How to find Index of Element in Vector in R ?

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 createdv <- c(0, 1, 2, 3, 4,       5, 6, 7, 8, 9)  # which function is used # to get the indexwhich(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 createdv <- c(1, 2, 4, 1, 6, 2, 4, 4, 6)  # which function is used to# get every index of given elementwhich(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 createdv <- 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() vectorwhich(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 createdv <- c(1, 2, 4, 1, 6,       2, 4, 4, 6)  # which is used to get# the index of multiple elementswhich(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 createdv <- c(0, 1, 2, 3, 4,       5, 6, 7, 8, 9)  # which function is# used to get the indexmatch( 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 createdv <- c( 1, 2, 4, 1,       6, 2, 4, 4, 6)  # match() function is used# to get the first index of # elementsmatch(c( 4, 6), v)
Output: 
3 5
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