Calculate proportions within subsets of a data frame

Question

I am trying to obtain proportions within subsets of a data frame. For example, in this made-up data frame: ``` DF<-data.frame(category1=rep(c("A","B"),each=9), category2=rep(rep(LETTERS[24:26],each=3),2), animal=rep(c("dog","cat","mouse"),6),number=sample(18)) ``` I would like like to calculate the proportion of each of the three animals for each `category1` by `category2` combination (e.g., out of all animals that are both "A" and "X", what proportion are dogs?). With `prop.table` on column 4 of the data frame I can get the proportion that each row makes up of the total "number" column, but I have not found a way to do this for subsets based on category 1 and 2. I also tried splitting the data by `category1` and `category2` using this: ``` splitDF<-split(DF,list(DF$category1,DF$category2)) ``` And I was hoping I could then apply a function with `prop.table` to get the proportions of each animal within each split group, but I cannot get `prop.table` working because I can't seem to spe…

Accepted Answer

You can use function `ddply()` from library `plyr` to calculate proportions for each combination and then add new column to data frame.

```
 library(plyr)     
 DF<-ddply(DF,.(category1,category2),transform,prop=number/sum(number))
 DF
   category1 category2 animal number       prop
1          A         X    dog     17 0.44736842
2          A         X    cat      3 0.07894737
3          A         X  mouse     18 0.47368421
4          A         Y    dog      2 0.14285714
```

Calculate proportions within subsets of a data frame

Solution

Problem