[R] Conditional statement used in sapply()

[Erik Iverson]

Hello -

Altaweel, Mark R. wrote:
> 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.
> 

Before you think about sapply, what would you do if you had one element 
of this list.  Write a function to do that.

You wouldn't do :

if(x$Volume > 0)
   mean(x$Volume)

because x$Volume > 0 will create a logical vector greater than length 1 
(assuming x$Volume is greater than length 1), and then "if" will issue 
the warning.

You might do,

mean(x$Volume[x$Volume > 0])

and turn it into a function.

Then use sapply.

Hopefully that gets you started!

Erik