[R] Conditional statement used in sapply()

[Altaweel, Mark R.]

Hi,

I have data stored in a list that I would like to aggregate and perform some basic stats. However, I would like to apply conditional statements so that not all the data are used.  Basically, I want to get a specific variable, do some basic functions (such as a mean), but only get the data in each element's data that match the condition. The code I used is below:

> result<-sapply(res, function(.df) {   #res is the list containing file data
+ if(.df$Volume>0)mean(.df$Volume)  #only have the mean function calculate on values great than 0
+ })


I did get a numeric output; however, when I checked the output value the conditional was ignored (i.e. it did not do anything to the calculation)

I also obtained these warning statements:

Warning messages:
1: In if (.df$Volume > 0) mean(.df$Volume) :
  the condition has length > 1 and only the first element will be used
2: In if (.df$Volume > 0) mean(.df$Volume) :
  the condition has length > 1 and only the first element will be used

Please let me know what am I doing wrong and how can I apply a conditional statement to the sapply function.

Thanks

Mark