The “aggregation function missing” warning message occurs when you are trying to use the dcast function to melt long format data into molten data around a value variable. It happens when you forget to specifically set up the aggregation function. This message does an accurate job of describing the problem. Fixing this problem is a simple matter of adding the aggregation function to the dcast function.
Description of the R Warning
The “aggregation function missing” warning message occurs when aggregation is not included in the dcast function. The dcast function is a function from the reshape2 package. Defaulting to length is the response of the function to the situation. It will work with data frames consisting of multiple columns and column names. It is a surprisingly easy problem to fix because the message accurately describes the problem by adding the aggregation function. The only issue is that it may not be understood by someone who is unfamiliar with it. Once you are familiar with it, you are less likely to make this mistake in the first place.
Explanation of the R Warning
Here is an example of code that produces the “aggregation function missing” warning message. It is an amazingly easy problem to cause because it simply requires forgetting something.
> df = data.frame(ID = c(3,3,3,3,4,4,4,4),
+ Task = c(1,1,2,2,1,1,2,2),
+ Type = c(‘A’,’B’,’A’,’B’,’A’,’B’,’A’,’B’),
+ Freq = c(2,3,3,0,3,3,1,3))
> df
ID Task Type Freq
1 3 1 A 2
2 3 1 B 3
3 3 2 A 3
4 3 2 B 0
5 4 1 A 3
6 4 1 B 3
7 4 2 A 1
8 4 2 B 3
>
> df2 = reshape2::dcast(df, “Id” + “Task” ~ Type, value.var=”Freq”)
Aggregation function missing: defaulting to length
> df2
“Id” “Task” A B
1 Id Task 4 4
In this example, we are using the dcast function but did not include the aggregation function. It includes an id variable meaning that the rest of the format is correct. This application simply requires the aggregation function. It means that the problem will be easy to fix. However, you need to keep in mind that you are still getting results. Furthermore, if the aggregation function is included but set to NULL, it still produces our warning message. This shows that you can include the function incorrectly and still get our warning message. It is an important part of understanding and fixing this problem, but once we understand it, it should be an easy problem to fix.
How to fix the R Warning
Here is an example of some code that fixes the “aggregation function missing” warning message. Fixing this problem is quite easy, but it adds complexity to the program.
> df = data.frame(ID = c(3,3,3,3,4,4,4,4),
+ Task = c(1,1,2,2,1,1,2,2),
+ Type = c(‘A’,’B’,’A’,’B’,’A’,’B’,’A’,’B’),
+ Freq = c(2,3,3,0,3,3,1,3))
> df
ID Task Type Freq
1 3 1 A 2
2 3 1 B 3
3 3 2 A 3
4 3 2 B 0
5 4 1 A 3
6 4 1 B 3
7 4 2 A 1
8 4 2 B 3
>
> df2 = reshape2::dcast(df, “Id” + “Task” ~ Type, value.var=”Freq”, fun.aggregate = length)
> df2
“Id” “Task” A B
1 Id Task 4 4
In this example, we insert the aggregation function into the dcast function and set it to length. there are other values that you can set this function to create different effects. The result is that there is no warning message, and it produces the expected results. The variations that you can get by adjusting aggregation are many allow you to adjust the data for the situation. While fixing this problem is simple, it takes some effort to learn these functions well enough to get the results that you are looking for.
Fixing this warning message is simple, but it does require understanding how these functions work. Because you can get this message whether aggregation is missing or set to null, it is an easy message to get but it is also easy to fix. The simple solution is setting it to length, but you have other options. If you are going to use these functions, it is critical for you to learn how to use them properly, because otherwise, you can get this warning message and more.
The key point is properly applying aggregation to the dcast function so that you get to results you are looking for without any warnings or errors. Once you learn how to use them properly, you will be able to avoid this warning message, because creating the function properly will become a normal part of using it. This is mainly a problem that comes about from a lack of experience and understanding of the functions you are working with. Once you understand them, and why this problem occurs you will likely avoid it.