如何遍历具有已排序数字索引的数据框唯一行的列值，并在熊猫中进行重复？

Question

I have a pandas DataFrame with the sorted, numerical index with duplicates, and the column values are identical for the same values of the index in the given column. I would like to iterate through the values of the given column for the unique values of the index.

Example

df = pd.DataFrame({'a': [3, 3, 5], 'b': [4, 6, 8]}, index=[1, 1, 2])

   a  b
1  3  4
1  3  6
2  5  8

I want to iterate through the values in column a for the unique entries in the index - [3,5] .

When I iterate using the default index and print the type for column a , I get the Series entries for the duplicate index entries.

for i in df.index:
    cell_value = df['a'].loc[i]
    print(type(cell_value))

Output:

<class 'pandas.core.series.Series'>
<class 'pandas.core.series.Series'>
<class 'numpy.int64'>

Answer 1

First remove duplicated index by mask and assign positions by arange , then select with iloc :

arr = np.arange(len(df.index))
a = arr[~df.index.duplicated()]
print (a)
[0 2]

for i in a:
    cell_value = df['a'].iloc[i]
    print(type(cell_value))

<class 'numpy.int64'>
<class 'numpy.int64'>

No loop solution - use boolean indexing with duplicated and inverted mask by ~ :

a = df.loc[~df.index.duplicated(), 'a']
print (a)
1    3
2    5
Name: a, dtype: int64

b = df.loc[~df.index.duplicated(), 'a'].tolist()
print (b)
[3, 5]

print (~df.index.duplicated())
[ True False  True]

Answer 2

Try np.unique :

_, i = np.unique(df.index, return_index=True)
df.iloc[i, df.columns.get_loc('a')].tolist() 

[3, 5]

Answer 3

This seems an XY Problem if, as per your comment, same index means same data.

You also don't need a loop for this.

Assuming you want to remove duplicate rows and extract the first column only (ie 3, 5), the below should suffice.

res = df.drop_duplicates().loc[:, 'a']

# 1    3
# 2    5
# Name: a, dtype: int64

To return types:

types = list(map(type, res))

print(types)
# [<class 'numpy.int64'>, <class 'numpy.int64'>]

Answer 4

Another solution using groupby and apply:

df.groupby(level=0).apply(lambda x: type(x.a.iloc[0]))
Out[330]: 
1    <class 'numpy.int64'>
2    <class 'numpy.int64'>
dtype: object

To make your loop solution to work, create a temp df:

df_new = df.groupby(level=0).first()
for i in df_new.index:
    cell_value = df_new['a'].loc[i]
    print(type(cell_value))

<class 'numpy.int64'>
<class 'numpy.int64'>

Or to use drop_duplicates()

for i in df.drop_duplicates().index:
    cell_value = df.drop_duplicates()['a'].loc[i]
    print(type(cell_value))

<class 'numpy.int64'>
<class 'numpy.int64'>