CODEWITHSUNDEEP
rsapplyeuclidean-distance
Sid
Sid

Reputation: 65

sapply in R over a vector with individual elements of another vector

I have two vectors x and cen. Say the length of x is 10 and the length of cen is 3. I want to find the Euclidean distance between x and individual points of cen.

The Euclidean distance function is defined as : euc.dist <- function(x1, x2) sqrt(sum((x1 - x2) ^ 2))

I saw that sapply works with the negation "-" operator like so:

> x 
 [1] 23  4 65  8  9 23 90 76 55  7
> cen
 [1] 23  4 65
> sapply(x,"-",cen)
     [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
[1,]    0  -19   42  -15  -14    0   67   53   32   -16
[2,]   19    0   61    4    5   19   86   72   51     3
[3,]  -42  -61    0  -57  -56  -42   25   11  -10   -58

Here each each value within the cen vector operates over whole x vector.

But when I try to use the euc.dist function instead of "-" then I only get a single vector as an output instead of a 3x10 matrix.

> sapply(x,"euc.dist",cen)
 [1]  46.09772  63.89053  74.06079  59.07622  57.93962  46.09772 111.84811  90.07774  61.03278  60.24118

Any reasons as to why this is happening? Do I need to define the euc.dist function in a different manner? Any help would be appreciated. Thanks in advance.

Upvotes: 1

Views: 153

Answers (2)

Marat Talipov
Marat Talipov

Reputation: 13314

Your euc.dist function is not vectorize, as could be easily tested by:

euc.dist(x[1],cen)
[1] 46.09772

whereas what you probably want is:

> Vectorize(euc.dist)(x[1],cen)
[1]  0 19 42

(Use ?Vectorize for the details).

So, in your case a simple solution would be:

> sapply(x,Vectorize(euc.dist),cen)

     [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
[1,]    0   19   42   15   14    0   67   53   32    16
[2,]   19    0   61    4    5   19   86   72   51     3
[3,]   42   61    0   57   56   42   25   11   10    58

Upvotes: 2

Shape
Shape

Reputation: 2952

how about this? since you want all factor combinations

datas <- expand.grid(x = x,cen = cen)
datas$euclid = apply(datas,1,function(rowval) euc.dist(rowval[1], rowval[2]))

Upvotes: 1

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