How to find the row total of columns having same name in R matrix?

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To find the row total of columns having same name in R matrix, we can follow the below steps −

First of all, create a matrix with some columns having same name.
Then, use tapply along with colnames and sum function to find the row total of columns having same name.

Example

Create the matrix

Let’s create a matrix as shown below −

M<-matrix(rpois(100,10),ncol=4)
colnames(M)lt;-c("C1","C1","C2","C2")
M

Output

On executing, the above script generates the below output(this output will vary on your system due to randomization) −

      C1 C1 C2 C2
[1,]  12 15  6 16
[2,]   2 12 13 10
[3,]   8 11 10 11
[4,]   9 11  6  9
[5,]  11 12  4 10
[6,]  12 12  8 12
[7,]  14  8  5  5
[8,]   9 15  9 13
[9,]   8 14  8  6
[10,]  8 14 16  7
[11,]  8 12 18 18
[12,] 18  8 15 12
[13,]  9 13 14 10
[14,]  8 11  8 10
[15,] 11 17 12  7
[16,]  8 13 12  6
[17,] 11 14  8 15
[18,] 12  6 10  6
[19,]  4  8 10  8
[20,] 17 13  7 16
[21,]  9  9 20  5
[22,] 12  8 13 14
[23,]  9 12 12  5
[24,]  7 14 13 13
[25,] 10  7  7 13

Find the row total of columns having same name

Using tapply along with colnames and sum function to find the row total of columns having same name in matrix M −

M<-matrix(rpois(100,10),ncol=4)
colnames(M)<-c("C1","C1","C2","C2")
t(apply(M,1, function(x) tapply(x,colnames(M),sum)))

Output

      C1 C2
[1,]  25 18
[2,]  21 20
[3,]  17 19
[4,]  25 14
[5,]  14 17
[6,]  27 20
[7,]  20 20
[8,]  23 23
[9,]  21 21
[10,] 16 19
[11,] 17 21
[12,] 29 18
[13,] 23 24
[14,] 21 19
[15,] 15 19
[16,] 18 17
[17,] 19 15
[18,] 22 16
[19,] 24 14
[20,] 23 25
[21,] 16 21
[22,] 24 20
[23,] 23 18
[24,] 20 23
[25,] 23 17