How to find the groupwise common value for a data.table object?

To find the groupwise common value for a data.table object, we can use Reduce function with intersect function.

For example, if we have a data.table object called DT that contains a numerical column say Num and a categorical column say C where C exists at the first position then the groupwise common value can be found by using the command given below −

Reduce(intersect,DT[,.(list(unique(Num))),C]$V1)

Example

Consider the below data.table object −

Group<-sample(LETTERS[1:4],20,replace=TRUE) Rate<-rpois(20,1) library(data.table) DT1<-data.table(Group,Rate) DT1

output

The following dataframe is created −

   Group Rate 1:  A    4 2:  C    0 3:  D    0 4:  C    0 5:  A    3 6:  A    1 7:  D    1 8:  B    0 9:  A    1 10: A    0 11: C    3 12: B    2 13: B    1 14: C    2 15: D    3 16: B    1 17: A    0 18: C    1 19: A    1 20: C    1

In order to find the groupwise common value of Rate for all groups in Group column, add the following code to the above snippet −

Example

Reduce(intersect,DT1[,.(list(unique(Rate))),Group]$V1) 

Output

If you execute all the above given snippets as a single program, it generates the following Output −

[1] 1 0

Example

Consider the below data.table object −

Category<-sample(c("I","II","III"),20,replace=TRUE) Rank<-sample(1:3,20,replace=TRUE) DT2<-data.table(Category,Rank) DT2

output

The following dataframe is created −

  Category Rank 1:    I    1 2:  III    2 3:  III    3 4:    I    2 5:    I    3 6:  III    3 7:  III    2 8:  III    2 9:  III    2 10:  II    3 11: III    2 12:  II    1 13: III    1 14:  II    3 15:   I    1 16: III    2 17: III    1 18:  II    1 19:   I    1 20: III    3

To find the groupwise common value of Rank for all groups in Category column, add the following code to the above snippet −

Example

Reduce(intersect,DT2[,.(list(unique(Rank))),Category]$V1) 

Output

If you execute all the above given snippets as a single program, it generates the following Output −

[1] 1 3

Example

Consider the below data.table object −

Class<-sample(c("First","Second","Third"),20,replace=TRUE) Score<-sample(0:2,20,replace=TRUE) DT3<-data.table(Class,Score) DT3

output

The following dataframe is created −

    Class  Score 1:  First   2 2:  Second  0 3:  First   1 4:  Third   2 5:  Second  1 6:  Third   1 7:  Third   0 8:  Third   1 9:  Third   2 10: Third   1 11: Second  0 12: Third   0 13: First   2 14: First   1 15: First   1 16: First   0 17: Second  2 18: Second  2 19: First   0 20: Second  1

To find the groupwise common value of Score for all groups in Class column, add the following code to the above snippet −

Example

Reduce(intersect,DT3[,.(list(unique(Score))),Class]$V1) 

Output

If you execute all the above given snippets as a single program, it generates the following Output −

[1] 2 1 0