The building blocks.
WHAT A VARIABLE IS
A named reference to a value.
WHAT DIFFERS BETWEEN LANGUAGES
Whether the type is declared Whether the reference can be reassigned Whether the value can be changed
WHAT THAT LAST DISTINCTION MEANS
A constant reference may still point at something changeable.
WHAT PRIMITIVE TYPES USUALLY EXIST
Integers, of various sizes Floating point numbers Boolean values Characters and text A representation of absence
WHAT COLLECTIONS USUALLY EXIST
Ordered sequences Key and value mappings Sets, holding unique values Fixed-size groupings
WHAT TO CHOOSE BY
What operations you perform most.
WHY
Looking up by key in a sequence requires examining every item; in a mapping it does not.
WHAT THAT MEANS
The wrong structure turns fast code into slow code, at scale.
WHAT TO BE CAREFUL WITH
Integer overflow, in languages that permit it Floating point for money, in every language Comparing floating point values for equality
WHAT TO UNDERSTAND FOR YOUR LANGUAGE
Which types are copied and which are referenced.
