R help - Oct 2007 - how to know whether Vector B is a subset of Vector A

Hi,

I'm quite fresh to R, and a layman of English as well. I hope I can make you
understood.

Now I have two vectors A and B. Is there any quick way to know whether B is
a subset of A? and If B is a subset of A, can I know easily which elements
in A (the index of A) equals to B's elements accordingly?

For example,
> a<-1:20
> b=c(2,5,9,7,4,8,3)
>
Question 1: we know b is a subset of a, but how does R know that?

Question 2: since we know b is a subset of a, then which a's elements equals
to b? I do it like this:

test=0
> for (i in 1:length(b)) test[i]=which(a==b[i])
> test
>
Is there any easier way to know these indexes?

Thanks a lot for you helps in advance

-- 
Samuel Wu

	[[alternative HTML version deleted]]

Samuel wrote:
> Hi,
>
> I'm quite fresh to R, and a layman of English as well. I hope I can
make you
> understood.
>
> Now I have two vectors A and B. Is there any quick way to know whether B is
> a subset of A? and If B is a subset of A, can I know easily which elements
> in A (the index of A) equals to B's elements accordingly?
>
> For example,
>
>   
>> a<-1:20
>> b=c(2,5,9,7,4,8,3)
>>
>>     
>
> Question 1: we know b is a subset of a, but how does R know that?
>
> Question 2: since we know b is a subset of a, then which a's elements
equals
> to b? I do it like this:
>
> test=0
>   
>> for (i in 1:length(b)) test[i]=which(a==b[i])
>> test
>>
>>     
>
> Is there any easier way to know these indexes?
>
> Thanks a lot for you helps in advance
>
>   
> a<-1:20
> b<-c(2,5,9,7,4,8,3)
> !length(setdiff(b,a))
[1] TRUE
> a %in% b
 [1] FALSE  TRUE  TRUE  TRUE  TRUE FALSE  TRUE  TRUE  TRUE FALSE FALSE FALSE
[13] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
> which(a %in% b)
[1] 2 3 4 5 7 8 9


-- 
   O__  ---- Peter Dalgaard             ?ster Farimagsgade 5, Entr.B
  c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
 (*) \(*) -- University of Copenhagen   Denmark          Ph:  (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)                  FAX: (+45) 35327907

On Fri, 2007-10-12 at 18:47 +0800, Samuel wrote:
> Hi,
> 
> I'm quite fresh to R, and a layman of English as well. I hope I can
make you
> understood.
> 
> Now I have two vectors A and B. Is there any quick way to know whether B is
> a subset of A? and If B is a subset of A, can I know easily which elements
> in A (the index of A) equals to B's elements accordingly?
> 
> For example,
> 
> > a<-1:20
> > b=c(2,5,9,7,4,8,3)
> >
> 
> Question 1: we know b is a subset of a, but how does R know that?
> 
> Question 2: since we know b is a subset of a, then which a's elements
equals
> to b? I do it like this:
> 
> test=0
> > for (i in 1:length(b)) test[i]=which(a==b[i])
> > test
> >
> 
> Is there any easier way to know these indexes?
> 
> Thanks a lot for you helps in advance

You can use %in% to determine whether or not the elements of 'b' are in
'a':

a <- 1:20
b <- c(2, 5, 9, 7, 4, 8, 3)
 
> b %in% a
[1] TRUE TRUE TRUE TRUE TRUE TRUE TRUE

To get the indices of b's elements in 'a', you can do:
> which(a %in% b)
[1] 2 3 4 5 7 8 9


If you want to know whether or not ALL of the elements of 'b' are in
'a', you add the use of all() to the first example:
> all(b %in% a)
[1] TRUE

See ?"%in%" and ?all

HTH,

Marc Schwartz