For example I have simple DF:
import pandas as pd
from random import randint
df = pd.DataFrame({'A': [randint(1, 9) for x in xrange(10)],
'B': [randint(1, 9)*10 for x in xrange(10)],
'C': [randint(1, 9)*100 for x in xrange(10)]})
Can I select values from ‘A’ for which corresponding values for ‘B’ will be greater than 50, and for ‘C’ – not equal 900, using methods and idioms of Pandas?
Answers:
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Method 1
Sure! Setup:
>>> import pandas as pd
>>> from random import randint
>>> df = pd.DataFrame({'A': [randint(1, 9) for x in range(10)],
'B': [randint(1, 9)*10 for x in range(10)],
'C': [randint(1, 9)*100 for x in range(10)]})
>>> df
A B C
0 9 40 300
1 9 70 700
2 5 70 900
3 8 80 900
4 7 50 200
5 9 30 900
6 2 80 700
7 2 80 400
8 5 80 300
9 7 70 800
We can apply column operations and get boolean Series objects:
>>> df["B"] > 50 0 False 1 True 2 True 3 True 4 False 5 False 6 True 7 True 8 True 9 True Name: B >>> (df["B"] > 50) & (df["C"] == 900) 0 False 1 False 2 True 3 True 4 False 5 False 6 False 7 False 8 False 9 False
[Update, to switch to new-style .loc]:
And then we can use these to index into the object. For read access, you can chain indices:
>>> df["A"][(df["B"] > 50) & (df["C"] == 900)] 2 5 3 8 Name: A, dtype: int64
but you can get yourself into trouble because of the difference between a view and a copy doing this for write access. You can use .loc instead:
>>> df.loc[(df["B"] > 50) & (df["C"] == 900), "A"]
2 5
3 8
Name: A, dtype: int64
>>> df.loc[(df["B"] > 50) & (df["C"] == 900), "A"].values
array([5, 8], dtype=int64)
>>> df.loc[(df["B"] > 50) & (df["C"] == 900), "A"] *= 1000
>>> df
A B C
0 9 40 300
1 9 70 700
2 5000 70 900
3 8000 80 900
4 7 50 200
5 9 30 900
6 2 80 700
7 2 80 400
8 5 80 300
9 7 70 800
Note that I accidentally typed == 900 and not != 900, or ~(df["C"] == 900), but I’m too lazy to fix it. Exercise for the reader. :^)
Method 2
Another solution is to use the query method:
import pandas as pd
from random import randint
df = pd.DataFrame({'A': [randint(1, 9) for x in xrange(10)],
'B': [randint(1, 9) * 10 for x in xrange(10)],
'C': [randint(1, 9) * 100 for x in xrange(10)]})
print df
A B C
0 7 20 300
1 7 80 700
2 4 90 100
3 4 30 900
4 7 80 200
5 7 60 800
6 3 80 900
7 9 40 100
8 6 40 100
9 3 10 600
print df.query('B > 50 and C != 900')
A B C
1 7 80 700
2 4 90 100
4 7 80 200
5 7 60 800
Now if you want to change the returned values in column A you can save their index:
my_query_index = df.query('B > 50 & C != 900').index
….and use .iloc to change them i.e:
df.iloc[my_query_index, 0] = 5000
print df
A B C
0 7 20 300
1 5000 80 700
2 5000 90 100
3 4 30 900
4 5000 80 200
5 5000 60 800
6 3 80 900
7 9 40 100
8 6 40 100
9 3 10 600
Method 3
And remember to use parenthesis!
Keep in mind that & operator takes a precedence over operators such as > or < etc. That is why
4 < 5 & 6 > 4
evaluates to False. Therefore if you’re using pd.loc, you need to put brackets around your logical statements, otherwise you get an error. That’s why do:
df.loc[(df['A'] > 10) & (df['B'] < 15)]
instead of
df.loc[df['A'] > 10 & df['B'] < 15]
which would result in
TypeError: cannot compare a dtyped [float64] array with a scalar of type [bool]
Method 4
You can use pandas it has some built in functions for comparison. So if you want to select values of “A” that are met by the conditions of “B” and “C” (assuming you want back a DataFrame pandas object)
df[['A']][df.B.gt(50) & df.C.ne(900)]
df[['A']] will give you back column A in DataFrame format.
pandas gt function will return the positions of column B that are greater than 50 and ne will return the positions not equal to 900.
All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0