LabVIEW for Dummiees
Section 2

Back to the Beginning

For, While, and Sequence Loops

These are very similar to the for and while loops encountered in your first computing class.
A For Loop performs an operation for a certain number of times, say ten. The big N in the upper left hand corner is the set number of times for the loop to execute.
A While Loop performs an operation while a certain condition is true, say the end of the file has not been reached. The little round arrow in the lower right hand corner is the condition, that is, to keep the loop executing or not. This has to be wired with a Boolean operator or the loop will execute only once, negating the purpose of making a loop. Usually uses a comparison, Boolean logic operator, or simple control.
The little blue boxed i in the lower left hand corner is the iteration number. It represents the number of times the loop has been executed. Needed if you want an operation to occur at a certain step.

A Sequence Loop, which looks like a piece of film, performs an operation after one has been completed, say after acquiring the data, analyze the data. To add a Sequence frame, right click on the frame and choose Add Frame After or Add Frame Before or Duplicate Frame. They will be used in sequential order according to the numbering scheme on the top center. Many times you will want the data from a previous frame to be used in a later sequence, Add Sequence Local, where you can attach any wire to be brought out in a later frame. An arrow will show the direction of data flow. If you move a sequence local, it will move the same way in the other frames. Little programming tip: If you are acquiring data aver a several minutes and need to enter some physical constants needed after acquisition is complete, place the control for the physical constants inside the later sequence, not outside the loop. This way while the data is acquiring, your data can be entered SAVING TIME! This works because LabVIEW hasn't started on your data yet.

You may notice some problems when an Array enters a For Loop. What happens is the array is auto-indexed, i.e. the array's value at the index of the iteration is used. If you don't like, click on the little black "tunnel", and choose Disable Indexing. The Loop can also be used to build an array, by indexing the values to the iteration number. Look at this for a comparison. As you can see, the while and for loops both have this feature. Once again:
Enable Indexing allows you to build arrays for output, or auto-index input arrays
Disable Indexing allows you to just take the values as they come in or out.
By placing a Formula Node in the For Loop, and array can be evaluated, the number of times to evaluate is the size of the array, and the array will be automatically rebuilt upon exiting.

In For and While Loops a Shift Register can be added to the loop by right clicking on the loop and selecting Add Shift Register. This is invaluable when you need to use data from a previous loop in order to calculate the next loop. By choosing Add Element, you can use the element from two iterations ago. For example, by adding two elements, you can use the data from the previous three iterations.

Arrays

Arrays are invaluable. Nearly every application will need them. You can define vectors (a 1-D array) or matrices (a 2-D array) or a general N dimensional array, like tensors. Although Arrays may resemble vectors and matricies, they are not the same thing. This is how the basics work. Same principals for subtraction and division.

Scalar + Scalar = Scalar; 1+1=2
Scalar + Array  = Array;  1+[1,2,3]=[2,3,4]
Array  + Array  = Array;  [1,2,3]+[4,5,6]=[5,7,9], but [1,2]+[3,4,5]=[4,6]
Scalar * Scalar = Scalar; 4*5=20
Scalar * Array  = Array;  4*[4,5,6]=[16,20,24]
Array  * Array  = Array;  [1,3,3]*[4,6,2]=[4,18,6], but [1,3]*[4,6,2]=[4,18]

1. Build array. First choose the build array element. Go to the bottom corner of the node and click and drag till it's doubled. The regualr mouse arrow will change to a little corner. Create two numbers, and wire them to each of the inputs, and create and indicator. Run and on the front panel you will see the numbers combined into a 1-D array. Try with Boolean values and strings.
2. Initialize Array is invaluable in loops especially with a Shift Register. First connect an initializing value to the element data, usually zero. If you need an N-D array, enlarge it like the build array. Enter the dimension size(s) which is the length (and width) of the array. Create an indicator and see if it worked. A little note, you can't initialize a random array by connecting the random operator to the element data. It will be random, but the array will have the same value.
3. Index Array allows you to find the value at an index. Simply connect the array and list what index you want. If you need a column in a matrix you can choose the Disable Indexing on the boxes in the lower left corner to make a 2-D matrix into a 1-D vector. Be careful when using, it can be a bit tricky, check with known values before you have rely on the program. See this on extracting a row/column from a matrix.
4. Transpose 2D Array simply flips the 2-D array over. That is, the rows and columns are interchanged.
5. Array Size simply tells the size of the array. If it is a 1-D array with 15 elements, it returns 15. A 2-D array with 10 by 5 elements returns [10,5]
6. Interleave 1D Arrays - Takes two arrays of the same size and type and interleaves them. For example, interleaving the two arrays [1,2,3] and [0,6,9] would result [1,0,2,6,3,9]. Order is important. If the sizes are unequal, the size will be only twice the smallest array. Try it and see.
7. Decimate Arrays - Undoes the Interleave Arrays operation.
8. Array Max & Min - Finds the Max and Min values and the indexes.
9. Sort 1D Array - Sorts the Array in Accending Order. Either String or Numeric.
10. Add Array Elements & Multiply Array Elements - found under the Numeric pallet allow you to either find the sum or product of all the elements in an array.

