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Variables

Variables are names that refer to values. Assigning a variable binds a name to a value; later references to the name use the bound value. Variables make programs readable and let us reuse computed results. Variables can be reassigned to new values, and in both Python & Matlab, they can refer to values of different types over time (dynamic typing).

Variables vs. Values

Let's return to our cooking analogy: variables are like labels on bowls, and
values are the dough inside.

  • Putting dough into a bowl = assigning a value to a variable.
  • Moving the label to a different bowl = rebinding the variable to a new value.
  • Scooping dough into a new bowl = "shallow" copying a value.
  • Replicating the dough in a new bowl = "deep" copying a value.
  • Two labels on the same bowl = two variables referencing the same mutable object = changing the dough through one label changes both variables.
# Simple assignment
x = 10

# Rebinding and dynamic typing
x = "now a string"

# Multiple assignment / unpacking
a, b = 1, 2
a, b = b, a  # swap without a temp variable

# Type hints (optional)
y: int = 5

% Assignment
x = 10;

% Rebinding and dynamic typing
x = 'now a string';

% Multiple assignment is usually done element-wise or via arrays
a = 1; b = 2;

Gotchas

  1. Case-Sensitivity: Variables names are case-sensitive and must start with a letter or underscore
  2. Reserved keywords: Special reserved words (e.g., if, for, while) cannot be used as variable names.
  3. Shadowing: Don't use names that shadow other things (e.g., list, str, sum). You may no longer be able to access them!
  4. Mutability: Always assume variables are mutable by default; unexpected sharing happens when multiple variables refer to the same object!