Merge pull request JuliaLang#14937 from stevengj/dictfilter

improved Dict filtering

base/dict.jl

@@ -241,14 +241,27 @@ function merge(d::Associative, others::Associative...)
 end
 
 function filter!(f, d::Associative)
+    badkeys = Array(keytype(d), 0)
     for (k,v) in d
-        if !f(k,v)
-            delete!(d,k)
-        end
+        # don't delete!(d, k) here, since associative types
+        # may not support mutation during iteration
+        f(k,v) || push!(badkeys, k)
+    end
+    for k in badkeys
+        delete!(d, k)
     end
     return d
 end
-filter(f, d::Associative) = filter!(f,copy(d))
+function filter(f, d::Associative)
+    # don't just do filter!(f, copy(d)): avoid making a whole copy of d
+    df = similar(d)
+    for (k,v) in d
+        if f(k,v)
+            df[k] = v
+        end
+    end
+    return df
+end
 
 eltype{K,V}(::Type{Associative{K,V}}) = Pair{K,V}
 
@@ -866,6 +879,16 @@ function next{K,V}(t::WeakKeyDict{K,V}, i)
 end
 length(t::WeakKeyDict) = length(t.ht)
 
+# For these Associative types, it is safe to implement filter!
+# by deleting keys during iteration.
+function filter!(f, d::Union{ObjectIdDict,Dict,WeakKeyDict})
+     for (k,v) in d
+        if !f(k,v)
+            delete!(d,k)
+        end
+     end
+     return d
+ end
 
 immutable ImmutableDict{K, V} <: Associative{K,V}
     parent::ImmutableDict{K, V}

test/dict.jl

@@ -439,3 +439,10 @@ let d = ImmutableDict{UTF8String, UTF8String}(),
     @test_throws KeyError d[k1]
     @test_throws KeyError d1["key2"]
 end
+
+# filtering
+let d = Dict(zip(1:1000,1:1000)), f = (k,v) -> iseven(k)
+    @test filter(f, d) == filter!(f, copy(d)) ==
+          invoke(filter!, (Function, Associative), f, copy(d)) ==
+          Dict(zip(2:2:1000, 2:2:1000))
+end