python - How are iloc and loc different?

Question

Can someone explain how these two methods of slicing are different?
I've seen the docs, and I've seen these answers, but I still find myself unable to understand how the three are different. To me, they seem interchangeable in large part, because they are at the lower levels of slicing.

For example, say we want to get the first five rows of a DataFrame. How is it that these two work?

df.loc[:5]
df.iloc[:5]

Can someone present three cases where the distinction in uses are clearer?

---

Once upon a time, I also wanted to know how these two functions differ from df.ix[:5] but ix has been removed from pandas 1.0, so I don't care anymore.

Answer 1

Label vs. Location

The main distinction between the two methods is:

• loc gets rows (and/or columns) with particular labels.

• iloc gets rows (and/or columns) at integer locations.

To demonstrate, consider a series s of characters with a non-monotonic integer index:

>>> s = pd.Series(list("abcdef"), index=[49, 48, 47, 0, 1, 2]) 
49    a
48    b
47    c
0     d
1     e
2     f

>>> s.loc[0]    # value at index label 0
'd'

>>> s.iloc[0]   # value at index location 0
'a'

>>> s.loc[0:1]  # rows at index labels between 0 and 1 (inclusive)
0    d
1    e

>>> s.iloc[0:1] # rows at index location between 0 and 1 (exclusive)
49    a

Here are some of the differences/similarities between s.loc and s.iloc when passed various objects:

<object>	description	s.loc[<object>]	s.iloc[<object>]
0	single item	Value at index label 0 (the string 'd')	Value at index location 0 (the string 'a')
0:1	slice	Two rows (labels 0 and 1)	One row (first row at location 0)
1:47	slice with out-of-bounds end	Zero rows (empty Series)	Five rows (location 1 onwards)
1:47:-1	slice with negative step	three rows (labels 1 back to 47)	Zero rows (empty Series)
[2, 0]	integer list	Two rows with given labels	Two rows with given locations
s > 'e'	Bool series (indicating which values have the property)	One row (containing 'f')	NotImplementedError
(s>'e').values	Bool array	One row (containing 'f')	Same as loc
999	int object not in index	KeyError	IndexError (out of bounds)
-1	int object not in index	KeyError	Returns last value in s
lambda x: x.index[3]	callable applied to series (here returning 3rd item in index)	s.loc[s.index[3]]	s.iloc[s.index[3]]