[R] Help with ddply/summarize

[David L Carlson]

I think this is what you want:

> MyVar <- 1:10
> MyVar
 [1]  1  2  3  4  5  6  7  8  9 10
> mean(MyVar)
[1] 5.5
> txt <- "MyVar"
> mean(txt)
[1] NA
Warning message:
In mean.default(txt) : argument is not numeric or logical: returning NA
> mean(get(txt))
[1] 5.5


-------------------------------------
David L Carlson
Department of Anthropology
Texas A&M University
College Station, TX 77840-4352




-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf Of John Posner
Sent: Thursday, November 13, 2014 5:32 PM
To: 'r-help at r-project.org'
Subject: [R] Help with ddply/summarize

I have a straightforward application of ddply() and summarize():

   ddply(MyFrame, .(Treatment, Week), summarize, MeanValue=mean(MyVar))

This works just fine:

   Treatment     Week MeanValue
1    MyDrug  BASELINE      5.91
2    MyDrug    WEEK 1      4.68
3    MyDrug    WEEK 2      4.08
4    MyDrug    WEEK 3      3.67
5    MyDrug    WEEK 4      2.96
6    MyDrug    WEEK 5      2.57
7    MyDrug    WEEK 6      2.50
8    Placebo BASELINE      8.58
9    Placebo   WEEK 1      8.25
...

But I want to specify the variable (MyVar) as a character string:

   ddply(MyFrame, .(Treatment, Week), summarize, MeanValue=mean("MyVar"))

(Actually, the character string "MyVar" will be selected from a vector of character strings.)

The code above produces no joy:

   Treatment     Week MeanValue
1    MyDrug  BASELINE        NA
2    MyDrug    WEEK 1        NA
3    MyDrug    WEEK 2        NA
4    MyDrug    WEEK 3        NA
...
I tried a few things, including:

  as.name("MyVar")
  as.quoted("MyVar")

... but they all produced the name results: NAs

I'm obviously thrashing around in the dark! Any advice would be greatly appreciated.

-John


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