For loop: Difference between revisions

Latest revision as of 15:00, 7 May 2023

Object information
Owning palette(s)	Structures palette
Type	Structure
Requires	Basic Development Environment
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The For Loop is a structure that will execute code contained within a finite number of times. The number of times that the loop will execute is initially determined by the numeric value written to the "N" terminal of the loop, or by the size of an array wired as an input to the loop (when indexing is enabled for the input tunnel). The latter method of specifying the number of iterations is similar to the For-Each structure in many text-based programming languages.

For Loop - Will execute "your code" ten times.

Usage

Outputs from a For Loop will be an array by default unless alternate behavior for the tunnel is selected by right-clicking on the tunnel and selecting a behavior under "Tunnel Mode".

How do I stop a For Loop?

In LabVIEW 8.20 and earlier, there was no way to stop the execution of a For Loop until the requisite number of iterations have been met. In those earlier LabVIEW versions, a While Loop structure would have to be used, in addition to having input and/or output tunnels with indexing enabled.

In LabVIEW 8.5, a conditional terminal was made available to a For Loop structure, which functions similar to that which is used in a While Loop structure. The conditional terminal can be made visible by right-clicking on the border of a For Loop and selecting "Conditional Terminal". This conditional terminal is useful if a condition has been met that would make further iterations of the loop unnecessary. A simple example of this could be if indexing a 10 element array in search of a particular value, then finding the desired value at the fourth element. The loop could then be stopped without having to continue to iterate through the remaining elements.

Best practice

Why doesn't my For Loop return a value?

As a good programming rule in LabVIEW, NEVER output a value from a For Loop with indexing disabled. When the loop does not execute (0 iteration) a non indexing output stays undefined and will hold any garbage left there by previous memory usage. This is because the wire output from the For Loop has no code or data source to get the value from when the loop does not execute. Instead, use a shift register (SR) to output the value and at the left SR enter a default value for the case when the loop does not execute. When the For Loop does not execute this default value of the left SR is passed to the right SR. Similarly passing a Refnum through a For Loop that never executes (0 iterations) destroys the reference. Either pass the reference using a SR as described above or wire around the loop. Note that While Loop always execute at least once so the outputs are always defined.

Tips and tricks

Wire an error cluster to the conditional terminal to terminate the loop on an error.

History

Version	Change(s)
	In LabVIEW 2020 the Iteration Terminal visibility can be toggled.
	In LabVIEW 8.5 the Conditional Terminal was added to allow For Loops to end early.

