The which() function in R returns the position of elements in a logical vector that are TRUE.
This tutorial provides several examples of how to use this function in practice.
Example 1: Find Elements in a Vector
The following code shows how to find the position of all elements in a vector that are equal to 5:
#create data data <- c(1, 2, 2, 3, 4, 4, 4, 5, 5, 12) #find the position of all elements equal to 5 which(data == 5) [1] 8 9
We can see that the elements in positions 8 and 9 in the vector are equal to the value 5.
We can also find the position of all elements in a vector that are not equal to 5:
#find the position of all elements not equal to 5 which(data != 5) [1] 1 2 3 4 5 6 7 10
We can also find which elements are between two values or outside of two values:
#find the position of all elements with values between 2 and 4 which(data >= 2 & data <= 4) [1] 2 3 4 5 6 7 #find the position of all elements with values outside of 2 and 4 which(data < 2 | data > 4) [1] 1 8 9 10
Example 2: Count Occurrences in a Vector
The following code shows how to use the length() function to find the number of elements in a vector that are greater than some value:
#create data data <- c(1, 2, 2, 3, 4, 4, 4, 5, 5, 12) #find number of elements greater than 4 length(which(data > 4)) [1] 3
We can see that there are 3 elements in this vector with values greater than 4.
Example 3: Find Rows in a Data Frame
The following code shows how to return the row in a data frame that contains the max or min value in a certain column:
#create data frame
df <- data.frame(x = c(1, 2, 2, 3, 4, 5),
y = c(7, 7, 8, 9, 9, 9),
z = c('A', 'B', 'C', 'D', 'E', 'F'))
#view data frame
df
x y z
1 1 7 A
2 2 7 B
3 2 8 C
4 3 9 D
5 4 9 E
6 5 9 F
#return row that contains the max value in column x
df[which.max(df$x), ]
x y z
6 5 9 F
#return row that contains the min value in column x
df[which.min(df$x), ]
x y z
1 1 7 A
Example 4: Subset by Rows in a Data Frame
The following code shows how to subset a data frame by rows that meet a certain criteria:
#create data frame
df <- data.frame(x = c(1, 2, 2, 3, 4, 5),
y = c(7, 7, 8, 9, 9, 9),
z = c('A', 'B', 'C', 'D', 'E', 'F'))
#view data frame
df
x y z
1 1 7 A
2 2 7 B
3 2 8 C
4 3 9 D
5 4 9 E
6 5 9 F
#return subset of data frame where values in column y are greater than 8
df[which(df$y > 8), ]
x y z
4 3 9 D
5 4 9 E
6 5 9 F
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