diff --git a/fluents/dataset.py b/fluents/dataset.py
index d871b4e..61af574 100644
--- a/fluents/dataset.py
+++ b/fluents/dataset.py
@@ -224,6 +224,22 @@ class Dataset(object):
         """
         return copy.deepcopy(self)
 
+    def subdata(self, dim, idents):
+        """Returns a new dataset based on dimension and given identifiers.
+        
+        """
+        ds = self.copy()
+        indices = ds.get_indices(dim, idents)
+        ax = [i for i, name in enumerate(ds._dims) if name == dim][0]
+        subarr = ds._array.take(indices, ax)
+        for k, v in ds._map[dim].items():
+            if k not in idents:
+                del ds._map[dim][k]
+        ds.shape = tuple(len(ds._map[d]) for d in ds._dims)
+        ds.set_array(subarr)
+        return ds
+    
+        
     def transpose(self):
         """Returns a copy of transpose of a dataset.
 
@@ -569,7 +585,7 @@ def write_ftsv(fd, ds, decimals=7, sep='\t', fmt=None, sp_format=True):
 
     fd.write("# name: %s" % ds.get_name() + '\n')
     # xy-node-positions
-    if ds.nodepos != None:
+    if type == 'network' and ds.nodepos != None:
         fd.write("# nodepos:")
         node_dim = ds.get_dim_name(0)
         for ident in ds.get_identifiers(node_dim, sorted=True):