I have one dataframe like this with the first column is the index (i.e., index = [1,2,3]):

    1      2      3
1   0      0.43   0.61
2   0.88   0      0.12
3   0.33   0.95   0

The new dataframe should look like the below:

Link      Value
1_1       0
1_2       0.43
1_3       0.61
2_1       0.88
2_2       0
2_3       0.12
3_1       0.33
3_2       0.95
3_3       0

Any help is highly appreciated! Thank you!