Tracing Assignment Statements

Tracing means playing computer. You go line by line and keep a table with one column per variable, adding a row every time a value changes. When you hit an assignment statement like a <- 2, you don't check whether it's true the way you'd check an equation. You just compute the right side using whatever values are currently stored, then overwrite the left side variable with that result. The old value of a is gone the instant the new one is stored.

This is why order flips everything. If a is 1, then b is set to a, b copies the value 1 right then, as a snapshot. If a changes to 2 after that, b does not follow along. b only updates if another assignment statement explicitly sets it again. Swapping two variables shows why this matters: you cannot just say x <- y and y <- x, because the first line destroys x's original value before the second line can use it. That's why swaps need a temporary variable to hold one value safely while the other gets overwritten.

This topic is the foundation for everything else in the unit. Branches and loops are built entirely out of sequences of assignment statements, so tracing them carefully, one line and one updated value at a time, is the same skill you'll use to trace a loop that runs five times or a conditional that updates a counter. Get comfortable with the table-and-rows method here and loop tracing later will feel like the same move repeated.