External Links

NI.com Tutorial on For Loops and While Loops

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+{{TOCright}}
+{{LabVIEW Palette Object Information|palette=Functions Palette/Programming/Structures{{!}}Structures palette|type=structure|icon=Functions Palette - Programming - Structures - For Loop.png}}
-== What does 0ms wait function do in a while loop ==
+The '''For Loop''' is a structure that will execute code contained within a finite number of times.  The number of times that the loop will execute is initially determined by the numeric value written to the "N" terminal of the loop, or by the size of an array wired as an input to the loop (when indexing is enabled for the input tunnel).  The latter method of specifying the number of iterations is similar to the For-Each structure in many text-based programming languages.
-If you have multiple loops in your application that don't need to run as fast as possible, then placing a 1ms delay will limit your loops to at most 1000 iterations per second. Without a delay, the loop rate can be in the millions of iterations per second range depending on what is inside it. So that means that your CPU has about 1/1000th as much work to do and can go off and tend to other tasks. If your loop was already taking several ms, then the 1ms delay is likely in parallel and it won't affect the loop speed noticeably. So placing the delay in the loop can drop your CPU usage noticably and allow time for the OS to do other work like send messages that your application may be waiting for, etc.
-But what does a wait of 0 ms do? Let's consider the LV execution system to be a xerox copying machine. Everyone that has something to copy heads for the copying machine and lines up. If you have never had the pleasure of waiting at a copy machine, then consider any other time consuming task where people wait in line. Everytime a LV diagram executes a wait function, it is like releasing the copier and staying out of the line for some time delay. After the delay has passed, you will get back in line to make your next copy. A wait of 0 ms is like immediately going to the end of the line to let a smaller copy-task take place. If nobody is in line behind you, you immediately start your next copy task. If someone is in line, it lets them take a turn.
+[[File:ForLoop.PNG|thumb|For Loop - Will execute "your code" ten times.]]
+[[File:ForLoop Indexing.png|thumb|For Loop With Indexed Input - Will execute "your code" ten times.]]
+[[File:ForLoop Indexing Conditional.png|thumb|For Loop With Conditional Terminal - Will execute until first TRUE element, which would be at index 4.  The remaining elements will not be indexed.]]
-This 0 ms wait is a pretty cool little trick to make LabVIEW loop parallelism less chunky. It naturally adds some overhead because the loops have some setup time, but when the loops are doing significant work it is tiny. Use it whenever you think you need to, but beware that if some loops don't have any delay, they are still going to
+== Usage ==
+Outputs from a For Loop will be an array by default unless alternate behavior for the tunnel is selected by right-clicking on the tunnel and selecting a behavior under "Tunnel Mode".
-hog the CPU, and the wait of 0 ms may turn into much larger waits because the loops with no waits play by different rules.
+=== How do I stop a For Loop? ===
+In [[LabVIEW 8.20]] and earlier, there was no way to stop the execution of a For Loop until the requisite number of iterations have been met.  In those earlier LabVIEW versions, a [[While Loop structure]] would have to be used, in addition to having input and/or output tunnels with indexing enabled.
-== Is the while loop really a while loop? ==
+In [[LabVIEW 8.5]], a conditional terminal was made available to a For Loop structure, which functions similar to that which is used in a [[While Loop structure]].  The conditional terminal can be made visible by right-clicking on the border of a For Loop and selecting "Conditional Terminal".  This conditional terminal is useful if a condition has been met that would make further iterations of the loop unnecessary.  A simple example of this could be if indexing a 10 element array in search of a particular value, then finding the desired value at the fourth element.  The loop could then be stopped without having to continue to iterate through the remaining elements.
-No. Strictly speaking the while loop is a "do while" (in C) or repeat until (in PASCAL) programming structure. In other words the loop always executes at least once, and the Boolean test for continuation is done at the end of the loop execution. With a true while loop in a text based program the test is done prior to executing any of the commands contained within the loop.
-To create a "real" while loop wire the output of your terminating condition to a case structure surrounding the rest of the code within the loop.
+== Best practice ==
+=== Why doesn't my For Loop return a value? ===
+As a good programming rule in LabVIEW, NEVER output a value from a For Loop with indexing disabled. When the loop does not execute (0 iteration) a non indexing output stays undefined and will hold any garbage left there by previous memory usage. This is because the wire output from the For Loop has no code or data source to get the value from when the loop does not execute. Instead, use a shift register (SR) to output the value and at the left SR enter a default value for the case when the loop does not execute. When the For Loop does not execute this default value of the left SR is passed to the right SR. Similarly passing a [[Refnum]] through a For Loop that never executes (0 iterations) destroys the reference. Either pass the reference using a SR as described above or wire around the loop. Note that [[While Loop]] always execute at least once so the outputs are always defined.
-This is of course true, but then there is no real difference between a while loop as you describe it and a for loop in C either. Of course the syntax changes but the functionality is really equivalent.
+== Tips and tricks ==
-And instead of putting the case structure inside the while loop you could just as easily put the while loop in the case structure and get the same behaviour and IMO it looks more clear what is happening.
+* Wire an [[error cluster]] to the conditional terminal to terminate the loop on an error.
-Also true, but then you loose having a shift register from the loop hold variable(s) that are part of the exit test, unless you then wrap the whole thing in another loop.
+== History ==
+{| class="wikitable"
+! Version
+! Change(s)
+|-
+|[[File:LV2020.png|frameless|border|64x64px|LabVIEW 2020|link=LabVIEW 2020]]
+| In [[LabVIEW 2020]] the Iteration Terminal visibility can be toggled.
+|-
+|[[File:LV8-2013.png|frameless|border|64x64px|LabVIEW 8.5|link=LabVIEW 8.5]]
+| In [[LabVIEW 8.5]] the Conditional Terminal was added to allow For Loops to end early.
+|}
-[QUOTE]
+== See Also ==
-and the Boolean test for continuation is done at the end of the loop execution.[/quote]
+* [[ForLoop class]]
-This is a little misleading, particularly for new [[LabVIEW]] programmers. The boolean test for continuation is done as soon as possible... it only waits until the end of loop execution if there is a data dependency. With no dependacy, the loop will decide whether to execute again before/in parallel with any other code in the loop.
+== External Links ==
+* [http://www.ni.com/white-paper/7588/en/ NI.com Tutorial on For Loops and While Loops]
-I have seen plenty of people getting one extra iteration than they expected because of this.
+[[Category:Structures Palette]]