R help - Aug 2008 - Conditional statement used in sapply()

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

Hi Mark, 

How about this?

result <- sapply(split(res, res$Volume>0)$`TRUE`, mean)

There is one thing I'm not sure: is res$Volume a vector or single
numeric?

-gary

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
On Behalf Of Altaweel, Mark R.
Sent: Wednesday, August 13, 2008 6:03 PM
To: r-help at r-project.org
Subject: [R] Conditional statement used in sapply()


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

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> -----Original Message-----
> From: r-help-bounces at r-project.org
[mailto:r-help-bounces at r-project.org]
> On Behalf Of Altaweel, Mark R.
> Sent: Wednesday, August 13, 2008 3:03 PM
> To: r-help at r-project.org
> Subject: [R] Conditional statement used in sapply()
> 
> 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
> + })
> 
You probably want something such as
result<-sapply(res, function(.df) {  mean(.df$Volume[.df$Volume>0]) })  

HTH
Steve McKinney
> 
> 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
> 
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-
> guide.html
> and provide commented, minimal, self-contained, reproducible code.

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