[R] mean of a column in a list of data frames

[Duncan Murdoch]

On 05/02/2017 7:01 AM, Brandon Payne wrote:
> I have a list of data frames,
>
>     ownersList <- list(exp2004owners,exp2005owners,
>                    exp2006owners,exp2007owners,
>                    exp2008owners,exp2009owners,
>                    exp2010owners,exp2011owners,
>                    exp2012owners,exp2013owners,
>                    exp2014owners)
>
>  I want to take the mean of the first column $grossIncome.
>  I can access the first column with
>
> lapply(ownersList, "[[", 1)                      ##works
>
> But I can't take the mean of that.
> mean(lapply(ownersList, "[[", 1))          ##not working

lapply returns a list of the first columns, and mean() doesn't know what 
to do with that.
>
> There must be a more idiomatic way to write this with map or apply.

Yes, take the mean inside lapply:

lapply(ownersList, function(x) mean(x[[1]]))

Not tested.

Duncan Murdoch

>
> ownersIncome<- c(mean(ownersList[[1]]$grossIncome),
>                  mean(ownersList[[2]]$grossIncome),
>                  mean(ownersList[[3]]$grossIncome),
>                  mean(ownersList[[4]]$grossIncome),
>                  mean(ownersList[[5]]$grossIncome),
>                  mean(ownersList[[6]]$grossIncome),
>                  mean(ownersList[[7]]$grossIncome),
>                  mean(ownersList[[8]]$grossIncome),
>                  mean(ownersList[[9]]$grossIncome),
>                  mean(ownersList[[10]]$grossIncome),
>                  mean(ownersList[[11]]$grossIncome))
>
> I tried a for loop, which also didn't work.
>
> aList<-
>     for(i in 1:3){
>     mean(ownersList[[i]]$grossIncome)
> }

for loops don't have a useful value:  it's always NULL.  This would have 
worked as

aList <- list()
for(i in 1:3){
     aList[[i]] <- mean(ownersList[[i]]$grossIncome)
}


Duncan Murdoch