內(nèi)容說明ni培訓(xùn)lesson file io techniques

1、Lesson 6File I/O TechniquesFile FormatsBinary FilesTDMS FilesA. File FormatsAt their lowest level, all files written to your computers hard drive are a series of bits2BinaryTDMSASCIIA. File FormatsASCIITDMSDirect BinaryNumeric PrecisionGoodBestBestShare data Best (Any program easily)Better (NI Progr

2、ams easily)Good (only with detailed metadata)EfficiencyGoodBestBestIdeal UseShare data with other programs when file space and numeric precision are not importantShare data with programs when storing simple array data and metadataStore numeric data compactly with ability to random access B. Binary F

3、ilesUse Binary File functions to interact directly with a binary file4B. Binary FilesBits/Bytes?A bit is a single binary valueEach bit is either on or off and is represented by a 1 or a 0A byte is a series of 8 bits0bit00000000byte5B. Binary FilesStoring Boolean ValuesLabVIEW represents Boolean valu

4、es as 8-bit values in a binary fileEight zeroes represents False 00000000Any other value represents True00000001, 01000110, 11111111, and so onFiles are divided into byte-sized chunks, making them much easier to read and process6B. Binary FilesStoring Boolean ValuesFile ContentsMethod A00000001 0000

5、0001 00000000 00000001 00000000 00000001Method B001010117B. Binary FilesStoring IntegersBinary ValueU8 Value000000000000000011000000102111111112558B. Binary FilesStoring IntegersMulti-byte integers are broken into separate bytes and are stored in files in either little-endian or big-endian byte orde

6、rUsing the Write to Binary File function, you can choose whether you store your data in little-endian or big-endian format9U32 ValueLittle-endian ValueBig-endian Value1000000010000000000000000 0000000000000000 0000000000000000 00000001B. Binary FilesStoring Other Data TypesStrings are stored as a se

7、ries of unsigned 8-bit integers, each of which has a value in the ASCII Character Code Equivalents TableThis means that no difference exists between writing strings with the Binary File Functions and writing them with the Text File FunctionsClusters are best represented in binary files by using Data

8、log Files10B. Binary FilesStoring ArraysArrays are represented as a sequential list of the elementsElement representation depends upon the element typeA header contains a 32-bit integer representing the size of each dimensionExample A 2D array with a header contains: row integer, column integer, the

9、n array data11B. Binary FilesSequential/Random AccessTwo methods of accessing data: Sequential AccessRead each item in order, starting at the beginning of a fileRandom AccessAccess data at an arbitrary point within the file12B. Binary FilesSequential AccessTo sequentially access all of the data in a

10、 file, you can call the Get File Size function and use the result to calculate the number of items in the file, based upon the size of each item and the layout of the fileYou can then wire the number of items to the count terminal of the Read from Binary File function13B. Binary FilesRandom AccessUs

11、e the Set File Position function to set the read offset to the point in the file you want to begin readingThe offset is in bytes; therefore, you must calculate the offset based upon the layout of the file14B. Binary FilesDatalogDatalog is a specific type of binary file, designed for storing a list o

12、f records to a fileEach record is represented by a cluster and can contain multiple pieces of data with any data type15B. Binary FilesDatalog Random Access16Exercise 6-1Bitmap File Writer VIUse Binary File I/O to write a file with a specified format.17Exercise 6-1Bitmap File Writer VIWould the gener

13、ated bitmap file still be correct if you swapped the BITMAPFILEHEADER VI and the BITMAPINFOHEADER VI? Why?It was not necessary to specify the byte-order in the Write to Binary File functions. Why not?C. TDMS FilesTDMSTechnical Data Management StreamingUse TDMS files for the following purposes: To st

14、ore test or measurement dataTo create a structure for grouping your dataTo store information about your dataTo read and write data at high speeds19C. TDMS FilesFile FormatTDMS fileBinary file (.tdms) that contains data and stores properties about the dataTDMS_Index fileBinary index file (*.tdms_inde

15、x) that provides consolidated information on all the attributes and pointers in the TDMS file Speeds up access to the data while readingAutomatically regenerated if lostTDMS file format internal structure is publicly documented20C. TDMS FilesData HierarchyChannel Stores measurement signals or raw da

16、ta in a TDMS fileEach channel can have properties describing the dataThe data stored in the signal is stored as binary data on disk to conserve disk space and improve efficiencyChannel GroupSegment of a TDMS file that contains properties and one or more channelsUse channel groups to organize your da

17、ta and to store information that applies to multiple channels21C. TDMS FilesAccessing TDMS FilesUse the Write to Measurement File and Read from Measurement File Express VIs Allows you to quickly save and retrieve data from TDMS format Very little control over your data grouping and propertiesUse the

18、 TDM Streaming APISet of functions for opening, writing to, reading from, and closing TDMS filesAllows you to organize your data into channel groups and channelsUse the TDM Excel Add-In ToolAllows you to read TDMS files in Microsoft Excel22C. TDMS FilesTDM Streaming API 23C. TDMS FilesWrite DataStre

19、ams data to the specified TDMS fileData subset to write is determined by group name and channel name(s) inputs24C. TDMS FilesRead DataReads the specified TDMS file and returns data from the specified channel and/or channel group25C. TDMS FilesSet PropertiesSets the properties of the TDMS file, chann

20、el group, or channel26C. TDMS FilesGet PropertiesReturns the properties of the TDMS file, channel group, or channel27C. TDMS FilesFile ViewerOpens TDMS file and presents the file data in the TDMS File Viewer dialog box28C. TDMS FilesTDM Excel Add-In ToolExercise 6-2Read TDMS FilesLearn how to read d

21、ata from a TDMS file.30Exercise 6-2Read TDMS FilesHow difficult would it be to implement the logger and reader using ASCII files? Custom Binary files?SummaryQuizYou need to store test results and organize the data into descriptive groups. In the future, you need to efficiently view the test results

22、by group. Which file storage format should you use?Tab-delimited ASCIICustom binary formatTDMSDatalog32SummaryQuiz AnswerYou need to store test results and organize the data into descriptive groups. In the future, you need to efficiently view the test results by group. Which file storage format shou

23、ld you use?Tab-delimited ASCIICustom binary formatTDMSDatalog33SummaryQuizYou need to write a program which saves Portable Network Graphics (PNG) image files. Which file storage method should you use?Storage file VIsBinary file functionsASCII file VIsDatalog file VIs34SummaryQuiz AnswerYou need to w

24、rite a program which saves Portable Network Graphics (PNG) image files. Which file storage method should you use?Storage file VIsBinary file functionsASCII file VIsDatalog file VIs35SummaryQuizYou need to store data that other engineers will later analyze with Microsoft Excel. Which file storage format should you use?Tab-delimited ASCIICustom binary formatTDMSDatalog36SummaryQuiz AnswerYou need to store data that other engineers will later analyze with Microsoft Excel. Which file storage format should you use?Tab-delimited ASCIICustom binary formatTDMSDatalog37SummaryQuiz00