Saturday, September 25, 2010

String Functions

Use the String functions located on the Functions>>All Functions>>String palette to edit and manipulate strings on the block diagram. String functions include the following:
  • String Length: Returns in length the number of characters (bytes) string, including space characters. For example, the String Length function returns a length of 19 for the following string:
         The quick brown fox
  • Concatenate Strings: Concatenates input strings and 1D arrays of strings into a single output string. For array inputs, this function concatenates each element of the array. Add inputs to the function by right-clicking an input and selecting Add Input from the shortcut menu or by resizing the function. For example, concatenate the previous string with the following array of strings:
    TABLE 1
    The Concatenate Strings function returns the following string:
         The quick brown fox jumped over the lazy dog.
  • String Subset: Returns the substring of the input string beginning at offset and containing length number of characters. The offset of the first character in string is 0. For example, if you use the previous string as the input, the String Subset function returns the following substring for an offset of 4 and a length of 5:
  • Match Pattern: Searches for regular expression in string beginning at offset, and if it finds a match, splits string into three substrings. If no match is found, match substring is empty and offset past match is -1. For example, use a regular expression of : and use the following string as the input:
         VOLTS DC: +1.22863E+1;
    The Match Pattern function returns a before substring of VOLTS DC, a match substring of :, an after substring of +1.22863E+1;, and an offset past match of 9.

Converting Numeric Values to Strings with the Build Text Express VI

Use the Build Text Express VI to convert numeric values into strings. The Build Text Express VI, located on the Functions>>Output palette, concatenates an input string. If the input is not a string, this Express VI converts the input into a string based on the configuration of the Express VI.
When you place the Build Text Express VI on the block diagram, the Configure Build Text dialog box appears. The dialog box shows the Express VI configured to accept one input,voltage, and change it to a fractional number with a precision of 4. The input concatenates on the end of the string Voltage is. A space has been added to the end of the Voltage is string.
Figure 1
Figure 1 (ConfigBuildText.png)
This configuration produces the block diagram shown in Figure 2. A probe has been added to view the value of the output string. The Build Text Express VI concatenates theBeginning Text input, in this case the voltage value, at the end of the configured text.
Figure 2
Figure 2 (BuildTextEx.png)

Converting Strings to Numeric Values with the Scan From String Function

The Scan From String function converts a string containing valid numeric characters, such as 0-9+-eE, and period (.), to a numeric value. This function scans the input stringand converts the string according to format string. Use this function when you know the exact format of the input text. This function can scan input string into various data types, such as numeric or Boolean, based on the format string. Resize the function to increase the number of outputs.


For example, use a format string of %f, an initial search location of 8, and VOLTS DC+1.28E+2 as the input string, to produce an output of 128, as shown in the block diagram shown in Figure 3. Change the precision of the output by changing the precision of the indicator.
Figure 3
Figure 3 (ScanStringEx.png)
In format string% begins the format specifier and f indicates a floating-point numeric with fractional format. Right-click the function and select Edit Scan String from the shortcut menu to create or edit a format string. The Edit Scan String dialog box shows a configuration for the format string %4f.
Figure 4
Figure 4 (editformatex.png)
Refer to the LabVIEW Help for more information about format specifier syntax.

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