Often you may want to find the index of a specific element in a vector in R.

For example, you may want to know which index position in a vector contains the value 5.

Or, you may wish to know which index positions(s) in a vector contain the value 3 *or* 5.

You can use the following methods to find the index of specific elements in a vector in R:

**Method 1: Find Index of First Occurrence of Element in Vector**

which(4 == my_vector)[[1]]

This particular example will return the first index position of the first occurrence of the value **4** in the vector named **my_vector**.

By using the **[[1]]** notation, we are able to extract only the first occurrence of the value **4** if it happens to occur multiple times in the vector.

**Method 2: Find Index of All Occurrences of Element in Vector**

which(4 == my_vector)

This particular example will return the index position of all occurrences of the value **4** in the vector named **my_vector**.

**Method 3: Find Index of First Occurrence of Multiple Elements in Vector**

match(c(4,10), my_vector)

This particular example will return the index position of the first occurrence of both the value **4** and the value **10** in the vector named **my_vector**.

**Method 4: Find Index of All Occurrences of Multiple Elements in Vector**

which(my_vector %in% c(4,10))

This particular example will return index position of all occurrences of the value **4** and the value **10** in the vector named **my_vector**.

The following examples show how to use each of these methods in practice.

**Example 1: Find Index of First Occurrence of Element in Vector**

We can use the following syntax to define a vector named **my_vector** and then find the index position of the first occurrence of the value **4** in the vector:

#create vector of values my_vector <- c(3, 4, 4, 6, 2, 10, 15, 12, 4, 7, 10, 15) #find index position of first occurrence of the value 4 which(4 == my_vector)[[1]] [1] 2

This tells us that the first occurrence of the value **4** in the vector occurs in index position **2**.

**Example 2: Find Index of All Occurrences of Element in Vector**

We can use the following syntax to define a vector named **my_vector** and then find the index position of all occurrences of the value **4** in the vector:

#create vector of values my_vector <- c(3, 4, 4, 6, 2, 10, 15, 12, 4, 7, 10, 15) #find index position of all occurrences of the value 4 which(4 == my_vector) [1] 2 3 9

This tells us that the value **4** occurs in in index positions **2**, **3** and **9** in the vector.

**Example 3: Find Index of First Occurrence of Multiple Elements in Vector**

We can use the following syntax to define a vector named **my_vector** and then find the index position of the first occurrence of the value **4** and the value **10** in the vector:

#create vector of values my_vector <- c(3, 4, 4, 6, 2, 10, 15, 12, 4, 7, 10, 15) #find index position of first occurrence of 4 and 10 match(c(4,10), my_vector) [1] 2 6

This tells us that the value **4** first occurs in index position **2** and the value **10** first occurs in index position **6**.

**Example 4: Find Index of First Occurrence of Multiple Elements in Vector**

We can use the following syntax to define a vector named **my_vector** and then find the index positions of all occurrences of the value **4** and the value **10** in the vector:

#create vector of values my_vector <- c(3, 4, 4, 6, 2, 10, 15, 12, 4, 7, 10, 15) #find index positions of all occurrences of 4 and 10 which(my_vector %in% c(4,10)) [1] 2 3 6 9 11

This tells us that the values **4** and **10** occur in index positions **2**, **3**, **6**, **9** and **11** in the vector.

**Additional Resources**

The following tutorials explain how to perform other common tasks in R:

How to Sort a Matrix in R

How to Remove NA from Matrix in R

How to Convert Data Frame to Matrix in R

How to Convert a Table to a Matrix in R