Iterates over upper triangular indices.

uppertriangular.jl

#upper triangular indices - iterates over upper triangular indices in a list of
#indices.
type uppertriangular; iterable; end

Base.start(x::uppertriangular) = (1, 1)
function Base.next(x::uppertriangular, state)
  (idx1, idx2) = state
  next1 = idx1
  next2 = idx2 + 1
  if next2 > length(x.iterable)
    next1 += 1
    next2 = next1
  end
  ((x.iterable[idx1], x.iterable[idx2]), (next1, next2))
end

function Base.done(x::uppertriangular, state)
  (state[1] > length(x.iterable))
end

function Base.length(x::uppertriangular)
  (x.length * x.length + x.length) / 2
end

###########################################
# examples

for (x,y) in uppertriangular(1:3)
  print("<$x, $y> ")
end
println()

# output:
# <1, 1> <1, 2> <1, 3> <2, 2> <2, 3> <3, 3>

for (x,y) in uppertriangular([2, 7, 5])
  print("<$x, $y> ")
end
println()

# output:
# <2, 2> <2, 7> <2, 5> <7, 7> <7, 5> <5, 5>