Linear search

To find something in an unsorted list, you check each element in turn until you spot it. The subtle part is what you report: a search returns where the value is — its index — not the value itself, which you already knew. Step through and watch found.

Find WHERE a target value sits in the list, and report its index (or -1 if it is not there).

Linear search — value where the index belongs execution-derived · CPython

same data, two zoom levels

state after line 1 runs

nums

48151623

step 1 / 15

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What you are looking at

The list is [4, 8, 15, 16, 23] and we are looking for 15. The right answer is its index, 2. found starts at -1, a sentinel that means "not found yet" — no real index is negative, so it can never be mistaken for an answer.

Buggy — found = nums[i] stores the value that matched, so it ends at 15: the thing we searched for, not where it lives. Useless as a position.
Procedural — found = i records the index, so it ends at 2. One character apart, and only the state pane makes the difference undeniable.
Compact — found = nums.index(target) scans and returns the position (2) in one call. Concise, but it raises an error if the target is absent — guard it with target in nums when that can happen.

Every state you see came from running the program under CPython's tracer at build time — see how GlassBox stays honest.