Pandas Delete rows from dataframe based on condition

Consider this code:

from StringIO import StringIO
import pandas as pd

txt = """a, RR
10, 1asas
20, 1asasas
30,
40, asas
50, ayty
60, 2asas
80, 3asas"""
frame = pd.read_csv(StringIO(txt), skipinitialspace=True)

print frame,"\n\n\n"

l=[]
for i,j in frame[~ frame['RR'].str.startswith("1", na=True)]['RR'].iteritems():
    if j.startswith(('2','3')):
         if frame[frame['RR'].str.startswith("1", na=False)]['RR'].str.match("1"+j[1:], as_indexer = True).any():
            l.append(i)
    else:
        if frame[frame['RR'].str.startswith("1", na=False)]['RR'].str.match("1"+j, as_indexer = True).any():
            l.append(i)
 frame = frame.drop(frame.index[l])
 print frame

What I am doing here is,

Loop through dataframes to drop any RR which already has 1RR in dataframe
If RR has 2 or 3 at start , then drop if that RR has 1RR[1:] in dataframe.
If RR startswith 1 or is NaN do not touch it.

The code is working fine but this dataframe will have up to 1 million entries and I don't think this code is optimised. As I have just started with pandas I have limited knowledge. Is there any way we can achieve this without iteration. Does pandas have any in-built utility to do this?

Solution

First, keep all strings starting with 1 or nan:

keep = frame['RR'].str.startswith("1", na=True)
keep1 = keep[keep]  # will be used at the end

Second, keep strings starting with 2 or 3 that are not in the first dataframe rr1:

rr1 = frame.loc[frame['RR'].str.startswith("1", na=False), 'RR']
keep2 = ~frame.loc[
            (frame['RR'].str.startswith("2")) | (frame['RR'].str.startswith("3")), 'RR'
        ].str.slice(1).isin(rr1.str.slice(1))

Third, keep other strings that are not in rr1 after adding a leading 1:

import numpy as np
keep3 = ~("1" + frame.loc[
            ~frame['RR'].str.slice(0,1).isin([np.nan, "1", "2", "3"]), 'RR'
        ]).isin(rr1)

Finally, put everything together:

frame[pd.concat([keep1, keep2, keep3]).sort_index()]