Variables & Assignment
A variable is a name attached to a value the program is keeping track of. Assignment — usually
written with = — stores a value under that name; later, using the name reads the value back out.
score = 5 store 5 under the name "score"
score = score + 10 read score (5), add 10, store 15 back
The crucial, sometimes confusing point: score = score + 10 isn't a claim that the two sides are
equal. It's a command — compute the right side, then put the result into the box on the left. The
name stays the same; the value inside changes. What kind of value a variable can hold is governed by
its type.
A running program keeps its data in two regions of memory. Small, short-lived values — a loop counter, a number, a reference — sit on the stack: a region that grows and shrinks automatically as functions are called and return, making it very fast but tied to the function's lifetime. Larger or longer-lived data — a list that grows, an object whose size isn't known ahead of time — is put on the heap, a general pool you allocate from explicitly (or that a garbage collector manages for you).
So a variable often doesn't hold the big thing directly; it holds a small value on the stack that points to the real data on the heap. That's why, in many languages, copying a variable copies the pointer, not the data — and two variables can end up referring to the very same object.