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

To find the row mean 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 mean function to find the row mean of columns having same name.

## Example

#### Create the matrix

Let’s create a matrix as shown below −

M<-matrix(rpois(100,5),ncol=4) colnames(M)<-c("A","B","B","A") M

## Output

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

A B B A [1,] 4 10 2 5 [2,] 5 1 5 7 [3,] 6 4 5 2 [4,] 7 4 3 6 [5,] 2 7 6 7 [6,] 3 6 4 7 [7,] 7 7 3 2 [8,] 8 2 4 2 [9,] 7 5 7 7 [10,] 3 7 5 3 [11,] 3 10 3 1 [12,] 6 8 6 2 [13,] 6 5 2 7 [14,] 5 3 3 4 [15,] 4 4 1 11 [16,] 10 7 9 6 [17,] 2 6 4 6 [18,] 8 6 11 10 [19,] 3 4 3 5 [20,] 6 4 2 2 [21,] 7 3 4 5 [22,] 4 4 4 9 [23,] 2 4 3 6 [24,] 0 3 3 5 [25,] 2 4 3 2

**Find the row mean of columns having same name**

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

M<-matrix(rpois(100,5),ncol=4) colnames(M)<-c("A","B","B","A") t(apply(M,1, function(x) tapply(x,colnames(M),mean)))

## Output

A B [1,] 4.5 6.0 [2,] 6.0 3.0 [3,] 4.0 4.5 [4,] 6.5 3.5 [5,] 4.5 6.5 [6,] 5.0 5.0 [7,] 4.5 5.0 [8,] 5.0 3.0 [9,] 7.0 6.0 [10,] 3.0 6.0 [11,] 2.0 6.5 [12,] 4.0 7.0 [13,] 6.5 3.5 [14,] 4.5 3.0 [15,] 7.5 2.5 [16,] 8.0 8.0 [17,] 4.0 5.0 [18,] 9.0 8.5 [19,] 4.0 3.5 [20,] 4.0 3.0 [21,] 6.0 3.5 [22,] 6.5 4.0 [23,] 4.0 3.5 [24,] 2.5 3.0 [25,] 2.0 3.5

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