Strings

"This is a test to see the 	 tab and 
end of line backslash codes."

• Concatenate Strings is used to combine multiple strings and can be expanded in the same way as an array. If you need a delimiter between strings, be sure to include it in the Concatenate Strings. Very useful when need to combine a descriptive string with an array.
• Match Pattern - is very useful when extracting an alphanumeric character string from a file, such as contains: name \s 10 20 30 .
  1. name \s 10 20 30 is the String
  2. \s is the Regular Expression
  3. name is the Before Substring
  4. \s is the Match Substring
  5. 10 20 30 is After Substring
  6. The Offset is where to start.
• Format Into String - takes a number and converts it into a string. You can't just wire a number with a string. Also, you need to set the format string.
  • Format - determines to what precision you want the number represented. Even if it is only an integer, there are the invisible zeros. These are some you may need:
    %.3f which is a 3 decimal floating point number. This is the default when saving files.
    %.0f - an integer
    %.5e - a floating point number to 5 decimal places in scientific notation
    If you need others look in the String Syntax Elements Table >> Conversion Codes
• Array to Spreadsheet String - Works like the Format Into String except you need to insert a delimiter, but the default is Tab. A problem you may encounter is the need to include the data from several arrays on one line in a file. However, the array is converted with an end of line break. The way to remove this is by using the Match Pattern, using the EOL character as the Regular Expression, and the data is contained in the Before Substring. Then simply use Concatenate Strings to combine the strings. Be sure to include Tabs between the strings and an EOL at the end.
  
• Spreadsheet String to Array - Transforms a numeric delimited string into an array. I use the way it appears in my altered Read from Text File.vi.

File Creation

• Have all the necessary data included in your file(s)
• Know where all the data is being placed

1. Write to Spreadsheet File.vi
2. Read from Spreadsheet File.vi
3. Write Characters to File.vi
4. Read Characters from File.vi

• FILE PATH - The location of the file you want to read or write, such as C:\Windows\Desktop\myfile.txt
  If you are working with a large number of files that need to combine strings to make a file name, you need to use the String to File Path
• DELIMITER - The special pattern between the data entries, defaulted to Tab, no real reason the change unless you are using another program to read the file that needs another delimiter. For example, Mathematica needs comma delimited files.
• APPEND TO FILE - Put new data in the file, defaulted to False. Change to True especially if you are acquiring data over a period of time
• CHARACTER STRING - The data has been converted to an alphanumeric sequence, needed if data contains words and numbers.
• EOF (End of File) If you are reading the lines from a file, in a loop you will need to know when the end has been reached. NOTE: If this is in a loop you will need the Boolean NOT to keep the loop operating.
• Format - See under strings
• 1D Data and 2D Data - are used when you are using Array data in vector or matrix form. NOTE: If you are extracting the data with a mixture of types, numeric and symbolic, only the numeric will be extracted correctly.
• TRANSPOSE - flip the array over. Use when 1-D array very large (>256) or you just want a column vector. Useful when you are going to analyze the data later in a spreadsheet.

Time and Dialog

When you have written your programs you sometimes need to inform others about errors when entering data. This is done by using a Dialog Box

Graphing

1. Waveform which only needs the Y component, i.e. uses a 1-D array. It graphs the array's data over time.
2. X,Y Plot - Plots X vs. Y The X-Y plot is a Cluster consisting of Bundled Elements. This is a bit difficult to explain. So I suggest you look at this picture. Try reproducing it.
3. Intensity - Plots the values of a matrix, i.e. uses a 2-D array. Kind of like making a picture.

1. Label & Caption are basically the same thing allowing you to name your graph
2. Legend is all important for your display of data. You can choose
   • Common Plots - Waveform, points, points and line, bar graph, base fill. I used the default usually
   • Point Style, Line Style, Line Width, & Bar Plots, I think are self explanatory
   • Fill Base Line - If you want to fill the region under the graph. I never used it.
   • Interpolation - Only points, or connect the dots either as a square wave or as a even wave
   • Color - Change the plots color. Very useful when examining more than one data set at a time
   • X-Scale & Y-Scale I never did anything with it
3. Pallet - Allows you to zoom, set the precision and expression of the axes, or to auto-scale
4. Cursor Display - Move a cursor around the graph to study the plot. Lock to the plot, to point or allow to drag (the padlock). The little rhombus on the right allows you to make minor adjustments either up, down, right, or left by clicking. Only on appears on graphs. By using attribute nodes you record certain points. Coming later
5. X-Scale & Y-Scale - Show the X or Y scale
6. Scrollbar - Allows you to scroll around the graph in the x direction. Only appears on Charts.
7. Chart History Length - Charts have a buffer size (default 1024 data points) so you can input numbers and they will be displayed as a graph. You can change it, but don't be ridiculous, the buffer is usually sufficient.

You can also change the graph ranges by clicking on the upper and lower bounds and replacing the numbers. If you want a logarithmic plot, you can either insert a Logarithm Base 10, or choose X Scale, Y Scale >> Formatting... to change the number to log, also once there you can change the digits of precision, the notation, and if you want graph lines.

Continue to Section 3