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Date/Time Scales
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Time Scales
Data is often grouped according to the time period in which they
were registered. It is therefore necessary to provide a mechanism
to group time data into categories. The range of the data is
divided into a number of intervals, where each interval becomes a
category in a time scale. This type of time scale is implemented by
the DateTimeScale
class. This class inherits from CategoricalScale,
but provides some additional functionality.
Constructing Time Scales
The DateTimeScale class has four constructors. They
come in two pairs, each pair offering one way of defining the
intervals that make up the scale.
The first constructor takes one argument: a
DateTime array that contains the boundaries of the
intervals. The values in this array must be in ascending order, or
an ArgumentException will be thrown.
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DateTime start = new DateTime(2005, 11, 15);
DateTime[] bounds = new DateTime[] {start, start.AddDays(10),
start.AddDays(20), start.AddDays(30), start.AddDays(40)};
DateTimeScale scale1 = new DateTimeScale(bounds); |
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Dim start As DateTime = New DateTime(2005, 11, 15)
Dim bounds As DateTime() = New DateTime() {start, start.AddDays(10), _
start.AddDays(20), start.AddDays(30), start.AddDays(40)}
Dim scale1 As DateTimeScale = New DateTimeScale(bounds) |
The second constructor also has a DateTime array as
its first argument, but has one additional argument: a
SpecialBins value that specifies which special
intervals to include in the scale.
The possible values are as follows:
| Name |
Description |
None |
No special intervals are included. |
BelowMinimum |
There is a special interval for values below the scale's
minimum value. |
AboveMaximum |
There is a special interval for values above the scale's
maximum value. |
OutOfRange |
There is a special interval for values that are outside the
scale's range. |
Missing |
There is a special interval for missing values. |
Table 1. Values of the SpecialBins enumeration.
If BelowMinimum is included, an interval with lower
bound DateTime.MinValue is inserted before all other
intervals. If AboveMaximum is included, an interval
with upper bound DateTime.MaxValue is added at the
end. The following creates a scale with intervals of one month for
the period November 15, 2005 to March 15, 2006:
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DateTime start = new DateTime(2005, 11, 15);
DateTime[] bounds = new DateTime[] {start, start.AddDays(10),
start.AddDays(20), start.AddDays(30), start.AddDays(40)};
DateTimeScale scale2 = new DateTimeScale(bounds, SpecialBins.BelowMinimum); |
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Dim start As DateTime = New DateTime(2005, 11, 15)
Dim bounds As DateTime() = New DateTime() {start, start.AddDays(10), _
start.AddDays(20), start.AddDays(30), start.AddDays(40)}
Dim scale2 As DateTimeScale = New DateTimeScale(bounds, SpecialBins.BelowMinimum) |
The third constructor takes three arguments. The first two are
the lower bound of the first interval, and the upper bound of the
last interval. The third argument is a DateTimeUnit
value that specifies the size of the intervals. The possible values
are:
| Member Name |
Description |
| Unknown |
The time unit is unknown. |
| Hour |
The time unit is one hour |
| Day |
The time unit is one day. |
| Week |
The time unit is one week. |
| Month |
The time unit is one month. |
| Quarter |
The time unit is one quarter. |
| Year |
The time unit is one year. |
Table 2. Values of the
DateTimeUnit enumeration.
The boundaries of the intervals are set on whole time units. For
example, if the lower bound is on the 15th of march, and the time
unit is DateTimeUnit.Month, then the first interval
will span the period March 15 - March 31.
The fourth constructor has one additional argument: a
SpecialBins value
that indicates which special values should be tabulated in addition
to those within the specified interval.
The code below creates a scale with five intervals for values
between 50 and 100:
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DateTime fromDate = new DateTime(2005, 11, 15);
DateTime toDate = new DateTime(2006, 3, 15);
DateTimeScale scale3 = new DateTimeScale(fromDate, toDate, DateTimeUnit.Month); |
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Dim fromDate As DateTime = New DateTime(2005, 11, 15)
Dim toDate As DateTime = New DateTime(2006, 3, 15)
Dim scale3 As DateTimeScale = New DateTimeScale(fromDate, toDate, DateTimeUnit.Month) |
The above scale is made up of 5 intervals. The first and last
intervals cover only a part of a month. The three middle intervals
each cover a full month.
Properties and Methods
The Count
property returns the number of intervals in the scale. The
IsOrdered
property indicates whether the scale is ordered or unordered. It
always returns true.
The GetBounds
method returns a DateTime array containing the
boundaries of the intervals. If the BelowMinimum or
AboveMaximum intervals were included, the return value
includes these intervals as well.
The GetLowerBound
and GetUpperBound
methods take one argument and return the lower and upper bound of
the interval with the specified index.
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Console.WriteLine(scale3.GetLowerBound(2)); // Prints '1/1/2006'
Console.WriteLine(scale3.GetUpperBound(2)); // Prints '2/1/2006' |
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Console.WriteLine(scale3.GetLowerBound(2)) ' Prints '1/1/2006'
Console.WriteLine(scale3.GetUpperBound(2)) ' Prints '2/1/2006' |
The GetCaption
method returns the text label of the level at the specified level
index. The GetCaptions
method returns a string array containing the captions for each
level.
The GetEnumerator
method returns an IEnumerator object that can be used
to iterate through the levels of the scale.
Mapping Values to Intervals
Time scales convert dates and times to intervals. The
Map
method provides this functionality.
This method is overloaded. The first overload takes any object
as its only argument. This object is converted to a number using
the Convert.ToDateTime method. If this conversion
succeeds, the index of the interval containing the number is
returned. If the number is outside the scale, -1 is returned.
The second overload takes an array of objects and returns an
integer array of the indexes corresponding to those objects. Each
of these methods takes an optional IFormatProvider
argument that is used to convert the object value to a number.
The DateTimeScale class also provides type-safe
overloads. One maps a number to the index of the interval. The
second takes an array and returns an array of indexes.
The example below uses the scale defined earlier. Each line
prints the same value ('2'):
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Console.WriteLine(scale3.Map(new DateTime(2006, 1, 6)));
Console.WriteLine(scale3.Map("1/6/2006"));
IFormatProvider provider = System.Globalization.CultureInfo.CreateSpecificCulture("NL-BE").DateTimeFormat
Console.WriteLine(scale3.Map("6/1/2006", provider)); |
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Console.WriteLine(scale3.Map(New DateTime(2006, 1, 6)))
Console.WriteLine(scale3.Map("1/6/2006"))
Dim provider As IFormatProvider = _
System.Globalization.CultureInfo.CreateSpecificCulture("NL-BE").DateTimeFormat
Console.WriteLine(scale3.Map("6/1/2006", provider)); |
In the last line, the value is provided in a format that uses a
month-day-year format. The right DateTimeFormat is
required to make the result correct.
Up: Categorical Variables Next: Categorical Variables Previous: Numerical Scales Contents
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