop.
But I still get the same error fck. message
can you please navigate me ?
for (Mesic in monthnumbers){
for (Stanica in TVstations){
Client<-data[data$month ==Mesic & data$Channel_group1 ==Stanica & data$Brand == brand, ]
if (nrow(Client)!=0)
###some code
Client_AGG<-aggregate(formula= Client$BUYING_GRPs ~ Client$Brand,data= Client,FUN = sum)
###some code
}
}
}

Answer 1

Like Gregor commented, there is likely a better way of going about this.
But a quick patch might be to put the error check before the Client<- line because that's where it's looking for a channel that doesn't exist. Check if data$Channel_group1 == Stanica even exists before trying to get data from it.
Another option using for loops is to cycle through what you know is there with something like this:
subsetindex <- unique(data[ ,c('month','Channel_group1')])
for(i in 1:nrow(subsetindex)){
Client<-data[data$month ==subsetindex[i,'month'] & data$Channel_group1 ==subsetindex[i,'Channel_group1'] & data$Brand == brand, ]
#other code
}