Comparer class

A Comparer compares two values and tells which of them is smaller. It implements IComparer, so it can be given to Sort and BinarySearch of an ArrayList in place of a comparer that the program writes.

The class is predeclared: Comparer.Compare(a, b) can be called without creating an object, and Comparer is itself a value that can be passed as a comparer.

Debug.Print Comparer.Compare(1, 2)
Debug.Print Comparer.Compare(5, 5)
Debug.Print Comparer.Compare("pear", "apple")
Debug.Print Comparer.Compare(#1/2/2026#, #1/1/2026#)

Sort without a comparer compares items with the < operator, which needs no help from a Comparer for numbers and strings. A Comparer is useful for the other kinds of value (an array or an object), and as the base of a comparer that changes the result, such as one that sorts in descending order.

How two values are compared

Compare takes the first rule that applies:

  1. It subtracts the second value from the first and takes the sign of the difference (Sgn). This is the comparison of numbers and of dates. Two strings that both look like numbers are compared as numbers too, so "10" is greater than "9", and a Boolean counts as its number, so True (-1) is smaller than False (0).
  2. When the subtraction raises an error, two values that are not objects are compared with the < and > operators. The comparison of strings is by their characters, so an upper-case letter is smaller than a lower-case one. A text is greater than a number, and a Null is equal to every value.
  3. When one of the two values is an array, the first element of the array stands for it: Compare compares X(0) with Y(0), or with the other value when it is not an array.
  4. When the first value is an object, the result is that of its CompareTo method called with the second value. The result is returned as the object gives it, not reduced to -1, 0 or 1.
  5. An error raised by the comparison of rule 2, 3 or 4 reaches the caller of Compare, and a Sort or BinarySearch that called it stops with the same error. Errors come from an object with no CompareTo (error 438), from a value compared with such an object, from an array with no elements (error 9) and from a CompareTo that raises one.

Writing a comparer

A comparer is any object that implements IComparer. Its Compare returns a negative number when the first value goes before the second, zero when they are equal and a positive number when the first goes after. A comparer that gives the result of a Comparer the other way round sorts in descending order:

Class DescendingComparer
    Implements IComparer

    Public Function Compare(X As Variant, Y As Variant) As Long Implements IComparer.Compare
        Return Comparer.Compare(Y, X)
    End Function
End Class

Dim list As New ArrayList
list.AddRange Array(3, 1, 2)
list.Sort 0, , New DescendingComparer

Dim v As Variant
For Each v In list
    Debug.Print v
Next

Methods

Compare

Compares two values.

Syntax: object.Compare ( X, Y ) As Long

X
required The first value.
Y
required The second value.

Returns a negative number when X is smaller than Y, 0 when they are equal and a positive number when X is greater. For values that are not objects the result is -1, 0 or 1. See How two values are compared for the rules.

See Also