Fö 6: Filhantering. DN1212 Numeriska metoder och grundläggande programmering. Pawel Herman. KTH/CB

Transcription

1 Fö 6: Filhantering DN1212 Numeriska metoder och grundläggande programmering KTH/CB 22 februari Pawel 2013Herman

2 Lecture outline Recapitulation of what we already know about I/O Matlab format with save and load Handling ASCII files with textread and fileread with handles text (formatted) files (fprintf, fscanf, textscan, fgets, fgetl) binary files (fread, fwrite) GUI based approach to importing data with uiimport Fö 6 Filhantering 2

3 What have we already learnt about I/O? input to read in data from the standard input a = input( Input a variable: ) s = input( Write a name:, s ) disp, fprintf to write text data to the standard output disp( Text to write ) fprintf( The value is %f \n, a) 3

4 Saving and loading Matlab variables save filename [variable_list] [options] load filename [variable_list] [options] variable_list are variables in the workspace by default, the entire workspace is saved options: -mat (Matlab format: filename.mat), -ascii(text format) -double (16 digits) -append (only for save) 4

5 Saving and loading Matlab variables Functions can be also called using the following syntax: save/load(filename,[variable_list], [options]) However, all parameters must be string variables or literals, e.g. save( myfile, variable1, variable2, -mat ); Unnecessary options since.mat extension is assumed by default 5

6 Saving and loading Matlab variables Functions can be also called using the following syntax: save/load(filename,[variable_list], [options]) However, all parameters must be string variables or literals, e.g. save( myfile, variable1, variable2, -mat ); when using load with ASCII data, the number of elements in rows in the file must agree textread is more flexible, offers richer functionality for ASCII still, however, when possible it is recommended to rely on *.mat file. 6

7 The textread function [a,b,c, ]=textread(filename, format, n) James Jones O Yes Sally Smith A No [name, surname, blood, coefficient, age, answer ] = textread( test.dat, %s %s %s %f %d %s ) The number of output arguments must correspond to the number of formatting symbols and columns in the file. 7

8 The textread function [a,b,c, ]=textread(filename, format, n) James Jones O Yes Sally Smith A No [name, surname, blood, age, answer ] = textread( test.dat, %s %s %s %*f %d %s ) The elements (columns) can be skipped, then the number of output arguments must correspond to the number of unskipped columns 8

9 Additional ASCII I/O functions str = fileread(filename) returns the entire content of a file as a string result = dlmread(filename, delimiter) reads ASCII delimited file to a variable result delimiter can be '\t or, or ; etc. (e.g., dlmread( myfile.txt, \t )) dlmwrite(filename, array, attribute) write ASCII delimited file attribute delimiter newline (can be pc or unix ) precision (compare with fread/fwrite) 9

10 Introduction to Matlab file processing Matlab s mechanisms are universal and allow to operate on I/O devices (disk, memory stick, standard I/O etc.) File handle or file id (fid) is a file identifier fid 0 is reserved for stdin,1forstdout and fid 2 for stderr file is assigned a fid once it is opened it is very important that files are closed after having been used (variable fid cannot be lost between fopen and fclose) 10

11 Opening and closing files [fid, message] = fopen(filename, permission) permission r, r+, w, w+ a, a+ format modifiers t, b are combined with access permission status = fclose(fid) status=0 when the file has been successfully closed, otherwise 1 fclose( all ) closes all files except stdout and stderr 11

12 Binary I/O functions save data count = fwrite(fid, array, precision, skip) count is the number of values written to a file array is written in a column order precision describes data format (see next slide) skip the number of bytes to skip in the output file before each write operation (the number of elements to skip must be translated to the number of bytes, this depends on precision) 12

13 Binary I/O functions fwrite precision 13

14 Binary I/O functions load data [array, count] = fread(fid, size, precision, skip) precision specifies the data format in the file and in Matlab disk_precision => array_precision (compatibility of disk precision with precision when using fwrite is very important) single, single=>single, *single size n readexactlynvalues inf read until the end of file [n m] readnxmvaluesandformatthemasannxmarray 14

15 Formatted I/O functions fprintf and fscanf count = fprintf(fid, format,val1,val2, ) fprintf without fid argument prints out to stdout fprintf is analogous to string function sprintf [array, count] = fscanf(fid, format, size) size is the same as in fread (n, inf or [n m]) fscanf is analogous to string function sscanf 15

16 Formatting string in fprintf and fscanf %-10.3f Modifier Overall field width Modifier: the number is adjusted/justified from the right side + there is a sign +/ preceding the number 0 instead of spaces 0 s are used Precision number of decimal digits Format descriptor (see next slide) 16

17 Formatting options (1) 17

18 Formatting options (2) The following special signs can be part of the formatting string: \n new line \t horizontal tab \b backspace \\ an ordinary backslash (\) symbol \ or an apostrophe or single quote ( ) %% an ordinary percent (%) symbol 18

19 Formatted I/O functions Functions for reading a line from a text file line = fgetl(fid) read line excluding end of line character line = fgets(fid) read line including end of line character line is 1iffgets or fgetl encounters the end of a file end of line character is different in Windows and Linux after reading a line, position in the file is set to the next line while ~feof(fid) line = fgetl(fid); process line end 19

20 The textscan function the concept similar to textread but faster & more flexible a=textscan(fid, format,n) > it can also be used on str instead of fid a=textscan(fid, format, param, value, ) a=textscan(fid, format, N, param, value, ) when N is missing or is 1, textscan reads until the end of the file it can start at any position and does not reset position in a file unlike textread (with textscan position in the file is controlled) it outputs a single cell array unlike textread for options offering enormous flexibility check help textscan 20

21 File positioning and status functions (1) Verification whether a variable, file etc. of a given name exists on a disk (file) or in the workspace (variable) status = exist( name, type ) type canbe var, file, dir, builtin example: if ~exist( myfile.txt, file ), disp( File does not exist ); end Special care has to be paid to a file path file separators on a path differ between platforms (in Windows \ and in Unix/Linus it is / ) use filesep current path can be obtained with pwd path_with_filename =[pwd filesep filename] 21

22 File positioning and status functions (2) I/O error indicator is updated by every I/O operation [message, errnum] =ferror(fid) information (message & error number) about the most recent error it can be called once an error is detected message =ferror(fid, clear ) clears error indicator for fid It is possible to check we the end of a file has been reached (if yes, 1 is returned as eofstat, otherwise 0) eofstat = feof(fid) 22

23 File positioning and status functions (3) Position in a file in terms of bytes from the beginning position = ftell(fid) Position can be reset to the beginning frewind(fid) Position can be moved with reference to the origin: beginning ( bof ), current position ( cof ) or end of file ( eof ) status = fseek(fid, offset, origin) origin is in bytes (forward + or backward ) status becomes 0 if the operation was successful 23

24 GUI based data import Import Wizard can be used to import the data function call: uiimport, structure = uiimport from Matlab menu (File / Import) 24

25 Summary (1) For data used only in Matlab environment, load and save offer flexibility and rich functionality For well arranged tabular mixed type data in text files, load (-ascii), textread and textscan are recommended textscan requires special care because it deals without any warning (not necessarily in a desirable way) with data that is not well arranged in a tabular fashion For ASCII delimited files with numeric content it is recommended to rely on dlmwrite and dlmread (also load) 25

26 Summary (2) More universal writing to/reading from ASCII formatted input can be performed with fprintf / fscanf Binary files should be processed with fread and fwrite relies on handles so such functions as fprintf/fscanf, fwrite/fread, textscan have to be opened with fopen andclosedwithfclose Additional functionality in file processing is offered by file positioning and status functions including error handling 26