EET 2259 Unit 13 Strings and File I/O

1. EET 2259 Unit 13Strings and File I/O • Read Bishop, Chapter 9. • Lab #13 and Homework #13 due next week.

2. Strings • A string is a sequence of characters. • Characters include letters, numerals, punctuation marks, spaces, and certain non-displayable characters such as linefeed and tab. (Bishop, p. 400)

3. Uses of Strings • Strings have several uses in LabVIEW: • Displaying text messages to the user or allowing the user to enter text. • Sending commands over cables to instruments to control those instruments. • Storing data in a file on a disk. (Bishop, p. 400)

4. String Display Modes • String controls and string indicators can be configured to display their contents in any of the following modes: • Normal Display (the default) • \ Codes Display • Password Display • Hex Display • Hex Display shows the ASCII codes for the characters.

5. String Functions • You’re already familiar with some of the functions, constants, and VIs on the Programming > String palette.

6. String Subset • The function named String Subset lets you pull out a piece of a string, starting at any point in the string that you want.

7. Numbers versus Numeric Strings • To understand many of the other functions and VIs on this palette, answer the following question: What’s the difference between the two indicators shown below?

8. Numbers Functions versus String Functions • Depending on what you’re trying to do with numeric information, you may want to have that information represented as numeric data or as string data. • Examples: What if you want to…. • Multiply it by 2? • Find out how many digits it contains? • Round it to the nearest integer? • See whether it contains a decimal point?

9. Converting Between Numbers and Numeric Strings • The functions on the Programming > String >String/Number Conversion palette let you convert between numeric data and string data. • Functions in the 1st row convert numbers to string. • Functions in the 2nd row convert strings to numbers.

10. Scan From String and Format Into String • I recommend instead using two “super-functions” that can do the work of all the other conversion functions: • Use Scan From String to convert strings to numbers. • Use Format Into String to convert numbers to strings.

11. format string Input • These two functions have a format string input that lets you say exactly how the number is formatted. • In the examplebelow, LabVIEW will format the number into a string containing at least 6 characters, with 3 digits following the decimal point.

12. Other String Functions • LabVIEW provides other useful string functions not discussed in the textbook, including: • String Length • Concatenate Strings • String Subset • To Upper Case & To Lower Case • Search & Replace String

13. File I/O • Functions and VIson the Programming > File I/O palette let you: • Read data from files on disk • Write data to files on disk • Move, copy, rename, and delete files (and directories) on disk • And more …

14. Which Kind of File? • This is a large and complex topic, because LabVIEW can read from or write to many different kinds of files. • These file types can be grouped into two broad categories: • Text files, whose contents are in ASCII format. You can open and read these files in a text editor or word processor. • Binary files, which look like gibberish if you open them in a text editor or word processor.

15. Choosing Text File versus Binary File • If you have to save LabVIEW data to a file, how do you decide which kind of file? It’s a trade-off. • The big advantage of text files is portability: you can open them in other programs. • The big advantage of binary files is efficiency: they take up less disk space and can be processed more quickly than text files.

16. Which Kind of Text File? • LabVIEW can work with the following specific kinds of text files: • Generic text files, which can contain any text • Spreadsheet files, which contain numbers in rows and columns • LabVIEW Measurement (.lvm) files • Configuration (.ini) files • XML files

17. Which Kind of Binary File? • LabVIEW can work with the following specific kinds of binary files: • Generic binary files. • LabVIEWTDMS(TechnicalData Management Streaming) files • LabVIEWDatalog Files • ZIP files

18. High-Level versus Intermediate-Level • The items on the File I/O palette can be categorized as high-level or intermediate-level. • The intermediate-level functions give you more control, but require more work on your part. • The high-level VIs are easier to use. They call the intermediate-level functions. (Bishop, p. 412)

19. Three Steps in File I/O • Most file I/O operations involve three steps: • Open an existing file or create a new file. • Read data from or write data to the file. • Close the file. • The high-level VIs combine all three of these steps into a single VI. • On the other hand, with intermediate-level functions, you must do each step separately, as on the next slide. (Bishop, pp. 410-411)

20. A Simple Text-File Example • Writing to a text file: • Reading it back from the file:

21. A Simple Binary-File Example • Writing to a binary file: • Reading it back from the file: