Searching a .txt file for a specific number and extracting the corresponding data

[perfringo]

Do you mean something like that:

>>> d = [
...     [1, 10],
...     [0.6, 20],
...     [2, 30],
...     [0.6, 40]
...     ]
>>> [row[1] for row in d if row[0] == 0.6]
[20, 40]