r - Grouped correlation with dplyr (works only on console)

Question

I'm trying to use dplyr to calculate grouped correlations, but something is clearly wrong since the code below works only in the console:

require(dplyr)
set.seed(123)
xx = data.frame(group = rep(1:4, 100), a = rnorm(400) , b = rnorm(400))
gp = group_by(xx, group)
summarize(gp, cor(a, b))

  group   cor(a, b)
1     1 -0.02073084
2     2  0.12803353
3     3  0.06236264
4     4 -0.06181904

If i use the same code in RStudio, i get:

   cor(a, b)
1 0.02739193

What's happening?

Answer 1

What you experience is related to having both plyr and dplyr loaded at the same time. Since both packages have summarize functions, there can be conflicts if you don't specify explicitly which package you want to use. For the example data, this means:

require(dplyr)
set.seed(123)
xx = data.frame(group = rep(1:4, 100), a = rnorm(400) , b = rnorm(400))

Using dplyr as intended:

gp = group_by(xx, group)
dplyr::summarize(gp, cor(a, b))
#Source: local data frame [4 x 2]
#
#  group   cor(a, b)
#1     1 -0.02073084
#2     2  0.12803353
#3     3  0.06236264
#4     4 -0.06181904

Or using plyr

gp = group_by(xx, group)
plyr::summarize(gp, cor(a, b))
#   cor(a, b)
#1 0.02739193

So either avoid loading both packages or specify the package by using package::function.