将行添加到数据帧python

我有一个包含4列的数据框,如下所示:

nb,state,freebk,freebs
901,1,6,14
903,0,2,18
904,1,10,20
905,1,15,5

我想将状态等于1的所有行添加到一个新的数据帧中

所需的新数据帧:

nb,state,freebk,freebs
    901,1,6,14
    904,1,10,20
    905,1,15,5

我尝试了以下方法

 openStationDF = pd.DataFrame(columns=['nb','state','freebk','freebs'])
    t = 0
    for i in range(len(stationDF)):
        station = stationDF.iloc[i]
        if station[0] == 1:
            openStationDF.loc[t] = station
            t = t + 1

此代码成功添加了state=1的站点,但实际情况是,站点的nb=NaN,站点名称从0开始,然后递增1。

nb         NaN
state      1.0
freebk     6.0
freebs    14.0
Name: 0, dtype: float64
nb         NaN
state      1.0
freebk    10.0
freebs    20.0
Name: 1, dtype: float64
nb         NaN
state      1.0
freebk    15.0
freebs     5.0
Name: 2, dtype: float64

我希望名称等于电台的nb(name=901…)

谢谢