Variables and Data

In the previous chapter we discussed programming languages.

Now we begin the core mechanics of writing programs.

To do anything useful, a program needs to work with data.

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Why Data Matters

Programs answer questions and perform actions using values.

Examples:

• temperature in a weather app
• account balance in a banking app
• username in a login form
• score in a game

Without data, program logic has nothing to operate on.

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What Is a Variable?

A variable is a named storage location for a value.

You can think of it as a labeled box in memory.

graph LR
  A[Variable Name: age] --> B[Stored Value: 21]

If the value changes later, the variable can be updated.

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Basic Data Types

Programs usually categorize values by type.

Common beginner types:

• number: 42, 3.14
• text/string: "hello"
• boolean: true or false

Types matter because they determine what operations are valid.

For example, adding two numbers makes sense. Adding a number directly to plain text may require conversion.

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Simple Examples

age = 21
name = "Ahnaf"
is_student = True

Here:

• age stores a number
• name stores text
• is_student stores a boolean

Programs combine these values to make decisions and produce output.

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Variables Change Over Time

One important idea is state.

Program state is the current set of variable values.

Example:

count = 0
count = count + 1
count = count + 1

The same variable now holds different values as execution progresses.

This changing state is central to most software behavior.

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Input, Processing, Output

Variables are often used in a simple flow:

1. Read input.
2. Store in variables.
3. Process values.
4. Produce output.

This pattern appears everywhere from small scripts to large systems.

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Key Ideas to Remember

• Programs need data to do meaningful work.
• Variables are named storage for values.
• Data types classify values and valid operations.
• Program state changes as variables are updated.
• Tracing variable values is a powerful way to understand code.

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What Comes Next

Once values are stored, programs must decide what to do with them.

Next we learn conditional logic:

how code makes decisions with if statements and comparisons.