Return True Indices of a Logical Object in R Programming – which() Function

which() function in R Language is used to return the indices of the object which return true for the logical operation passed as argument.

Syntax: which(x, arr.ind)

Parameters:
x: logical object
arr.ind: Boolean value to display indices

Example 1:

# R program to illustrate
# the use of which() function

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

# Calling which() function
which(x %% 2 == 0, arr.ind = TRUE)

Output:

     [, 1] [, 2] [, 3]
[1, ]    1    4    7
[2, ]    2    5    8
[3, ]    3    6    9
     row col
[1, ]   2   1
[2, ]   1   2
[3, ]   3   2
[4, ]   2   3

Here, in the above code, the which() function returns the indices of all the even numbers present in the matrix.

Example 2:

# R program to illustrate
# the use of which() function

# Using predefined dataset
BOD

# Calling which() function
which(BOD$demand == 19, arr.ind = TRUE)

Output:

  Time demand
1    1    8.3
2    2   10.3
3    3   19.0
4    4   16.0
5    5   15.6
6    7   19.8
[1] 3