sweep() in R

Functions

Published

September 24, 2022

In this blog, I’m going to expain the functinality of a R function, sweep().

Introduction

sweep() is a R function which is used to perform arithmetic operations (eg: + or -) for data matrix by row or column.

Function definition

sweep(x, MARGIN, STATS, FUN = "+")

Definition of parameters

x: data matrix.
MARGIN: specifies typically whether the operation should be applied by row or by column. MARGIN = 1 operates by row; MARGIN = 2 operates by column.
STATS: Initialize the value that should be used for the operation (e.g. the value that should be used in addition or subtraction).
FUN: The arithmetic operation that should be carried out (e.g. + or -).

Examples

Initialize the data matrix

data_matrix <- matrix(0, nrow = 5, ncol = 3)                          
data_matrix

     [,1] [,2] [,3]
[1,]    0    0    0
[2,]    0    0    0
[3,]    0    0    0
[4,]    0    0    0
[5,]    0    0    0

Example 1: Add a value by row

data_example_1 <- sweep(x = data_matrix, MARGIN = 1, STATS = 3, FUN = "+") 
data_example_1

     [,1] [,2] [,3]
[1,]    3    3    3
[2,]    3    3    3
[3,]    3    3    3
[4,]    3    3    3
[5,]    3    3    3

In here, we want to add the value, 3 for each element of the matrix by row. Therefore, MARGIN parameter should be 1, STATS should be 3, and the FUN should be +.

Example 2: Substract a value by column

data_example_1 <- sweep(x = data_matrix, MARGIN = 2, STATS = -3, FUN = "-")
data_example_1

     [,1] [,2] [,3]
[1,]    3    3    3
[2,]    3    3    3
[3,]    3    3    3
[4,]    3    3    3
[5,]    3    3    3

In this example, the output is same as in Example 1, but the operation is different. That is, in here, the specific value, - is subtracted by each element of the matrix by column.

Let’s see more complex example to understand what is happening.

Example 3: Add set of values by column

data_example_3 <- sweep(x = data_matrix, MARGIN = 2, STATS = c(1, 3, 5), FUN = "+")
data_example_3

     [,1] [,2] [,3]
[1,]    1    3    5
[2,]    1    3    5
[3,]    1    3    5
[4,]    1    3    5
[5,]    1    3    5

This example shows how to perform column-wise operation for a matrix. In here, the value of the first element of STATS, 1 is added to all the elements of first column, the value of the second element of STATS, 3 is added to all the elements of second column, and so on.

What if the number of elements in STATS parameter is less than the number of columns in the matrix.

Example 4

data_example_4 <- sweep(x = data_matrix, MARGIN = 2, STATS = c(1, 3), FUN = "+")

Warning in sweep(x = data_matrix, MARGIN = 2, STATS = c(1, 3), FUN = "+"): STATS
does not recycle exactly across MARGIN

data_example_4

     [,1] [,2] [,3]
[1,]    1    3    1
[2,]    3    1    3
[3,]    1    3    1
[4,]    3    1    3
[5,]    1    3    1

Consider the warning message that we are getting. But still, we are receiving an output. However, the operation recycled across the end of each column.

Usually you should try to avoid this by specifying the length of STATS equal to the number of rows/columns.