Computer Laboratory 7
CSCI 2041: Advanced Programming Principles
March 9, 2021
0. Introduction.
In this laboratory assignment, you will write functions that implement a hash table which resolves collisions by chaining. (You may have discussed such hash tables in a data structures course before CSCI 2041.) It is similar to the hash table that was discussed in the lectures for use in memoization. However, this hash table is implemented in a somewhat different way, using mutable objects that contain variables.
1. Theory.
A hash table is a data structure that maps keys to their values. It supports operations that (1) add a new key and value to the table, (2) change the value associated with a key, and (3) delete a key and its value entirely.
One way to implement a hash table uses an array. Each element of the array is a linear linked list of nodes, called a bucket. Each node in the bucket contains a key, its value, and a pointer to the next node in the bucket (if there is one). The pointer to the next node lets us access the remaining nodes in the bucket that follow the first node.
Now suppose that we want to find the value associated with a key. We call a hash function on the key, giving an index into the array. Then we obtain a bucket from the array at that index. We search the bucket for a node that contains the key. If we find it, then the node will also contain the key’s value. If we don’t find it, then the hash table assigns no value to the key.
The buckets are needed because the hash function may send many different keys to the same bucket. This is called a collision. However, if the hash function is correctly designed, then each key has an approximately equal probability of being sent to any bucket within the array. If we make the array big enough, then most buckets will either be empty, or will have only one or two nodes.
As a result, buckets can be searched very quickly. It is possible to implement a hash table so that its operations can be performed in O(1) key comparisons, on average.
2. Implementation.
For this laboratory assignment, you will implement a hash table as an array of buckets, as described in the previous section. You must write an OCaml type and a few OCaml functions, described below. Unlike most Ocaml code written for this course, you will use variables to avoid copying.
(‘key, ‘value) pair
The type pair describes a list of key-value pairs for each hash table bucket. It must have two constructors, called NoPair and Pair. The constructor NoPair makes a pair with no parts, representing an empty list of pair’s. The constructor Pair makes a pair with three parts: a key, a variable that holds a value, and a variable that holds a pointer to the next pair in the bucket. Since the value and the next pair are variables, they can be changed by assignment.
hashDelete table key
Use the function hash to find a bucket in table. Search that bucket for a Pair that contains key. If there is such a Pair, then delete it from the bucket. If there is no such Pair, then do nothing. Hint: do not copy the Pair’s in the bucket, but rather delete only the Pair that contains key. See below for a longer version of this hint.
hashGet table key
Use the function hash to find a bucket in table. Search that bucket for a Pair that contains key and its value. Return that value. If there is no such Pair, then raise the exception NoSuchKey. Hint: the value will be a variable, so make sure to return the variable’s value, not the variable itself.
hashHas table key
Use the function hash to find a bucket in table. Search that bucket for a Pair that contains key. If there is such a Pair, then return true, otherwise return false. Hint: this will be much like hashGet.
hashPut table key value
Use the function hash to find a bucket in table. Search that bucket for a Pair that contains key and a variable that contains its value. If there is such a Pair, then reset the variable to value. If there is no such Pair, then add a new Pair to the front of the bucket. It contains key and a variable whose value is value.
Here are two helper functions that may be useful. The function hashMake returns a hash table. It is implemented as an array of length modulus whose elements are empty buckets (NoPair). Here modulus is assumed to be a prime number.
let hashMake modulus =
Array.make modulus NoPair ;;
The function hash returns the index of a bucket within table that may have a Pair containing key. Here table was created by calling hashMake.
let hash table key =
abs ((Hashtbl.hash key) mod (Array.length table)) ;;
And here is a hint about how to write hashDelete. Use a helper function, which might be called hashDeleting, to search a bucket from table. The function hashDeleting has three possible cases.
1. If the bucket is empty, then return an empty bucket (NoPair).
2. If the first Pair in the bucket contains key, then return the rest of the bucket, not including the first Pair. There is a pointer to the rest of the bucket in the first Pair.
3. If the first Pair in the bucket does not contain key, then call hashDeleting on the rest of the bucket. Set pointer to the rest of the bucket, in the first Pair, to the result returned by hashDeleting.
The function hashDelete should use hash to find the bucket in table that may contain key. It then calls hashDeleting on that bucket. It finally replaces the original bucket in table with the result returned from hashDeleting. In this way, hashDelete can delete the first pair from the bucket, in the same way that it deletes any other Pair, without needing a special case.
3. Deliverables.
The file tests7.ml contains definitions of some helper functions along with some tests, worth 45 points. Insert your code into this file, then run it with OCaml. When you think your code is correct, submit it to Gradescope and/or Canvas. If you do not know how to submit your work, then please ask your lab TA’s. It must be submitted by 11:55 PM on Tuesday, March 16, 2021.
NOTE: increasing numbers of students have asked for more time to submit their work, claiming that they have ‘‘submitted the wrong file.’’ We are unsympathetic to most such claims, because this excuse is often used by cheaters to get additional time they are not entitled to. Please check that you have submitted the correct file when you turn in your work! This probably takes no more effort than a couple of mouse clicks. Thanks.