如何求变数之和？

Question

Let's say I have two columns of data. The first contains categories such as "First", "Second", "Third", etc. The second has numbers which represent the number of times I saw "First".

For example:

Category     Frequency
First        10
First        15
First        5
Second       2
Third        14
Third        20
Second       3

I want to sort the data by Category and sum the Frequencies:

Category     Frequency
First        30
Second       5
Third        34

How would I do this in R?

转载于:https://stackoverflow.com/questions/1660124/how-to-sum-a-variable-by-group

Answer 1

Using aggregate:

aggregate(x$Frequency, by=list(Category=x$Category), FUN=sum)
  Category  x
1    First 30
2   Second  5
3    Third 34

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In the example above, multiple dimensions can be specified in the list. Multiple aggregated metrics of the same data type can be incorporated via cbind:

aggregate(cbind(x$Frequency, x$Metric2, x$Metric3) ...

---

(embedding @thelatemail comment), aggregate has a formula interface too

aggregate(Frequency ~ Category, x, sum)

Or if you want to aggregate multiple columns, you could use the . notation (works for one column too)

aggregate(. ~ Category, x, sum)

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or tapply:

tapply(x$Frequency, x$Category, FUN=sum)
 First Second  Third 
    30      5     34 

---

Using this data:

x <- data.frame(Category=factor(c("First", "First", "First", "Second",
                                      "Third", "Third", "Second")), 
                    Frequency=c(10,15,5,2,14,20,3))