loc for label-based selection in Pandas - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the loc function in pandas do?

loc is used to select rows and columns by their labels (names) in a DataFrame.

beginner

How do you select a single row with label 'A' using loc?

You use df.loc['A'] to get the row labeled 'A'.

intermediate

How to select multiple rows labeled 'A' and 'B' and columns 'X' and 'Y' using loc?

Use df.loc[['A', 'B'], ['X', 'Y']] to select those rows and columns by label.

intermediate

What happens if you use loc with a label that does not exist?

It raises a KeyError because the label is not found in the DataFrame index or columns.

intermediate

Can loc select rows using a label range like 'A' to 'C'?

Yes, loc supports label slicing, so df.loc['A':'C'] selects rows from 'A' to 'C' inclusive.

What does df.loc['row1'] return?

A The first row by position

B The column named 'row1'

C The row with label 'row1'

D An error

loc selects rows by label, so it returns the row labeled 'row1'.

How do you select columns 'A' and 'B' for rows labeled 'x' and 'y'?

A df.loc['A':'B', 'x':'y']

B df.loc[['x', 'y'], ['A', 'B']]

C df.iloc[['x', 'y'], ['A', 'B']]

D df.iloc[0:2, 0:2]

loc uses labels, so you pass lists of row and column labels.

What error occurs if you use loc with a missing label?

A KeyError

B IndexError

C ValueError

D TypeError

Using a label not in the index or columns raises a KeyError.

Which of these selects rows from label 'a' to 'c' inclusive?

A df.iloc[0:3]

B df.iloc['a':'c']

C df.loc['a','c']

D df.loc['a':'c']

loc supports label slicing and includes the end label.

What does df.loc[:, 'col1'] select?

A All rows for column 'col1'

B All columns for row 'col1'

C Only the first row

D Only the first column

The colon : means all rows, so this selects all rows in column 'col1'.

Explain how to use loc to select specific rows and columns by their labels in a pandas DataFrame.

Think about how you pick items by name in a list or dictionary.

Describe what happens if you try to select a label that does not exist using loc.

What happens when you ask for a key that is not in a dictionary?