Access to Delimited Text Files

Transcription

1 Access to Delimited Text Files dlmread(filename, delimiter) reads ASCII-delimited file of numeric data. dlmwrite(filename, M, delimiter) writes the array M to the file using the specified delimiter to separate array elements. The default delimiter is the comma (,). dlmwrite(filename, M, -append ) appends the data to the end of the existing file. dlmread(filename, delimiter, R, C) reads data whose upper left corner is at row R and column C in the file. R and C start from 0. (R, C) = (0, 0) specifies the first value in the file. Zheng-Liang Lu 377 / 403

2 Example 1 >> M = gallery('integerdata', 100, [5 8], 0); 2 >> dlmwrite('myfile.txt', M, 'delimiter', '\t'); 1 >> dlmread('myfile.txt', 't') 2 >> dlmread('myfile.txt', '\t', 2, 3) Zheng-Liang Lu 378 / 403

3 textread textread is useful for reading text files with a known format. [A, B, C,...] =textread(filename, format, N) reads data from the file filename into the variables A, B, C, and so on, using the specified format, format for N lines. format determines the number and types of return arguments. (See next page.) If you drop N, then textread reads until the end of file. The common conversions are as follows: %d: as signed integer values. %f: as floating-point values. %s: as a white-space or delimiter-separated string. Zheng-Liang Lu 379 / 403

4 Example Create a mydata.dat with the following content: 1 Sally Level Yes 2 Arthur Level No 1 >> [names, types, x, y, answer] = textread('mydata.dat',... '%s %s %f %d %s') % normal usage 1 >> [names, types, x, answer] = textread('mydata.dat',... '%s Level%d %f %*d %s', 1) % check the difference! In %*f, * ignores the matching characters specified by *. Zheng-Liang Lu 380 / 403

5 Access to Excel Files xlsread(filename, sheet, xlrange) reads from the specified sheet and range. sheet can be the sheet name 1 or a sheet number in the excel file. xlrange is optional for the rectangular portion of the worksheet to read. For example, xlrange = B:B is used to import column B. To read a single value, use xlrange = B1:B1. 2 xlswrite(filename, A, sheet, xlrange) writes the array A to the specified range of the sheet. 1 The default sheet name is 工作表 1. 2 Contribution by Mr. Tsung-Yu Hsieh (MAT24409) on August 27, Zheng-Liang Lu 381 / 403

6 Example 1 >> values = {1, 2, 3; 4, 5, 'x'; 7, 8, 9}; 2 >> headers = {'First', 'Second', 'Third'}; 3 >> xlswrite('myexample.xlsx', [headers; values]); % write 1 >> subseta = xlsread('myexample.xlsx', 1, 'B2 : C3') % read 2 3 subseta = NaN Zheng-Liang Lu 382 / 403

7 Low-Level File I/O Low-level file I/O functions allow the most control over reading or writing data to a file. However, these functions require that you specify more detailed information about your file than the easier-to-use high-level functions, such as importdata. If the high-level functions cannot import your data, you may consider to use low-level file I/O. The normal procedure looks like: 1 Open a file. 2 Read or write data into the file. 3 Close the file. Zheng-Liang Lu 383 / 403

8 Open Files fid = fopen(filename, permission) opens the file, filename, for binary read access, and returns an integer as fid equal to or greater than 3. fid refers to the file identifier. permission: file access type, specified as a string. MATLAB reserves fids 1 and 2 for standard output on the screen and standard error, respectively. (You will see later.) If fopen fails to open the file with r, then fid is 1. You can fopen a new file with permission w. Be aware that you can repeatedly open a new file with the same file name. Zheng-Liang Lu 384 / 403

9 fopen Permission Codes r is to open file for reading. r+ additionally for writing. w is to open or create new file for writing. 3 w+ additionally for reading. a is to open or create new file for writing. Also, it appends data to the end of the file. a+ additionally for reading. 3 Be aware that it will discard existing contents, if any. Zheng-Liang Lu 385 / 403

