which()

which(x, arr.ind = FALSE, useNames = TRUE)

which() is a base R function that returns the indices of elements in a vector or array that are TRUE. It is essential for extracting positions of matching elements, making it a fundamental tool for data manipulation and conditional operations.

Syntax

which(x)
which(x, arr.ind = FALSE, useNames = TRUE)

Parameters

Parameter	Type	Default	Description
x	logical	required	A logical vector or array
arr.ind	logical	FALSE	If TRUE, return array indices for matrices
useNames	TRUE	logical	If TRUE, preserve names in the result

Examples

Basic usage

x <- c(5, 12, 7, 8, 3)

# Find which elements are greater than 7
which(x > 7)
# [1] 2 4

Finding NA positions

y <- c(1, NA, 3, NA, 5)

# Find positions of NA values
which(is.na(y))
# [1] 2 4

Working with character vectors

words <- c("apple", "banana", "cherry", "apple")

# Find positions of "apple"
which(words == "apple")
# [1] 1 4

Using with data frames

df <- data.frame(name = c("Alice", "Bob", "Charlie"), age = c(25, 30, 35))

# Find rows where age > 28
which(df$age > 28)
# [1] 2 3

Common Patterns

Combining with subsetting

values <- c(10, 20, 30, 40, 50)

# Get indices first, then subset
idx <- which(values > 25)
values[idx]
# [1] 30 40 50

Finding first or last match

z <- c(FALSE, FALSE, TRUE, TRUE, FALSE)

# First TRUE position
which(z)[1]
# [1] 3

# Last TRUE position
which(z)[length(which(z))]
# [1] 4

Using with which.min and which.max

numbers <- c(5, 2, 8, 1, 9)

# Find position of minimum
which.min(numbers)
# [1] 4

# Find position of maximum
which.max(numbers)
# [1] 5

See Also

• base::which.max
• base::match