diff --git a/funcat/api.py b/funcat/api.py
index 92259e9..dae564d 100644
--- a/funcat/api.py
+++ b/funcat/api.py
@@ -1,7 +1,7 @@
 # -*- coding: utf-8 -*-
 import numpy as np
 
-from .time_series import MarketDataSeries
+from .time_series import MarketDataSeries, NumericSeries
 from .func import (
     SumSeries,
     AbsSeries,
@@ -19,7 +19,7 @@
     llv,
     Ref,
     iif,
-)
+    barslast)
 from .context import (
     symbol,
     set_current_security,
@@ -59,6 +59,7 @@
 HHV = hhv
 LLV = llv
 IF = IIF = iif
+BARSLAST=barslast
 
 S = set_current_security
 T = set_current_date
@@ -90,6 +91,7 @@
     "HHV",
     "LLV",
     "IF", "IIF",
+    "BARSLAST",
 
     "S",
     "T",
@@ -101,4 +103,6 @@
     "set_start_date",
     "set_data_backend",
     "set_current_freq",
+
+    "NumericSeries",
 ]
diff --git a/funcat/func.py b/funcat/func.py
index aca5846..81a0e6b 100644
--- a/funcat/func.py
+++ b/funcat/func.py
@@ -19,6 +19,17 @@
 )
 
 
+# delete nan of series for error made by some operator
+def filter_begin_nan(series):
+    i = 0
+    for x in series:
+        if np.isnan(x):
+            i += 1
+        else:
+            break
+    return series[i:]
+
+
 class OneArgumentSeries(NumericSeries):
     func = talib.MA
 
@@ -29,6 +40,7 @@ def __init__(self, series, arg):
             try:
                 series[series == np.inf] = np.nan
                 series = self.func(series, arg)
+                series = filter_begin_nan(series)
             except Exception as e:
                 raise FormulaException(e)
         super(OneArgumentSeries, self).__init__(series)
@@ -64,6 +76,7 @@ def __init__(self, series, arg1, arg2):
             try:
                 series[series == np.inf] = np.nan
                 series = self.func(series, arg1, arg2)
+                series = filter_begin_nan(series)
             except Exception as e:
                 raise FormulaException(e)
         super(TwoArgumentSeries, self).__init__(series)
@@ -129,6 +142,8 @@ def CrossOver(s1, s2):
 
 
 def Ref(s1, n):
+    if isinstance(n, NumericSeries):
+        return s1[int(n.value)]
     return s1[n]
 
 
@@ -213,3 +228,24 @@ def iif(condition, true_statement, false_statement):
     series[cond_series] = series1[cond_series]
 
     return NumericSeries(series)
+
+
+@handle_numpy_warning
+def barslast(statement):
+    series = get_series(statement)
+    size = len(series)
+    end = size
+    begin = size - 1
+
+    try:
+        result = np.full(size, 1e16, dtype=np.int64)
+    except ValueError as e:
+        raise FormulaException(e)
+
+    for s in series[::-1]:
+        if s:
+            result[begin:end] = range(0, end - begin)
+            end = begin
+        begin -= 1
+
+    return NumericSeries(result)
diff --git a/funcat/time_series.py b/funcat/time_series.py
index e791bb7..e090d9a 100644
--- a/funcat/time_series.py
+++ b/funcat/time_series.py
@@ -204,6 +204,9 @@ def __invert__(self):
     def __repr__(self):
         return str(self.value)
 
+    def __int__(self):
+        return int(self.value)
+
 
 class NumericSeries(TimeSeries):
     def __init__(self, series=[]):
@@ -216,7 +219,13 @@ def series(self):
         return self._series
 
     def __getitem__(self, index):
-        assert isinstance(index, int) and index >= 0
+        assert (isinstance(index, int) and index >= 0) \
+               or (isinstance(index, NumericSeries))
+
+        if isinstance(index, NumericSeries):
+            index = int(index.value)
+            assert index >= 0
+
         return self.__class__(series=self.series[:len(self.series) - index], **self.extra_create_kwargs)
 
 
@@ -255,6 +264,10 @@ def __getitem__(self, index):
         if isinstance(index, int):
             assert index >= 0
 
+        if isinstance(index, NumericSeries):
+            index = int(index.value)
+            assert index >= 0
+
         if isinstance(index, six.string_types):
             unit = index[-1]
             period = int(index[:-1])
diff --git a/requirements.txt b/requirements.txt
index 4afefcc..70d5247 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -6,3 +6,5 @@ lxml
 requests
 TA-Lib
 cached-property
+bs4
+tushare
\ No newline at end of file
diff --git a/tests/test_api.py b/tests/test_api.py
index 8015417..ca1a83c 100644
--- a/tests/test_api.py
+++ b/tests/test_api.py
@@ -14,6 +14,7 @@ def test_000001():
     T("20161216")
     S("000001.XSHG")
 
+    assert np.equal(REF(C, BARSLAST(C > 0)).value, C.value)
     assert np.equal(round(CLOSE.value, 2), 3122.98)
     assert np.equal(round(OPEN[2].value, 2), 3149.38)
     assert np.equal(round((CLOSE - OPEN).value, 2), 11.47)