View source for Time index details

[[Category:Analytica User Guide]]
<breadcrumbs>Analytica User Guide > Dynamic Simulation > {{PAGENAME}}</breadcrumbs>

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==Reference to earlier time==
<code>Time - k</code> in the expression <code>var[Time - k]</code> refers to the position of the elements in the <code>Time</code> index, not values of <code>Time</code>.

For example, if <code>Time</code> equals [1990, 1994, 1998, 2002, 2006], then the value of <code>Gasprice[Time - 3]</code> in year 2006 would refer to the price of gasoline in 1994, not 2003. When you refer to the <code>Time</code> variable directly, not as an index, the expression refers to the values of <code>Time</code>. For example, the expression <code>(Time - 3)</code> in 2006 is 2003.

The offset, '''k''', can be an expression, and might even be indexed by <code>Time</code>. When '''k''' is indexed by <code>Time</code>, then the offset varies at different points in <code>Time</code>. However, <code>Slice(k, Time, t)</code> must be between 1 and ''t - 1''. It must be positive since the expression is not allowed to depend on values in the future (that have not yet been computed). It must be less than ''t - 1'' since the expression cannot depend on values “before the beginning of time.”

==Defining time==
There are three ways to define the <code>Time</code>index, each of which has different advantages:

* Sequence (the preferred method)
* List (numeric)
* List of labels (text)

===Time as a sequence===
Using the '''Sequence()''' function is the easiest way to define <code>Time</code>with equal intervals (see “[[Creating an index|Expression view]]” and “[[Defining a variable as an edit table]]”). The numeric values for <code>Time</code>can be used in other expressions.

'''Example:'''

[[File:Chapter17_4.png]]

===Time as a list (numeric)===
When <code>Time</code>is defined as a numeric list, it usually consists of increasing numbers. The intervals between entries can be unequal, and the values for <code>Time</code>can be used in other expressions.

'''Example:''' <code>Time</code>

[[File:Chapter17_5.png]]

When you use time periods that differ by a value other than 1, typing <code>(Time-1)</code> won’t provide the value of the previous time period. You can use the syntax <code>x[Time-1]</code> if you want to utilize a variable indexed by <code>Time</code>, but if you want to perform an operation that depends on the difference in time between the current time period and the last one, you must first create a node that uncumulates the <code>Time</code>index.

 YearsPassed: Uncumulate(Time)

[[File:Chapter17_6.png]]

Now you can include this node in a dynamic expression that depends on the time between time periods. The following definition is equivalent to the '''[[Dynamic  function#Dynamic(initial1, initial2..., initialn, expr)|Dynamic()]]''' definition but allows for changes in time period increments.

 Gasprice := Dynamic(1.2, Gasprice[Time - 1]*1.05^YearsPassed) &rarr;

[[File:Chapter17_7.png]]

===Time as a list of labels (text)===
When <code>Time</code>is defined as a list of labels, <code>Time</code>values cannot be used in other expressions as numbers.

The resulting graph of any '''Dynamic()''' function, with the ''x''-axis set to <code>Time</code>, shows the labels at equal ''x''-axis intervals.

'''Example:''' <code>Time</code>

[[File:Chapter17_8.png]]

 Gasprice := Dynamic(1.2, Gasprice[Time - 1]*1.05) &rarr;

[[File:Chapter17_9.png]]

==Using Time in a model==
You can use <code>Time</code>like any index variable; you can change only its title and definition. To include the <code>Time</code>node on a diagram:

# Open the Object window for <code>Time</code>by selecting '''Edit Time''' from the '''Definition''' menu.
# Select '''Make Alias''' from the Object menu (see “[[Alias nodes|An alias is like its original]]”).

When the <code>Time</code>node displays on a diagram, arrows from <code>Time</code>to all dynamic variables display by default.


==See Also==
<footer>Dynamic function / {{PAGENAME}} / Dynamic initial values</footer>