python - What is the best way to access values in a dataframe column? -

for example have

df=pd.dataframe({'a':[1,2,3]}) df[df['a']==3].a = 4 

this not assign 4 3

df[df['a']==3] = 4 

but works.

it confused me on how assignment works. appreciate if can give me references or explanation.

you not want use second method. returns dataframe subslice , assigns same value every single row.

for example,

df      b 0  1  4 1  2  3 2  3  6  df[df['a'] == 3]      b 2  3  6  df[df['a']==3] = 3  df      b 0  1  4 1  2  3 2  3  3 

the first method not work because boolean indexing returns copy of column (series), trying assign to, assignment fails:

df[df['a'] == 3].a = 4 /library/frameworks/python.framework/versions/3.4/lib/python3.4/site-packages/pandas/core/generic.py:3110: settingwithcopywarning:  value trying set on copy of slice dataframe. try using .loc[row_indexer,col_indexer] = value instead  see caveats in documentation: http://pandas.pydata.org/pandas-docs/stable/indexing.html#indexing-view-versus-copy   self[name] = value 

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so, options using .loc (access name) or iloc (access index) based indexing:

df.loc[df.a == 3, 'a'] = 4  df    0  1 1  2 2  4 

if passing boolean mask, cannot use iloc.