[R] t.test in a loop

[Thomas Lumley]

On Wed, 28 Jan 2009, Michael Pearmain wrote:

> Hi All,
> I've been having a little trouble with creating a loop that will run a a
> series of t.tests for inspection,
> Below is the code i've tried, and some checks i've looked at.
>
> I've used the get(paste()) idea as i was told previously that the use of the
> eval should try and be avoided.
>
> I've run a single syntax to check that my systax is correct and works
> without any problems
>> t.test(channel.data.train$News~channel.data.train$power)
>
> Can anyone offer any advice?

There's the additional problem that if your code worked it would do 16 t-tests but only report the last one.

Assuming you want them printed

for(v in names(channel.data.train)[1:16]) {
   print(v)
   print(t.test(channel.data.train[[v]]~channel.data.train$power)
}

or
for(v in names(channel.data.train)[1:16]){
   test <- eval(bquote(.(v)~power, data=channel.data.train)
   print(eval(test))
}

This sort of use of eval is fairly harmless.

        -thomas
> Many thanks
>
> Mike
>
>> str(channel.data.train$power)
> num [1:9913] 0 0 0 0 0 0 0 0 0 0 ...
>> summary(channel.data.train$power)
>   Min. 1st Qu.  Median    Mean 3rd Qu.    Max.
> 0.0000  0.0000  0.0000  0.2368  0.0000  1.0000
>> names(channel.data.train)
> [1] "News"              "Entertainment"     "Communicate"
> [4] "Lifestyle"         "Games"             "Music"
> [7] "Money"             "Celebrity"         "Shopping"
> [10] "Sport"             "Film"              "Travel"
> [13] "Cars"              "Property"          "Chat"
> [16] "Bet.Play.Win"      "config"            "exposed"
> [19] "site"              "referrer"          "started"
> [22] "last_viewed"       "num_views"         "secs_since_viewed"
> [25] "register"          "secs.na"           "power"
> [28] "tt"
>> for(i in names(channel.data.train[,c(1:16)])){
> +
> t.test(get(paste("channel.data.train$",i,"~channel.data.train$power",sep="")))
> + }
> Error in get(paste("channel.data.train$", i, "~channel.data.train$power",
> :
>  variable "channel.data.train$News~channel.data.train$power" was not found
>
>
>
> --
> Michael Pearmain
> Senior Analytics Research Specialist
>
>
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Thomas Lumley			Assoc. Professor, Biostatistics
tlumley at u.washington.edu	University of Washington, Seattle