Why can't I use negative indexing for this pandas dataframe column?

My dataframe is like this:

a = pd.DataFrame({
'Column A': [1, np.nan, 7],
'Column B': [np.nan, 2, 3],
'Column C': [np.nan, 2, np.nan]
})

I can use this index below and get the desired outcome 7.

a["Column A"][2]

However, when I want to find the last one, this one below doesn’t work. What could be the reason for that?

a["Column A"][-1]

>Solution :

Because indexing is not positional but by label. For positional indexing use iloc:

a["Column A"].iloc[-1]