calculate distance between each pair of coordinates in wide dataframe

Question

I want to calculate the distance between two linked set of spatial coordinates (program and admin in my fake dataset). The data are in a wide format, so both pairs of coordinates are in the same row.

library(sp)
set.seed(1)
n <- 100
program.id <- seq(1, n)
c1 <- cbind(runif(n, -90, 90), runif(n, -180, 180))
c2 <- cbind(runif(n, -90, 90), runif(n, -180, 180))
dat <- data.frame(cbind(program.id, c1, c2))
names(dat) <- c("program.id", "program.lat", "program.long", "admin.lat", "admin.long")
head(dat)
#       program.id program.lat program.long  admin.lat  admin.long
# 1              1   -42.20844     55.70061 -41.848523   62.536404
# 2              2   -23.01770    -52.84898 -50.643849 -145.851172
# 3              3    13.11361    -82.70635   3.023431   -2.665397
# 4              4    73.47740    177.36626 -41.588893  -13.841337
# 5              5   -53.69725     48.05758 -57.389701  -44.922049
# 6              6    71.71014   -103.24507   3.343705  176.795719

I know how to create a matrix of distances among program or admin using the sp package:

ll <- c("program.lat", "program.long")
coords <- dat[ll]
dist <- apply(coords, 1, 
              function(eachPoint) spDistsN1(as.matrix(coords),
                                            eachPoint, longlat=TRUE))

But what I want to do is create a nx1 vector of distances (dist.km) between each pair of coordinates and add it to dat.

#       program.id program.lat program.long  admin.lat  admin.long  dist.km
# 1              1   -42.20844     55.70061 -41.848523   62.536404   567.35
# 2              2   -23.01770    -52.84898 -50.643849 -145.851172  8267.86
# ...

Any suggestions? I've spent a while going through old SO questions, but nothing seems quite right. Happy to be proven wrong.

Update

@Amit's solution works for my toy dataset:

apply(dat,1,function(x) spDistsN1(matrix(x[2:3],nrow=1),x[3:4],longlat=TRUE))

But I think I need to swap the order of the lat, long the order of the lat long columns so long comes before lat. From ?spDistsN1:

pts: A matrix of 2D points, first column x/longitude, second column y/latitude, or a SpatialPoints or SpatialPointsDataFrame object

Also, unless I've misunderstood the logic, I think Amit's solution should grab cols [2:3] and [4:5], not [2:3] and [3:4].

My challenge now is applying this to my actual data. I've reproduced a portion below.

library(sp)
dat <- structure(list(ID = 1:4, 
                      subcounty = c("a", "b", "c", "d"), 
                      pro.long = c(33.47627919, 31.73605491, 31.54073482, 31.51748984), 
                      pro.lat = c(2.73996953, 3.26530095, 3.21327597, 3.17784981), 
                      sub.long = c(33.47552, 31.78307, 31.53083, 31.53083), 
                      sub.lat = c(2.740362, 3.391209, 3.208736, 3.208736)), 
                 .Names = c("ID", "subcounty", "pro.long", "pro.lat", "sub.long", "sub.lat"),     
                 row.names = c(NA, 4L), class = "data.frame")
head(dat) 
#     ID subcounty pro.long  pro.lat sub.long  sub.lat
#  1   1         a 33.47628 2.739970 33.47552 2.740362
#  2   2         b 31.73605 3.265301 31.78307 3.391209
#  3   3         c 31.54073 3.213276 31.53083 3.208736
#  4   4         d 31.51749 3.177850 31.53083 3.208736
apply(dat, 1, function(x) spDistsN1(matrix(x[3:4], nrow=1),
                                    x[5:6],
                                    longlat=TRUE)) 

I get the error: Error in spDistsN1(matrix(x[3:4], nrow = 1), x[5:6], longlat = TRUE) : pts must be numeric

I'm confused because these columns are numeric:

> is.numeric(dat$pro.long)
[1] TRUE
> is.numeric(dat$pro.lat)
[1] TRUE
> is.numeric(dat$sub.long)
[1] TRUE
> is.numeric(dat$sub.lat)
[1] TRUE

Answer 1

The problem you're having is thatapply(...) coerces the first argument to a matrix. By definition, a matrix must have all elements of the same data type. Since one of the columns in dat (dat$subcounty) is char, apply(...) coerces everything to char. In your test dataset, everything was numeric, so you didn't have this problem.

This should work:

dat$dist.km <- sapply(1:nrow(dat),function(i)
                spDistsN1(as.matrix(dat[i,3:4]),as.matrix(dat[i,5:6]),longlat=T))