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Emma Wilson
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Asked: 2022-05-18T20:44:16+00:00 In:

Stabilizing the error with the best answers: aggregation function missing: defaulting to length

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I get the error: aggregation function missing: defaulting to length when I try to run the program below:

Id Task Type Freq 
 3 1 A 2
 3 1 B 3
 3 2 A 3
 3 2 B 0
 4 1 A 3
 4 1 B 3
 4 2 A 1
 4 2 B 3
Id A B … Z 
 3 5 3 
 4 4 6
df_wide <- dcast(df, Id + Task ~ Type, value.var="Freq")

The error appears the system notifies as follows:

Aggregation function missing: defaulting to length

I tried to solve it with another sample. I got the reference in the community forum, but it still returned an invalid result. If someone knows the solution, please give me the support. Thanks!

aggregation function missing
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2 Answers

  1. Best Answer
    Expert

    The cause:

    You have got this error because the dcast function which is in the reshape2 package is being used to change a data frame from a length to width format. However, there are more than one value which can be set in the single cell of the width data frame. When many values from the value.var column that correspond to the same value(s) or combination of values appearing on the LHS of the dcast formula (for example, “Id”) are crammed into one cell by the combination of variables in the RHS of the calculation, fun.aggregate is necessary (for example, “Type”).

    Solution:

    The length() default value in dcast is instructive because it can point to the possibility of coupling in the data and identify length > 1 cases that could need special attention.
    Using list() as fun.aggregate would be more instructive because it displays which value.var values are related for each instance as the following:

    dcast(dt, Id ~ Type, fun.aggregate = list, value.var = 'Freq')

Id A B
1: 3 2,3 3,0
2: 4 3,1 3,3

    Table cells typically have a length of 1. Consequently, the defaulting issue in dcast can be resolved by changing the formula or by putting in place a length-one summarization (aggregation): operators, custom or ready-made functions that deliver a length-one result in each case and are appropriate for the task.

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  2. This warning is caused by fun.aggregate (see ?dcast).

    aggregation function needed if variables do not identify a single
    observation for each output cell. Defaults to length (with a message)
    if needed but not specified

    An aggregation function is required when more than one value is available for a single spot in the large dataframe.



    Based on your data,

    Use dcast(df, Id + Task ~ Type, value.var="Freq") to get: 

     Id Task A B
1 3 1 2 3
2 3 2 3 0
3 4 1 3 3
4 4 2 1 3

    This is because Id, Task and Type only have one value in Freq. This warning message is also displayed when you use dcast(df, Id ~ Type, value.var="Freq") 

    Aggregation function missing: defaulting to length
 Id A B
1 3 2 2
2 4 2 2

    Let’s now look at the top portion of your data. 

    Id Task Type Freq 
3 1 A 2
3 1 B 3
3 2 A 3
3 2 B 0

    This is why it is. There are two codes in Freq for each combination of Id & Type (for Id 3 2 & 3 for type A & Type 0), while only one can be put in this spot in wide dataframe for each value of type. dcast will combine these values into one value. length is the default aggregation function. However, you can specify other functions such as sum, mean or sd by using fun.aggregate.

    fun.aggregate = sum, for example, gives you: 

     Id A B
1 3 5 3
2 4 4 6

    There is no warning, dcast is being told exactly what to do when more than one value is present: return the total of all values.

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