rowMeans(): Calculating the Mean of rows of a Data Frame in R

The rowMeans() is a built-in, highly vectorized function in R that computes the arithmetic mean of values in each row of a numeric matrix or a data frame (with numeric-like columns).

Ensure that the object passed to rowMeans() is numeric or can be coerced to numeric. If it contains non-numeric columns, it will throw an error.

Also, the input should have at least two dimensions. For example, if it is a vector, it will throw the error as well.

In the above figure, we find the mean of each row.

Mean of the first row: 1 + 4 + 7 / 3 = 12/3 = 4

Mean of the second row: 2 + 5 + 8 / 3 = 15 / 3 = 5

Mean of the third row: 3 + 6 + 9 / 3 = 18 / 3 = 6

Here is the program:

# Creating a data frame
df <- data.frame(
  row1 = c(1, 2, 3),
  row2 = c(4, 5, 6),
  row3 = c(7, 8, 9)
)
print(df)

# Calculating the mean of every row using rowMeans() function
rowMeans(df)

# Output: [1] 4  5  6

Syntax

rowMeans(x, na.rm = FALSE,  dims = 1)

Parameters

Handling NA values

As you can see from the figure, we get the output NA for every row because every row contains NA values.

# Data frame with NA values
df <- data.frame(
  col1 = c(NA, 2, 3),
  col2 = c(4, NA, 6),
  col3 = c(7, 8, NA)
)

rowMeans(df)

# Output: [1] NA NA NA

You can exclude NA values while calculating the mean of every row by passing the na.rm = TRUE parameter.

# Create a data frame.
df <- data.frame(
  col1 = c(NA, 2, 3),
  col2 = c(4, NA, 6),
  col3 = c(7, 8, NA)
)

# Calculate the mean of data frame
rowMeans(df, na.rm = TRUE)

# Output: [1] 5.5   5.0   4.5

Calculating the mean of specific rows

To calculate the mean of specific rows, you need to use row indexing and pass their index to the method.

df <- data.frame(
  col1 = c(1, 2, 3),
  col2 = c(4, 5, 6),
  col3 = c(7, 8, 9)
)

rowMeans(df[c(1, 3), ])

# Output:
# 1    3
# 4    6

For row 1, the mean is 4, and for row 2, the mean is 6.

Working with a matrix

Matrix has rows and columns, and it works the same as a data frame because if you create a matrix with the same values as the above data frame, the output will be the same.

# Create a matrix.
mat <- matrix(1:9, 3, 3)

print(mat)

# Calculate the mean of each row.
cat("The mean of every row of matrix: ", rowMeans(mat), "\n")

# Output: The mean of every row of matrix: 4   5   6

Matrix with a single row

If your input is a matrix with a single row, the output will have only the row’s mean.

single_row <- matrix(5:8, nrow = 1)

print(single_row)

rowMeans(single_row)

# Output: [1] 6.5

Empty input

If your input is an empty data frame or matrix, the output will be numeric(0).

empty_mat <- matrix(numeric(0), nrow = 0, ncol = 2)

rowMeans(empty_mat)

# Output: numeric(0)

That’s it!