Youre going to implement a new version of the map

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You’re going to implement a new version of the Map using the ideas outlined above. Rather than making it a template, we’ll map string s to double s to make things a little simpler. Here is the class definition you’ll be working with (in practice, there would be containsKey , get , iterator , and mapAll methods as well, but you’re not going to implement or even use them, so they’re being omitted): class Map { public: Map(); ~Map(); bool put(string key, double value); private: struct bucket { bool occupied; string key; double value; }; bucket *buckets; // addresses the array where all data gets stored int numBuckets; // allocated length of the array int count; // number of meaningful key-value pairs in the map int hash(string key, int numBuckets); void rehash(); }; The buckets field points to an array of numBuckets bucket records. The occupied field keeps track of whether or not that particular bucket is occupied by real data. If true , then some meaningful key-value pair occupies the rest of the record; if false , the key and 8
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value fields are irrelevant and ignored. (Note that all key-value pairs reside directly within the array addressed by buckets .) a. Implement the Map constructor, which constructs a raw Map to be logically empty but with enough space for 64 key-value pairs. Then implement the destructor, which disposes of all resources maintained by the Map being destroyed. const int kInitNumBuckets = 64; Map::Map() { Map::~Map() { b. Using the supplied hash method, implement the put method, which ensures that the specified key is part of the Map and that it’s associated with the specified value . It uses the quadratic internal probing technique, as described above, to find a home for the new key-value pair. put returns true if the new key-value pairs gets inserted into a previously unoccupied bucket, and false if the new value replaces a previously inserted one. (Don’t worry about rehashing the Map if more then three quarters of the buckets are occupied. You’ll worry about that in part c.) const int kHashMultiplier = 716911; int Map::hash(string key, int numBuckets) { int hashcode = 0; for (int i = 0; i < key.size(); i++) { hashcode = hashcode * kHashMultiplier + key[i]; } return hashcode % numBuckets; } bool Map::put(string key, double value) { if (count > 3 * numBuckets / 4) { rehash(); // you’ll implement this helper method in part c } // complete the implementation using the next page and the space below c. Finally, implement the rehash method, which updates the Map so that it has twice as many buckets and all of its key-value pairs are rehashed to take up residence in a bucket where they could have resided had the new number of buckets been the number of buckets all along. (You’ll benefit by figuring out how to call put to help with the redistribution.) void Map::rehash() { 9
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  • Winter '08
  • Cain,G

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