How Can I check the occurences of the values in each individual in R? - r

Question

Let's say I have a data.frame that looks like this:
ID B
1 1
1 2
1 1
1 3
2 2
2 2
2 2
2 2
3 2
3 10
3 2
Now I want to check the occurrences of B under each ID, such as that for no. 1, 1 happens twice, 2 and 3 happens 1 time each. And in no. 2, only 2 happens 4 times. How should I accomplish this? I tried to use table in ddply but somehow it did not work. Thanks.

Answer 1

It seems like you may just want a table
> table(dat)
## B
## ID 1 2 3 10
## 1 2 1 1 0
## 2 0 4 0 0
## 3 0 2 0 1
Then the following shows that for ID equal to 1, there are two 1s, one 2, and one 3.
> table(dat)[1, ]
## 1 2 3 10
## 2 1 1 0

Answer 2

And here's an aggregate solution:
> with(data, aggregate(B, list(ID=ID, B=B), length))
ID B x
1 1 1 2
2 1 2 1
3 2 2 4
4 3 2 2
5 1 3 1
6 3 10 1

Answer 3

Here's an approach using "dplyr" (if I understood your question correctly):
library(dplyr)
mydf %.% group_by(ID, B) %.% summarise(count = n())
# Source: local data frame [6 x 3]
# Groups: ID
#
# ID B count
# 1 1 1 2
# 2 1 2 1
# 3 1 3 1
# 4 2 2 4
# 5 3 2 2
# 6 3 10 1
In "plyr", I guess it would be something like:
library(plyr)
ddply(mydf, .(ID, B), summarise, count = length(B))
In base R, you could do something like the following and just remove the rows with 0:
data.frame(table(mydf))
# ID B Freq
# 1 1 1 2
# 2 2 1 0
# 3 3 1 0
# 4 1 2 1
# 5 2 2 4
# 6 3 2 2
# 7 1 3 1
# 8 2 3 0
# 9 3 3 0
# 10 1 10 0
# 11 2 10 0
# 12 3 10 1

Answer 4

And the data.table solution because there must be:
data[, .N, by=c('ID','B')]
The above won't work if you try to apply it to a data.frame. It must be converted to a data.table first. With more recent versions of "data.table", this is most easily done with setDT (as recommended by David in the comments):
library(data.table)
setDT(data)[, .N, by=c('ID', 'B')]