如何将 M x M 熊猫数据帧转换为 NX 2 数据帧？

Question

I have a pandas DataFrame like below:

df = pd.DataFrame({"type": ["A", "B", "C"],
                   "A": [0, 0, 12],
                   "B": [1, 3, 0],
                   "C": [0, 1, 1]}
)

I want to transform this to a DataFrame that is NX 2, where I concatenate the column and type values with " - " as delimiter. The output should look like this:

pair    value
A - A   0
A - B   0
A - C   12
B - A   1
B - B   3
B - C   0
C - A   0
C - B   1
C - C   1

I don't know if there is a name for what I want to accomplish (I thought about pivoting but I believe that is something else), so that didn't help me in googling the solution for this. How to solve this problem efficiently?

Answer 1

1st set index as type and then unstack and convert the result to dataframe.

try:

x = df.set_index('type').unstack().to_frame('value')
x.index = x.index.map(' - '.join)
res = x.rename_axis('pair').reset_index()

res:

    pair    value
0   A - A   0
1   A - B   0
2   A - C   12
3   B - A   1
4   B - B   3
5   B - C   0
6   C - A   0
7   C - B   1
8   C - C   1

Answer 2

First melt the column type , then join variable , and type column with a hyphen - , and take the required columns only:

>>> out = df.melt(id_vars='type')
>>> out.assign(pair=out['variable']+'-'+out['type'])[['pair', 'value']]

  pair  value
0  A-A      0
1  A-B      0
2  A-C     12
3  B-A      1
4  B-B      3
5  B-C      0
6  C-A      0
7  C-B      1
8  C-C      1