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How to find the number of unique values for each column in data.table object in R?
To find the number of unique values for each column in data.table object, we can use uniqueN function along with lapply. For example, if we have a data.table object called DT that contains five columns each containing some duplicate values then the number of unique values in each of these columns can be found by using DT[,lapply(.SD,uniqueN)].
Example
Consider the below data.table object −
x1<-rpois(20,2) x2<-rpois(20,5) DT1<-data.table(x1,x2) DT1
Output
x1 x2 1: 3 11 2: 1 10 3: 3 5 4: 0 1 5: 0 7 6: 2 5 7: 2 4 8: 3 6 9: 2 4 10: 4 7 11: 1 6 12: 0 7 13: 2 5 14: 3 2 15: 2 2 16: 1 9 17: 1 2 18: 1 7 19: 2 7 20: 4 5
Finding the number of unique values in each column of DT1 −
Example
DT1[,lapply(.SD,uniqueN)]
Output
x1 x2 1: 5 9
Example
y1<-round(rnorm(20),1) y2<-round(rnorm(20),1) DT2<-data.table(y1,y2) DT2
Output
y1 y2 1: 1.0 -0.5 2: -1.1 0.5 3: 0.0 0.4 4: -1.0 0.1 5: -1.0 -1.4 6: 0.4 -0.7 7: 0.6 -0.2 8: 0.0 -0.3 9: 0.0 0.6 10: -0.2 -0.2 11: -0.2 1.8 12: 0.8 0.7 13: 0.5 0.6 14: -1.6 -0.4 15: 0.1 -0.2 16: 0.6 -1.3 17: 0.0 0.8 18: 1.4 -0.6 19: 0.5 -0.2 20: 0.9 -0.7
Finding the number of unique values in each column of DT2 −
Example
DT2[,lapply(.SD,uniqueN)]
Output
y1 y2 1: 13 15
Example
z1<-round(runif(20,2,5),1) z2<-round(runif(20,2,5),1) z3<-round(runif(20,2,5),1) z4<-round(runif(20,2,5),1) DT3<-data.table(z1,z2,z3,z4) DT3
Output
z1 z2 z3 z4 1: 3.3 3.2 4.6 3.4 2: 4.1 4.4 2.9 2.7 3: 2.3 4.4 4.6 3.6 4: 5.0 3.6 2.6 2.6 5: 4.2 4.1 2.8 4.2 6: 3.7 4.4 2.9 3.1 7: 3.1 3.1 2.0 4.6 8: 4.7 2.7 3.5 5.0 9: 2.1 3.0 4.0 3.7 10: 2.3 2.5 3.2 2.7 11: 4.1 2.1 2.7 2.3 12: 2.4 2.7 4.2 3.2 13: 4.4 3.7 3.5 4.3 14: 3.7 3.1 3.3 3.3 15: 4.3 4.1 4.4 3.4 16: 3.9 2.7 2.9 3.6 17: 2.1 3.6 2.2 4.1 18: 3.0 3.6 2.3 3.4 19: 4.1 3.3 4.3 4.5 20: 2.4 3.4 3.7 3.6
Finding the number of unique values in each column of DT3 −
Example
DT3[,lapply(.SD,uniqueN)]
Output
z1 z2 z3 z4 1: 14 12 16 15
Updated on: 16-Mar-2021
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