R help - Aug 2011 - help with "by" command

Hello,

I am a new user of R, and I'd be grateful if someone could help me with the
following:

I would like to compute the mean of variable "trust" in dataframe
"foo", but
separately for each level of variable V2.  That is, I'd like to compute the
mean of trust at each level of V2.

I have done this:
> tmp <- by(foo, foo$V2, function(x) mean(foo$trust, na.rm=T))
>tmp
Doing this does indeed give me a mean for variable trust at each level of V2
- but the problem is that I get the exact same mean for each level of V2. 
The means are not the really the same across levels of V2, but instead of
getting the mean for each level of V2 I get the overall mean for the
variable in the dataframe listed over and over for each level of V2.  

I have been attempting to figure this out for a while now, but I just can't
seem to figure it out.

Thanks for your time!
Ari

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Hi Ari,

Try this instead

with(foo, tapply(V2, trust, mean, na.rm = TRUE))

See ?tapply and ?with for more information.

HTH,
Jorge


On Wed, Aug 24, 2011 at 2:51 PM, amalka <> wrote:
> Hello,
>
> I am a new user of R, and I'd be grateful if someone could help me with
the
> following:
>
> I would like to compute the mean of variable "trust" in dataframe
"foo",
> but
> separately for each level of variable V2.  That is, I'd like to compute
the
> mean of trust at each level of V2.
>
> I have done this:
>
> > tmp <- by(foo, foo$V2, function(x) mean(foo$trust, na.rm=T))
> >tmp
>
> Doing this does indeed give me a mean for variable trust at each level of
> V2
> - but the problem is that I get the exact same mean for each level of V2.
> The means are not the really the same across levels of V2, but instead of
> getting the mean for each level of V2 I get the overall mean for the
> variable in the dataframe listed over and over for each level of V2.
>
> I have been attempting to figure this out for a while now, but I just
can't
> seem to figure it out.
>
> Thanks for your time!
> Ari
>
> --
> View this message in context:
> http://r.789695.n4.nabble.com/help-with-by-command-tp3766285p3766285.html
> Sent from the R help mailing list archive at Nabble.com.
>
> ______________________________________________
> R-help@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.
>
	[[alternative HTML version deleted]]

---------- Forwarded message ----------
From: Ken Hutchison <vicvoncastle@gmail.com>
Date: Wed, Aug 24, 2011 at 6:06 PM
Subject: Re: [R] help with "by" command
To: amalka <amalka@gmail.com>


?tapply
or more specifically
?ave
 Hope this helps,
     Ken


On Wed, Aug 24, 2011 at 2:51 PM, amalka <amalka@gmail.com> wrote:
> Hello,
>
> I am a new user of R, and I'd be grateful if someone could help me with
the
> following:
>
> I would like to compute the mean of variable "trust" in dataframe
"foo",
> but
> separately for each level of variable V2.  That is, I'd like to compute
the
> mean of trust at each level of V2.
>
> I have done this:
>
> > tmp <- by(foo, foo$V2, function(x) mean(foo$trust, na.rm=T))
> >tmp
>
> Doing this does indeed give me a mean for variable trust at each level of
> V2
> - but the problem is that I get the exact same mean for each level of V2.
> The means are not the really the same across levels of V2, but instead of
> getting the mean for each level of V2 I get the overall mean for the
> variable in the dataframe listed over and over for each level of V2.
>
> I have been attempting to figure this out for a while now, but I just
can't
> seem to figure it out.
>
> Thanks for your time!
> Ari
>
> --
> View this message in context:
> http://r.789695.n4.nabble.com/help-with-by-command-tp3766285p3766285.html
> Sent from the R help mailing list archive at Nabble.com.
>
> ______________________________________________
> R-help@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.
>
	[[alternative HTML version deleted]]

Thanks very much for the help.

I ended up getting it to work with one small change:

by(foo, foo$V2, function(foo) mean(foo$trust, na.rm=T)) 

thanks again,
Ari

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