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How to find Index of Element in Vector in R ?

  • Last Updated : 16 May, 2021

In this article, we will discuss How to find the index of element in vector in the R programming language. We can find the index of the element by the following functions –

  • which()
  • match()

Method 1: by using which()

which() function basically returns the vector of indexes that satisfies the argument given in the which() function.

Syntax: which(condition)

Example 1: We first create the vector of values (0,1,2,3,4,5,6,7,8,9), and then we try to get the index value of the element “5” with the help of which() function.



So, the condition which we used to get the index of element 5 is:

which(v == 5)

Code:

R




# vector created
v <- c(0, 1, 2, 3, 4,
       5, 6, 7, 8, 9)
  
# which function is used 
# to get the index
which(v == 5)

Output:

6

Example 2: In this example, we will try to get the index of the element which is repeated.

So, we will create a vector of repeated elements (1,2,4,1,6,2,4,4,6) now we try to find the index of 4 and which function returns a function that holds every index value of 4 elements. In our case that is 3 7 8

R




# vector created
v <- c(1, 2, 4, 1, 6, 2, 4, 4, 6)
  
# which function is used to
# get every index of given element
which(v == 4)

Output:



3 7 8

Example 3: In this example, we will try to get the first index of the element which is repeated.

So, to do this we will just use the [1] to basically get the first element of the vector created by the which() function. In our case, we will try to get the first index of element 4

R




# vector created
v <- c(1, 2, 4, 1, 6, 2, 4, 4, 6)
  
# which function is used to get 
# every index of given element
# [1] is used to get the first
# element of which() vector
which(v == 4)[1]

Output:

3

Example 4: In this example, we will try to get the index of the multiple elements using which() function.

So, to do this we will just give the values as an argument to the which() function. In our case, we will try to get the index of elements 4 and 6. So, they together used as a vector with the %in% and which function returns the vector of the index of both the elements.

R




# vector created
v <- c(1, 2, 4, 1, 6,
       2, 4, 4, 6)
  
# which is used to get
# the index of multiple elements
which(v %in% c(4, 6))

Output:

[1] 3 5 7 8 9

Method 2: by using match()

match() function basically returns the vector of indexes that satisfies the argument given in the match() function.



Syntax: match(element, vector_name)

Example 1: In our case, we first create the vector of values (0,1,2,3,4,5,6,7,8,9), and then we try to get the index value of the element “5” with the help of the match() function. 

So, the condition which we used to get the index of element 5 is:

match(5,v)

Code:

R




# vector created
v <- c(0, 1, 2, 3, 4,
       5, 6, 7, 8, 9)
  
# which function is
# used to get the index
match( 5 , v )

Output:

6

Example 2: In this example, we will try to get the first index of the multiple elements using the match() function.

So, to do this we will just give the values as an argument to the match() function. In our case, we will try to get the index of elements 4 and 6. So, they together used as a vector which function returns the vector of the first index of both the elements.

R




# vector created
v <- c( 1, 2, 4, 1,
       6, 2, 4, 4, 6)
  
# match() function is used
# to get the first index of 
# elements
match(c( 4, 6), v)

Output: 

3 5



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