# [R] Any better way of optimizing time for calculating distances in the mentioned scenario??

Purna chander chanderbio at gmail.com
Mon Oct 8 09:25:26 CEST 2012

```Dear All,

I'm dealing with a case, where 'manhattan' distance of each of 100
vectors is calculated from 10000 other vectors. For achieving this,
following 4 scenarios are tested:

1) scenario 1:

> d<-matrix(nrow=nrow(v),ncol=nrow(x))
> for (i in 1:nrow(v)){
+ d[i,]<- sapply(1:nrow(x),function(z){dist(rbind(v[i,],x[z,]),method="manhattan")})
+ }
> print(d[1,1:10])

time taken for running the code is :
real	1m33.088s
user	1m32.287s
sys	0m0.036s

2) scenario2:

> v<-as.matrix(v)
> d<-matrix(nrow=nrow(v),ncol=nrow(x))
> for (i in 1:nrow(v)){
+ tmp_m<-matrix(rep(v[i,],nrow(x)),nrow=nrow(x),byrow=T)
+ d[i,]<- rowSums(abs(tmp_m - x))
+ }
> print(d[1,1:10])

time taken for running the code is:
real	0m0.882s
user	0m0.854s
sys	0m0.025s

3) scenario3:

> v<-as.matrix(v)
> d<-matrix(nrow=nrow(v),ncol=nrow(x))
> for (i in 1:nrow(v)){
+ d[i,]<-sapply(1:nrow(x),function(z){dist(rbind(v[i,],x[z,]),method="manhattan")})
+ }
> print(d[1,1:10])

time taken for running the code is:
real	1m3.817s
user	1m3.543s
sys	0m0.031s

4) scenario4:
> v<-as.matrix(v)
> d<-dist(rbind(v,x),method="manhattan")
> m<-as.matrix(d)
> m2<-m[1:nrow(v),(nrow(v)+1):nrow(x)]
> print(m2[1,1:10])

time taken for running the code:
real	0m0.445s
user	0m0.401s
sys	0m0.041s

Queries:
1) Though scenario 4 is optimum, this scenario failed when matrix 'v'
having more no. of rows. An error occurred while converting distance
object 'd' to a matrix 'm'.
For E.g: > m<-as.matrix(d)
the above command resulted in error: "Error: cannot allocate
vector of size 922.7 MB".

So, what can be done to convert a larger dist object into a matrix or
how allocation size can be increased?

2) Here I observed that 'dist()' function calculates the distances
across all vectors present in a given matrix or dataframe. Is it not
possible to calculate distances of specific vectors from other vectors
present in a matrix using 'dist()' function? Which means, suppose if a
matrix 'x' having 20 rows, is it not possible using 'dist()' to
calculate only distance of 1st row vector from other 19 vectors.

3) Any other ideas to optimize the problem i'm facing with.

Regards,
Purnachander

```