foreach_cell iteration should continue until at least one site is found

src/apply.jl

@@ -92,13 +92,19 @@ function recursive_push!(v::Vector{Pair{T,T}}, (xs, ys)::Pair) where {T}
     return v
 end
 
-# for cells = function
+# for cells::Function
 function recursive_push!(v::Vector{SVector{L,Int}}, fcell::Function) where {L}
     iter = BoxIterator(zero(SVector{L,Int}))
+    keepgoing = true
     for cell in iter
-        fcell(cell) || continue
-        acceptcell!(iter, cell)
-        push!(v, cell)
+        found = fcell(cell)
+        if found || keepgoing
+            acceptcell!(iter, cell)
+            if found
+                push!(v, cell)
+                keepgoing = false
+            end
+        end
     end
     return v
 end

src/selectors.jl

@@ -70,11 +70,15 @@ isonsite(dr) = iszero(dr)
 function foreach_cell(f, sel::AppliedSiteSelector)
     lat = lattice(sel)
     cells_list = cells(sel)
-    if isempty(cells_list) # no cells specified
+    if isempty(cells_list)      # no cells specified
         iter = BoxIterator(zerocell(lat))
+        keepgoing = true        # will search until we find at least one
         for cell in iter
             found = f(cell)
-            found && acceptcell!(iter, cell)
+            if found || keepgoing
+                acceptcell!(iter, cell)
+                found && (keepgoing = false)
+            end
         end
     else
         for cell in cells_list

test/test_lattice.jl

@@ -168,6 +168,9 @@ end
     @test ls2[2] isa Tuple{SVector{2, Int}, Int}
     ls = ls1[sublats = :B]
     @test isempty(ls)
-    ls = lat[cells = n -> norm(n) < 4]
-    @test all(n -> norm(n) < 4, Quantica.cells(ls))
+    ls = lat[cells = n -> 5 < norm(n) < 10]
+    @test !isempty(Quantica.cells(ls)) && all(n -> 5 < norm(n) < 10, Quantica.cells(ls))
+    ls = lat[region = r -> 5 < norm(r) < 10]
+    @test !isempty(Quantica.cells(ls)) && all(r -> 5 < norm(r) < 10, Quantica.sites(ls))
+
 end