Test if a vector contains a given element

问题:

How to check if a vector contains a given value?

回答1:

Both the match() (returns the first appearance) and %in% (returns a Boolean) functions are designed for this.

v <- c('a','b','c','e')

'b' %in% v
## returns TRUE

match('b',v)
## returns the first location of 'b', in this case: 2

回答2:

is.element() makes for more readable code, and is identical to %in%

v <- c('a','b','c','e')

is.element('b', v)
'b' %in% v
## both return TRUE

is.element('f', v)
'f' %in% v
## both return FALSE

subv <- c('a', 'f')
subv %in% v
## returns a vector TRUE FALSE
is.element(subv, v)
## returns a vector TRUE FALSE

回答3:

The any() function makes for readable code

> w <- c(1,2,3)
> any(w==1)
[1] TRUE

> v <- c('a','b','c')
> any(v=='b')
[1] TRUE

> any(v=='f')
[1] FALSE

回答4:

I will group the options based on output. Assume the following vector for all the examples.

v <- c('z', 'a','b','a','e')

For checking presence:

%in%

> 'a' %in% v
[1] TRUE

any()

> any('a'==v)
[1] TRUE

is.element()

> is.element('a', v)
[1] TRUE

For finding first occurance:

match()

> match('a', v)
[1] 2

For finding all occurances as vector of indices:

which()

> which('a' == v)
[1] 2 4

For finding all occurances as logical vector:

==

> 'a' == v
[1] FALSE  TRUE FALSE  TRUE FALSE

Edit: Removing grep() and grepl() from the list for reason mentioned in comments

回答5:

You can use the %in% operator:

vec <- c(1, 2, 3, 4, 5)
1 %in% vec # true
10 %in% vec # false

回答6:

Also to find the position of the element "which" can be used as

pop <- c(3,4,5,7,13)

which(pop==13)

and to find the elements which are not contained in the target vector, one may do this:

pop <- c(1,2,4,6,10)

Tset <- c(2,10,7)   # Target set

pop[which(!(pop%in%Tset))]

回答7:

I really like grep() and grepl() for this purpose.

grep() returns a vector of integers, which indicate where matches are.

yo <- c("a", "a", "b", "b", "c", "c")

grep("b", yo)
[1] 3 4

grepl() returns a logical vector, with "TRUE" at the location of matches.

yo <- c("a", "a", "b", "b", "c", "c")

grepl("b", yo)
[1] FALSE FALSE  TRUE  TRUE FALSE FALSE

These functions are case-sensitive.