Often you may want to sum the values in a matrix in R in a specific way.

Fortunately this is easy to do and you can use the following methods to do so:

Method 1: Sum All Values in Matrix

#sum all values in matrix
sum(my_matrix)

This particular example will sum all of the values in the entire matrix of the matrix named my_matrix.

Method 2: Sum Values in Each Column of Matrix

#sum values in each column of matrix
colSums(my_matrix)

This particular example will return the sum of each column of the matrix named my_matrix.

Method 3: Sum Values in Each Row of Matrix

#sum values in each row of matrix
rowSums(my_matrix)

This particular example will return the sum of each row of the matrix named my_matrix.

The following examples show how to use each of these methods in practice with the following matrix in R named my_matrix:

#create matrix
my_matrix <- matrix(1:20, ncol=5)

#specify column names for matrix
colnames(my_matrix) <- c('A', 'B', 'C', 'D', 'E')

#view matrix
my_matrix

     A B  C  D  E
[1,] 1 5  9 13 17
[2,] 2 6 10 14 18
[3,] 3 7 11 15 19
[4,] 4 8 12 16 20

Example 1: Sum All Values in Matrix

We can use the following syntax to sum all of the values in the entire matrix:

#sum all values in matrix
sum(my_matrix)

     A B  D  E
[1,] 1 5 13 17
[2,] 2 6 14 18
[3,] 3 7 15 19
[4,] 4 8 16 20

This returns a value of 210, which represents the sum of all values in the entire matrix.

Example 2: Sum Values in Each Column of Matrix

We can use the following syntax to calculate the sum of values in each column of the matrix:

#sum values in each column of matrix
colSums(my_matrix)

A  B  C  D  E 
10 26 42 58 74

This returns the sum of each column in the matrix.

For example, we can see:

• The sum of values in column A is 10.
• The sum of values in column B is 26.
• The sum of values in column C is 42.
• The sum of values in column D is 58.
• The sum of values in column E is 74.

Example 3: Sum Values in Each Row of Matrix

We can use the following syntax to calculate the sum of values in each row of the matrix:

#sum values in each row of matrix
rowSums(my_matrix)

[1] 45 50 55 60

This returns the sum of each row in the matrix.

For example, we can see:

• The sum of values in row 1 is 45.
• The sum of values in row 2 is 50.
• The sum of values in row 3 is 55.
• The sum of values in row 4 is 60.

Note that we can also use the cbind function to bind these row sums to the end of the matrix if we’d like:

#sum values in each row of matrix and bind as new column to matrix
cbind(my_matrix, rowSums(my_matrix))

     A B  C  D  E   
[1,] 1 5  9 13 17 45
[2,] 2 6 10 14 18 50
[3,] 3 7 11 15 19 55
[4,] 4 8 12 16 20 60

The row sums now appear as the last column in the matrix.

Note: The cbind function is used to “column-bind” together objects in R.

Additional Resources

The following tutorials explain how to perform other common tasks in R:

How to Sort a Matrix in R
How to Remove NA from Matrix in R
How to Convert Data Frame to Matrix in R
How to Convert a Table to a Matrix in R