LabVIEW Wiki - User contributions [en]

Cluster data type

2010-11-15T18:04:06Z

Ygyluvyz:

A cluster is a [[LabVIEW]] [[Data_type|datatype]] that groups data elements of mixed types. A cluster is similar to a record or a struct in text-based
programming languages. Clusters can be stored solitarely in a [[ctl]].

Bundling several data elements into clusters eliminates wire
clutter on the block diagram and reduces the number of
connector pane terminals that subVIs need. The connector pane
has, at most, 28 terminals. If a
front panel contains more than
28 controls and indicators that
you want to use programmatically, you can group some of them into a
cluster and assign the cluster to a terminal on the connector
pane. Like an array, a cluster is either a control or an
indicator. A cluster cannot contain a mixture of controls and
indicators.

The color of the cluster on the [[Block_Diagram]] gives some insight on the included data-types. A brown cluster consists of numeric data types and numerical actions as 'plus', 'minus' and others can be applied to the cluster directly. Additionally the size of the cluster (bytewise) is fixed. A pink cluster has a variable size because of the use of (for instance) [[String|strings]] or [[Array|arrays]].

>== Creating Cluster Controls and Indicators ==

To create a cluster control or indicator,
select a cluster on the Controls>>All Controls>>Array
& Cluster palette, place it on the front panel,
and drag controls or indicators into the cluster
shell. Resize the cluster shell by dragging the
cursor while you place the cluster shell.

[[Image:cluster.png|center|frame|A cluster of three controls]]
*[http://wiki.lavag.org/Image:Create_cluster_video.gif Watch video of cluster creation process]

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==Creating Cluster Constants==

Create a cluster constant on the block diagram
by selecting a cluster constant on the
Cluster palette, placing it on the block
diagram, and dragging a constant into the
cluster shell.

If you have a cluster control or indicator on
the front panel and you want to create a cluster
constant containing the same elements on the block
diagram, you can either drag that cluster from the front
panel to the block diagram or right-click the cluster on the
front panel and select Create>>Constant from
the shortcut menu.

==Cluster Order==
Cluster elements have a logical order unrelated to their
position in the shell. The first object you place in the
cluster is element 0, the
second is element 1, and so
on. If you delete an element, the order adjusts
automatically.

The cluster order determines the order in which the elements
appear as terminals on the Bundle and
Unbundle functions on the block diagram.

You can view and modify the cluster order by right-clicking
the cluster border and selecting Reorder Controls In
Cluster from the shortcut menu.

The white box on each element shows its current place in the
cluster order. The black box shows the new place in the
order for an element. To set the order of a cluster element,
type the new order number in the Click to set
to text box and click the element. The cluster order
of the element changes, and the cluster order of other
elements adjusts. Save the changes by clicking the
Confirm button on the toolbar. Revert to the
original order by clicking the Cancel button.

Corresponding elements, determined by the cluster order,
must have compatible data types. For example, in one
cluster, element 0 is a
numeric control, and element 1
is a string control. In a second cluster, element
0 is a numeric indicator, and
element 1 is a string
indicator. The cluster control correctly wires to the
cluster indicator.

However, if you change the cluster order of the indicator so
the string is element 00, and
the numeric is element 11, the
wire from the cluster control to the cluster indicator
appears broken, indicating that the data types do not match.
[[Image:cluster order mismatch.png|center|frame|Broken wire due to cluster order mismatch]]
[[category:Data_types]]
[[category:LabVIEW fundamentals]]

NaN

2010-11-15T12:45:08Z

Ygyluvyz:

'''NaN''' is the notation in LabVIEW for the floating-point value ''Not-a-Number''. LabVIEW implements the [http://grouper.ieee.org/groups/754/ IEEE-754 standard] for floating-point calculations, which specifies unique values for the outputs of mathematical operations which have no meaningful numerical result. A few examples of calculations which produce NaN are:

*division of zero by zero
*square root of a negative number
*division of an infinity by an infinity
*logarithm (any base) of a non-positive number
*any calculation which includes NaN as one of its operands

The last example above implies that any calculation sequence which generates NaN as an intermediate step will propagate NaN through all subsequent calculations. While this is generally true, there are exceptions (the LabVIEW '''Power of X''' primitive returns one for zero raised to NaN).

One salient feature of the NaN value is that all comparison operations involving NaN return '''FALSE''', including the equality operator (in other words, NaN != NaN). LabVIEW provides the '''Not A Number/Path/Refnum?''' primitive, which is the proper way to test for NaN.
<center>
{| class="wikitable FCK__ShowTableBorders" align="center"
|-
! Not A Number/Path/Refnum? <br>primitive
|-
| [[Image:Nanfunc.png|center]]
|}
</center>
Floating-point controls, indicators, and block diagram constants support the direct entry of NaN (case-insensitive). Similarly, string-to-numeric and numeric-to-string primitives convert or format NaN without regard to format specifiers.

>== Use Cases ==

*indicating "missing" data in data-tables in files.
*used for lifting the "plotting pen" in graph, for instance if a value would be out of its allowed range.

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== External links ==

*[http://grouper.ieee.org/groups/754/ IEEE-754: Standard for Binary Floating-Point Arithmetic]
*[http://en.wikipedia.org/wiki/NaN Wikipedia entry on NaN]

Daemon

2010-11-15T07:28:35Z

Ygyluvyz:

A '''daemon''' is a process that runs in the background, not usually visible to a user. In LabVIEW, daemons are usually implemented as one of two types: self-launched or launched.

>==Self-launching daemon==
A ''self-launching daemon'' is a VI that executes, closes its own [[front panel]], and continues to run in the background. A VI normally ceases to execute when its front panel is closed. To avoid this, a daemon VI first opens a [[VI reference]] to itself. The VI can then use the Front Panel:Open method to set the state of the VI to "hidden." In LabVIEW versions earlier than 8.0, the Front Panel:Open method is not available, but the Front Panel Window:Open property can be used.

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==Launched daemon==
A ''launched daemon'' is a daemon VI that is opened by an intermediate, "launcher" VI. This allows a high-level VI to launch a background process and continue executing. In this case, the launcher VI opens a reference to the daemon VI and starts it using the Run VI method, with "Wait Until Done" set to False and "Auto Dispose Ref" set to True. The launcher VI must not close the VI reference to the daemon VI. Once the daemon VI has launched, the launcher VI can close itself.

==External Links==
* [http://zone.ni.com/devzone/cda/tut/p/id/5237 LabVIEW Application Design Patterns]

[[category:design patterns]]