Update u2/list_ops.py

u2/list_ops.py

@@ -1,8 +1,4 @@
-def length(l):
+
-    count = 0
-    for item in l:
-        count +=1
-    return count
 
 def append(list1, list2):
     result = []
@@ -12,3 +8,21 @@ def append(list1, list2):
         result += [item]
     return result
 
+def concat(lists):
+    result = []
+    for sublist in lists
+        result = append(result, sublist)
+    return result
+
+def filter(function, list):
+    result = []
+    for item in list:
+        if function(item):
+            result += [item]
+    return result
+
+def length(l):
+    result = 0
+    for item in l:
+        result +=1
+    return result