EET 2259 Unit 13 Strings and File I/O

Floyd, Digital Fundamentals, 10th ed

EET 2259 Unit 13Strings and File I/O

Read Bishop, Chapter 9.

Lab #13 and Homework #13 due next week.


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)


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)


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.

http://www.asciitable.com/


String Functions

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


String Subset Function

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


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?


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?


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.


Scan From String and Format Into String You can use two “super-functions” to 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.


format string Input These two functions have a format

string input that lets you say exactly how the number is formatted.

The example below, which uses the Format Into String function, will format the number into a string containing at least 6 characters, with 3 digits following the decimal point.


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


File I/O Functions and VIs on 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 …


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.


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.


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


Which Kind of Binary File? LabVIEW can work with the following

specific kinds of binary files: Generic binary files.

LabVIEW TDMS (TechnicalData Management Streaming) files

LabVIEW Datalog Files

ZIP files


High-Level versus Intermediate-Level

The items on the File I/O palette can be categorized as high-level or intermediate-level. The high-level VIs are

easier to use. They call the intermediate-level functions. The intermediate-level functions give you more control, but require more work on your part.

(Bishop, p. 412)


Three Steps in File I/O

Most file I/O operations involve three steps:

1. Open an existing file or create a new file.2. Read data from or write data to the file.3. 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)


A Simple Text-File Example Writing to a text file:

Reading it back from the file:


A Simple Binary-File Example Writing to a binary file:

Reading it back from the file:

EET 2259 Unit 13 Strings and File I/O

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