6.3.1. Comparing Records¶

If we want to sort some things, we have to be able to compare them, to decide which one is bigger. How do we compare two things? If all that we wanted to sort or search for was simple integer values, this would not be an interesting question. We can just use standard comparison operators like “<” or “>”. Even if we wanted to store strings, most programming languages give us built-in functions for comparing strings alphabetically. But we do not usually want to store just integers or strings in a data structure. Usually we want to store records, where a record is made up of multiple values, such as a name, an address, and a phone number. In that case, how can we “compare” records to decide which one is “smaller”? We cannot just use “<” to compare the records! Nearly always in this situation, we actually are interested in sorting the records based on the values of one particular field used to represent the record, which itself is something simple like an integer. This field is referred to as the key for the record.

Likewise, if we want to search for a given record in a database, how should we describe what we are looking for? A database record could simply be a number, or it could be quite complicated, such as a payroll record with many fields of varying types. We do not want to describe what we are looking for by detailing and matching the entire contents of the record. If we knew everything about the record already, we probably would not need to look for it. Instead, we typically define what record we want in terms of a key value. For example, if searching for payroll records, we might wish to search for the record that matches a particular ID number. In this example the ID number is the search key.

To implement sorting or searching, we require that keys be comparable. At a minimum, we must be able to take two keys and reliably determine whether they are equal or not. That is enough to enable a sequential search through a database of records and find one that matches a given key. However, we typically would like for the keys to define a total order, which means that we can always tell which of two keys is greater than the other. Using key types with total orderings gives the database implementor the opportunity to organize a collection of records in a way that makes searching more efficient. An example is storing the records in sorted order in an array, which permits a binary search. Fortunately, in practice most fields of most records consist of simple data types with natural total orders. For example, integers, floats, doubles, and character strings all are totally ordered.

But if we want to write a general purpose sorting or searching function, we need a general way to get the key for the record. We could insist that every record have a particular method called .key(). That seems like a good name for it!

Some languages like Java and C++ have special infrastructure for supporting this (such as the Comparable interface in Java, which has the .compareTo() method for defining the exact process by which two objects are compared). But many languages like Processing and JavaScript do not.

But what if the programmer had already used that method name for another purpose? An even bigger problem is, what if the programmer wants to sort the record now using one field as the key, and later using another field? Or search sometimes on one key, and at other times on another? The problem is that the “keyness” of a given field is not an inherent property within the record, but rather depends on the context. So, you cannot always count on being able to use your favorite method name (or even the comparable interface) to extract the desired key value.

Another, more general approach is to supply a function or class—called a comparator—whose job is to extract the key from the record. A comparator function can be passed in as a parameter, such as in a call to a sorting function. In this case, the comparator function would be invoked on two records whenever they need to be compared. In this way, different comparator functions can be passed in to handle different record types or different fields within a record. In Java (with generics) or C++ (with templates), a comparator class can be a parameter for another class definition. For example, a BST could take a comparator class as a generics parameter in Java. This comparator class would be responsible for dealing with the comparison of two records.

Unfortunately, while flexible and able to handle nearly all situations, there are a few situations for which it is not possible to write a key extraction method. In that case, a comparator will not work. 1

One good general-purpose solution is to explicitly store key-value pairs in the data structure. For example, if we want to sort a bunch of records, we can store them in an array where every array entry contains both a key value for the record and a pointer to the record itself. This might seem like a lot of extra space required, but remember that we can then store pointers to the records in another array with another field as the key for another purpose. The records themselves do not need to be duplicated. A simple class for representing key-value pairs is shown here.

// KVPair class definition
public class KVPair implements Comparable {
  Comparable theKey;
  Object theVal;

  KVPair(Comparable k, Object v) {
    theKey = k;
    theVal = v;
  }

  public int compareTo(Object it) throws ClassCastException {
    if (it instanceof KVPair) // Compare two KVPair objects
      return theKey.compareTo(((KVPair)it).key());
    else if (it instanceof Comparable) // Compare against a key value
      return theKey.compareTo(it);
    else
      throw new ClassCastException("Something comparable is expected.");
  }

  public Comparable key() {
    return theKey;
  }

  public Object value() {
    return theVal;
  }

  public String toString() {
    String s = "(";
    if (theKey != null) { s += theKey.toString(); }
    else { s += "null"; }
    s += ", ";
    if (theVal != null) { s += theVal.toString(); }
    else { s += "null"; }
    s += ")";
    return s;
  }
}

// KVPair class definition
public class KVPair<K extends Comparable<K>, E> implements Comparable<KVPair<K, E>> {
  K theKey;
  E theVal;

  KVPair(K k, E v) {
    theKey = k;
    theVal = v;
  }

  // Compare KVPairs
  public int compareTo(KVPair<K,E> it) {
    return theKey.compareTo(it.key());
  }

  // Compare against a key
  public int compareTo(K it) {
    return theKey.compareTo(it);
  }

  public K key() {
    return theKey;
  }

  public E value() {
    return theVal;
  }


  public String toString() {
    String s = "(";
    if (theKey != null) { s += theKey.toString(); }
    else { s += "null"; }
    s += ", ";
    if (theVal != null) { s += theVal.toString(); }
    else { s += "null"; }
    s += ")";
    return s;
  }
}

The main places where we will need to be concerned with comparing records and extracting keys is for various dictionary implementations and sorting algorithms. To keep them clear and simple, visualizations for sorting algorithms will usually show them as operating on integer values stored in an array. But almost never do people really want to sort an array of integers. But to be useful, a real sorting algorithm typically has to deal with the fact that it is sorting a collection of records. A general-purpose sorting routine meant to operate on multiple record types would have to be written in a way to deal with the generic comparison problem. To illustrate, here is an example of Insertion Sort implemented to work on an array that stores records that support the Comparable interface. Note that since KVPair is implemented to implement the Comparable interface, an array of KVPair could be used by this sort function.

static <T extends Comparable<T>> void inssort(T[] A) {
  for (int i=1; i<A.length; i++) // Insert i'th record
    for (int j=i; (j>0) && (A[j].compareTo(A[j-1]) < 0); j--)
      Swap.swap(A, j, j-1);
}

static <T extends Comparable<T>> void inssort(T[] A) {
  for (int i=1; i<A.length; i++) { // Insert i'th record
    for (int j=i; (j>0) && (A[j].compareTo(A[j-1]) < 0); j--) {
      swap(A, j, j-1);
    }
  }
}

def inssort(A):
  for i  in range(len(A)):  # Insert i'th record
    j = i;
    while (j != 0) and (A[j] < A[j-1]):
      swap(A, j, j - 1)
      j -= 1

Here are some review questions to test your knowledge from this module.

1

One example of a situation where it is not possible to write a function that extracts a key from a record is when we have a collection of records that describe books in a library. One of the fields for such a record might be a list of subject keywords, where the typical record stores a few keywords. Our dictionary might be implemented as a list of records sorted by keyword. If a book contains three keywords, it would appear three times on the list, once for each associated keyword. However, given the record, there is no simple way to determine which keyword on the keyword list triggered this appearance of the record. Thus, we cannot write a function that extracts the key from such a record.