Pandas dataframe - 检查多行是否具有相同的值

Question

I have a DataFrame which looks like this:

Reference	Value
String1	1
String2	0
String3	-1
String2	1
String1	1
String3	0

Each reference can appear in the dataframe either once, two times, or three times; and can have either the same or different value associated. I would like to create another dataframe which tells me, for each Reference, do they all have the same value or not. So with the example above, I would like to get something like this:

Reference	Value
String1	Yes
String2	No
String3	No

(I put Yes and No as an example but it could be 1/0 or whatever else)

How can I do this?

My initial thought was to use a .groupby but then I didn't find any type of aggregation which would help me here...

Answer 1

You could use groupby + nunique to get a count of unique Values for each Reference. Then use np.where to assign Yes/No values depending on if the number of unique values is 1 or not:

out = df.groupby('Reference', as_index=False)['Value'].nunique()
out['Value'] = np.where(out['Value'].eq(1), 'Yes', 'No')

Output:

  Reference Value
0   String1   Yes
1   String2    No
2   String3    No