match() {base}

match() function returns a vector of the positions of the matches of the first argument in the second argument.

match(x, table, nomatch = NA_integer_, incomparables = NULL)

The parameters are:

 -x:vector to be matched

 -table: other vector the be matched against

 -nomatch: value to show when no match is found

 -incomparables: vector of values that can not be matched

x = c(10:20)
y = c(15:30)
x

##  [1] 10 11 12 13 14 15 16 17 18 19 20

y

##  [1] 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

match(x,y)

##  [1] NA NA NA NA NA  1  2  3  4  5  6

The function returns a vector with a length equals to x, and each value in x has to be match in a position in the vector y. The function returns a vector of the positions of each value of x in the vector y. The vector shows 5 values of NA since 10:15 are not found in y, and then shows the values of the positions of the values 15:20 in y.

match(y,x)

##  [1]  6  7  8  9 10 11 NA NA NA NA NA NA NA NA NA NA

It is showed a vector of the positions of each value of y in the vector x. Only the first 6 values of y are found in the vector x, the numbers returned by the function are the positions of the values in the vector x. For example, the first value in y is match with the sixth value in x.

Instead of returning NA when the values don’t match we can give a specific value for the values that have no match:

match(x,y, nomatch = 0)

##  [1] 0 0 0 0 0 1 2 3 4 5 6

Using the parameter incomparables we can give a vector of the values that we don’t want to be matched:

x

##  [1] 10 11 12 13 14 15 16 17 18 19 20

y

##  [1] 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

#if we don't want to match the values 15,16,17:
match(x,y, incomparables = c(15,16,17)) 

##  [1] NA NA NA NA NA NA NA NA  4  5  6

#if we don't want to match the values 15,16,17:
match(x,y, nomatch = 0, incomparables = c(15,16,17)) 

##  [1] 0 0 0 0 0 0 0 0 4 5 6

#if we don't want to match the values 18,19,20:
match(x,y, nomatch = 0, incomparables = c(18,19,20)) 

##  [1] 0 0 0 0 0 1 2 3 0 0 0

Related to this functions we have the function: %in%, which instead of returning the positions that match the first with the second vector, returns a logical vector indicating if there is a match or not.

match(x,y)

##  [1] NA NA NA NA NA  1  2  3  4  5  6

x %in% y

##  [1] FALSE FALSE FALSE FALSE FALSE  TRUE  TRUE  TRUE  TRUE  TRUE  TRUE

match(y,x)

##  [1]  6  7  8  9 10 11 NA NA NA NA NA NA NA NA NA NA

y %in% x

##  [1]  TRUE  TRUE  TRUE  TRUE  TRUE  TRUE FALSE FALSE FALSE FALSE FALSE
## [12] FALSE FALSE FALSE FALSE FALSE