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For Loops
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A For Loop executes a subdiagram a set number of
times. Figure 1 shows a For Loop in
LabVIEW, a flow chart equivalent of the For Loop
functionality, and a pseudo code example of the functionality of
the For Loop.
 |
The For Loop is located on the
Functions>>All Functions>>Structures palette. You also
can place a While Loop on the block diagram,
right-click the border of the While Loop, and
select Replace with For Loop from the shortcut menu
to change a While Loop to a For
Loop. The value in the count terminal (an
input terminal), shown in Media 2, indicates how
many times to repeat the subdiagram.
The iteration terminal (an output terminal), shown
in Media 3, contains the number of completed
iterations. The iteration count always starts at zero. During
the first iteration, the iteration terminal returns
0.
The For Loop differs from the While
Loop in that the For Loop executes a set
number of times. A While Loop stops executing the
subdiagram only if the value at the conditional terminal exists.
The For Loop in Figure 2 generates a
random number every second for
 |
Wait Functions
The Wait Until Next ms Multiple function, shown
in Media 5, monitors a millisecond counter and
waits until the millisecond counter reaches a multiple of the
amount you specify. Use this function to synchronize
activities. Place this function within a loop to control the
loop execution rate.
The Wait (ms) function, shown in Media 6, adds the wait time to the code execution
time. This can cause a problem if code execution time is
variable.
Note:
TheTime Delay Express VI, located on the
Functions>>Execution Control palette, behaves
similar to the Wait (ms) function with the
addition of built-in error clusters. Refer to Clusters for more information about error
clusters.
Wait Functions
LabVIEW can represent numeric data types as signed or unsigned integers (8-bit, 16-bit, or 32-bit), floating-point numeric values (single-, double-, or extended-precision), or complex numeric values (single-, double-, or extended-precision). When you wire two or more numeric inputs of different representations to a function, the function usually returns output in the larger or wider format. The functions coerce the smaller representations to the widest representation before execution, and LabVIEW places a coercion dot on the terminal where the conversion takes place.
For example, the For Loop count terminal is a
32-bit signed integer. If you wire a double-precision,
floating-point numeric to the count terminal, LabVIEW converts
the numeric to a 32-bit signed integer. A coercion dot appears
on the count terminal of the first For Loop, as
shown in Figure 3.
 |
If you wire two different numeric data types to a numeric function that expects the inputs to be the same data type, LabVIEW converts one of the terminals to the same representation as the other terminal. LabVIEW chooses the representation that uses more bits. If the number of bits is the same, LabVIEW chooses unsigned over signed.
In the example in Figure 4, a 32-bit
signed integer (I32) and a double-precision, floating-point
numeric value (DBL) are wired to the Divide
function. The 32-bit signed integer is coerced since it uses
fewer bits than the double-precision, floating-point numeric
value.
 |
To change the representation of a numeric object, right-click
the object and select Representation from the
shortcut menu. Select the data type that best represents the
data.ut data types.
When LabVIEW converts double-precision, floating-point numeric
values to integers, it rounds to the nearest integer. LabVIEW
rounds x.5 to the nearest even integer. For
example, LabVIEW rounds
Refer to the Data Types section of Introduction to LabVIEW, of this manual or to the LabVIEW Help for more information about data types.
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This work is licensed by National Instruments under a Creative Commons Attribution License (CC-BY 1.0), and is an Open Educational Resource.
Last edited by Elizabeth Gregory on Aug 12, 2004 11:04 pm GMT-5.
"A full introductory course on programming with LabVIEW."