Why not use hashing/hash tables for everything?
In computer science, it is said that the insert, delete and searching operations for hash tables have a complexity of O(1), which is the best. So, I was wondering, why do we need to use other data structures since hashing operations are so fast? Why can't we just simply use hashing/hash tables for everything?
Answer
Hash tables, on average, do have excellent time complexity for insertion, retrieval, and deletion. BUT:
Big-O complexity isn't everything. The constant factor is also very important. You could use hashtables in place of arrays, with the array indexes as hash keys. In either case, the time complexity of retrieving an item is O(1). But the constant factor is way higher for the hash table as opposed to the array.
Memory consumption may be much higher. This is certainly true if you use hash tables to replace arrays. (Of course, if the array is sparse, then the hash table may take less memory.)
There are some operations which are not efficiently supported by hash tables, such as iterating over all the elements whose keys are within a certain range, finding the element with the largest key or smallest key, and so on.
All of that aside, you do still have a good point. Hashtables have an extraordinarily broad range of suitable use cases. That's why they are the primary built-in data structure in some scripting languages, like Lua.