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which() Function in R

  • Last Updated : 23 Aug, 2021

which() function in R Programming Language is used to return the position of the specified values in the logical vector.

Syntax: which(x, arr.ind, useNames)

Parameters: This function accepts some parameters which are illustrated below:

  • X: This is the specified input logical vector
  • Arr.ind: This parameter returns the array indices if x is an array.
  • useNames: This parameter says the dimension names of an array.

Return value: This function returns the position of the specified values in the logical vector.

Example 1: Which() function applying to the alphabet

In the below example, which() function returns the alphabetical position of the specified letter. For example, a is the first letter in the alphabet sequence that’s why 1 is returned and z is the last letter in the sequence so 26 is returned.



R




# R program to illustrate
# which() function
  
# Calling the which function
# to return alphabetical position
# of the given alphabet
which(letters == "a")
which(letters == "d")
which(letters == "z")
which(letters == "p")
which(letters == "g")

Output :

[1] 1
[1] 4
[1] 26
[1] 16
[1] 7

Example 2:  which() function with vectors

In the below example, the position of some elements of the specified vector is being returned which the help of which() function.

R




# R program to illustrate
# which() function
  
# Creating a vector of some elements
vector <- c(3, 5, 1, 6, 12, 4)
  
# Getting the position of element 12
# in the above vector
which(vector == 12)
  
# Getting the position of element 1
# in the above vector
which(vector == 1)
  
# Getting the position of element 6
# in the above vector
which(vector == 6)
  
# Getting the position of elements
# those are greater than 5
which(vector > 5)

Output:

[1] 5
[1] 3
[1] 4
[1] 4 5

Example 3: which() function with dataframe

 In the below example, which() function is used to find the columns in a data frame with numeric values.

An Iris data set is used as a data frame that contains 4 columns for numerical values and 1 column for categorical values i.e., Species. The which() function find the columns name from the data set that contain numeric values.

R






# Considering “Iris” dataset
data_set <- datasets::iris
  
# Printing the Iris dataset values
# along with its 5 columns out of which
# 4 columns are numerical and 1 is categorical
# (Species)
head(data_set)
  
# Calling the which() function over
# the above specified data set that 
# returns the columns with numeric values
Result <- which(sapply(data_set, is.numeric))
  
# Printing the columns with numeric values
colnames(data_set)[Result]

Output:

Example 4: which() function with the matrix

In the below example, which() function is used to find the position of an element in the specified matrix.

Here the position of value 2 in the specified matrix is being calculated.

R




# Creating a matrix of 3 columns and 4 rows
Matrix <- matrix(rep(c(1, 2, 3), 4), nrow = 4)
  
# Printing the entire matrix with its values
Matrix
  
# Calling the which() function to find the 
# position of value 2 in the above matrix
which(Matrix == 2, arr.ind = T)

Output:




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