How to receive differences of intersecting SpatialLines in R? - r

Question

How to receive a list of SpatialLines of the differences of the intersecting SpatialLines only?
create SpatialLines:
#from the sp vignette:
l1 = cbind(c(1,2,3,4),c(3,2,2,4))
rownames(l1) = letters[1:4]
l2 = cbind(c(2,2,3,3),c(3,2,2,5))
rownames(l2) = letters[1:4]
l3 = cbind(c(1,2,3,4),c(1,2,2,1))
rownames(l3) = letters[1:4]
Sl1 = Line(l1)
Sl2 = Line(l2)
Sl3 = Line(l3)
Ll1 = Lines(list(Sl1), ID="a")
Ll2 = Lines(list(Sl2), ID="b")
Ll3 = Lines(list(Sl3), ID="c")
Sl = SpatialLines(list(Ll1,Ll2,Ll3))
resulting SpatialLines ("Sl") show intersections and differences.
Receiving the differences of all SpatialLines of the list can be achieved like this:
C = combn(1:length(Sl),2)
C2 = cbind(C,C[2:1,])
MyDiffs = apply(C2, 2, function(x){gDifference(Sl[x[1]], Sl[x[2]])})
see spacedman´s answer to this question
Looking for the differences of the intersecting SpatialLines only.
I was thinking about something like if the condition gIntersect=TRUE then apply gDifference(). However, I can´t find a way to do that in R.
Maybe there´s a smarter solution...
Edit:
The answer of bogdata works, but all differences appear twice.
Manipulating the matrix in a way that the lower triangular part get removed led to the result that some doubled differences are kept while others get removed.
library("reshape2")
# compute intersection matrix by ID
intersections <- gIntersects(Sl, byid=TRUE)
# set lower triangular part of matrix NA
intersections[lower.tri(intersections, diag = TRUE)] <- NA
# melt matrix into edge list (+remove NA)
intersections <- melt(intersections, na.rm=TRUE)
# compute differences
MyDiffs = apply(intersections, 1, function(x){gDifference(Sl[x[1]], Sl[x[2]])})
Any suggestions?

Answer 1

Just use gIntersects with byid=T and the melt function in reshape2:
library("reshape2")
# compute intersection matrix by ID
intersections <- gIntersects(Sl, byid=T)
# melt matrix into edge list
intersections <- melt(intersections)
# keep only intersecting lines, remove diagonals
intersections <- subset(intersections, Var1 != Var2 & value)
# compute differences
MyDiffs = apply(intersections, 1, function(x){gDifference(Sl[x[1]], Sl[x[2]])})