> If your entire list of receipts fits into memory users won't notice a difference between a list and a set. If you had said "small enough", I would agree, but

> If your entire list of receipts fits into memory users won't notice a difference between a list and a set. If you had said "small enough", I would agree, but linear walks of data that can fit in memory can easily be noticed by users, for large enough lists. (Very large lists can fit in memory.) But, at a higher level, yes, this is always true when people talk about performance: make sure what you're optimizing actually matters. I think a charitable reading of what ThePawnBreak said should assume one has already determined this particular operation matters for performance.