As mentioned in the chapter introduction, understanding the type of data stored in each column is fundamental. Computers and software tools need this information to perform operations correctly. Imagine trying to calculate the average price if the prices are stored as text like ' $\$ 19.99$' instead of as numbers, or trying to sort events chronologically if dates are just treated as plain text strings like 'Jan 1st'. Using the wrong data type can lead to errors, incorrect calculations, and misleading results.

Let's look at the most common data types you'll encounter when working with datasets:

Numeric Types

These represent numerical values and are essential for mathematical calculations.

Integers (int): These are whole numbers, positive or negative, without any decimal points. Think of counts, indices, or discrete quantities.
- Examples: 10, -5, 0, 1024
Floating-Point Numbers (float): These represent real numbers, including those with decimal points. They are used for measurements, percentages, or any value where precision beyond whole numbers is needed.
- Examples: 3.14, -0.5, 98.6, 2.71828

You can perform arithmetic operations like addition, subtraction, multiplication, and division on numeric types. Trying to add $5 + 10$ works as expected if both are numeric, but if '5' is stored as text, the operation might fail or produce an unexpected result like concatenating text ('510').

String Type (str)

Strings represent textual data. They are sequences of characters enclosed in quotes (single ' ' or double " "). Anything can be represented as a string, including names, addresses, descriptions, codes, or even numbers that you don't intend to use in calculations (like ZIP codes or ID numbers).

Examples: 'Hello World', "Data Science", '123 Main St', "ID-9876", 'True' (note: the word 'True', not the boolean value), '2023-10-26' (a date represented as text)

While strings can contain digits, they are treated as text characters, not numerical values. Mathematical operations typically don't apply directly to strings in a numerical sense.

Boolean Type (bool)

Booleans represent truth values, indicating one of two states: true or false. They are fundamental in logic, comparisons, and control flow.

Values: True, False

These often result from comparisons (e.g., is price > 100?) or represent binary states (e.g., is_subscribed, email_verified).

Datetime Types

These specialized types represent dates, times, or both. Storing dates and times in a proper datetime format allows for chronological sorting, calculating durations, extracting components (like year, month, day, hour), and performing time-based analysis. If dates are stored as strings ('October 26, 2023', '26/10/2023'), these operations become much harder or impossible without conversion.

Examples: 2023-10-26 (date), 14:30:00 (time), 2023-10-26 14:30:00 (datetime)

Categorical Types

While sometimes represented initially as strings, categorical data represents variables that belong to a fixed, limited number of categories or groups. Examples include user ratings ('Low', 'Medium', 'High'), product types ('Electronics', 'Clothing', 'Groceries'), or survey responses ('Agree', 'Neutral', 'Disagree'). Recognizing these can sometimes optimize storage and analysis, though for basic cleaning, handling them often involves ensuring the string representation is consistent.

A classification of common data types found in datasets.

Understanding these fundamental types is the first step. In the following sections, we'll see how to check the current data types in your dataset and, more importantly, how to convert columns to their correct type to ensure your data is ready for reliable analysis.