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

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

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