Yes, of course. There are many use cases. Yet, in practical terms - most (90%) of look ups do return a value. For 'small' N, you can consider everything O(1). I

Yes, of course. There are many use cases. Yet, in practical terms - most (90%) of look ups do return a value. For 'small' N, you can consider everything O(1). If you do know the use case - like most look ups fail, you can pick a proper impl. and tune it. Yet again, very few cycles are spent on small hash table look ups in general. Yet, a lot of (like really a lot) is spent on small hash table wasted memory. If you care about performance: measure, measure, measure... and know what you measure.