10 Common Commands in Low-Level File I/O feof(fid), which refers to end-of-file, returns 1 if a previous operation set the end-of-file indicator for the specified file. fgetl(fid) returns the next line of the specified file, removing the newline characters. If the line contains only the end-of-file marker, then the return value is 1. Zheng-Liang Lu 386 / 403

11 Example Write a script to show the content of a text file, say, fgetl.m. 1 clear; clc 2 3 f = fopen('fgetl.m', 'r'); 4 while ~feof(f) 5 disp(fgetl(f)); 6 end 7 fclose(f); Zheng-Liang Lu 387 / 403

12 Close Files fclose(fid) closes an opened file. fclose( all ) closes all opened files. fclose returns a status of 0 when the close operation is successful. Otherwise, it returns 1. Zheng-Liang Lu 388 / 403

13 fprintf Recall that fprintf(format, A 1,..., A n ) formats data and displays the results on the screen 4. fprintf(fid, format, A 1,..., A n ) applies the format to all elements of arrays A 1,..., A n in column order, and writes the data to a text file. format: to specify a format for the output fields; it is a string. A 1,..., A n : arrays for the output fields. Matlab reserves the file identifier number 1 and 2 for standard output on the screen, and standard error, respectively. fprintf(1, This is standard output!\n ); fprintf(2, This is standard error!\n ); sprintf(format, A 1,..., A n ), similar to fprintf but returns the results as a string. 4 Recall disp. Zheng-Liang Lu 389 / 403

14 Example fprintf can print multiple numeric values and literal text to the screen. 1 >> A = [ ; ]; 2 >> format = 'X is %4.2f meters or %8.3f mm.\n'; 3 >> fprintf(format, A); % print on the screen 4 5 X is 9.90 meters or mm. 6 X is 8.80 meters or mm. 7 X is 7.70 meters or mm. %4.2f specifies that the first value in each line of output is a floating-point number with a field width of four digits, including two digits after the decimal point. Can you explain %8.3f? Zheng-Liang Lu 390 / 403

15 Escape Characters %%: Percent character \\: Backslash \b: Backspace \n: New line \t: Horizontal tab Zheng-Liang Lu 391 / 403

16 format Integer, signed: %d or %i Integer, unsigned %u: Base 10 %o: Base 8 %x: Base 16 Floating-point number 5 %f: Fixed-point notation %e: Exponential notation, such as e+00 Characters %c: Single character %s: String Note that the % can be followed by an optional field width to handle fixed width fields. 5 See IEEE 754. Zheng-Liang Lu 392 / 403

17 Example: grep in UNIX/Linux-like Systems findstr(s 1, S 2 ) returns the starting indices of any occurrences of the shorter of the two strings in the longer. 1 function grep(f, pattern) 2 f = fopen(f, 'r'); 3 cnt = 0; 4 while ~feof(f) 5 t = fgetl(f); 6 cnt = cnt + 1; 7 w = findstr(t, pattern); 8 if ~isempty(w) 9 fprintf('%d: %s\n', cnt, t); 10 end 11 end 12 fclose(f); 13 end Zheng-Liang Lu 393 / 403

18 Exercise Write a program which produces a multiplication table into a text file Zheng-Liang Lu 394 / 403

19 1 clear; clc; 2 3 f = fopen('multiplicationtable.txt', 'w'); 4 for i = 1 : 9 5 for j = 1 : 9 6 fprintf(f, '%3d', i * j); 7 end 8 fprintf(f, '\n'); 9 end 10 fclose(f); Zheng-Liang Lu 395 / 403

20 fscanf A = fscanf(fid, format, size) reads data from the file specified by file identifier fid, converts it according to the specified format string, and returns it in matrix A. fscanf populates A in column order. fscanf can be used to skip specific characters in a sample file, and return only numeric data. Zheng-Liang Lu 396 / 403

21 Example 1 clear all; 2 clc 3 str = { 4 ['78' char(176) 'C']; 5 ['72' char(176) 'C']; 6 ['64' char(176) 'C']; 7 ['66' char(176) 'C']; 8 ['49' char(176) 'C']}; 9 % char(176) is the symbol of degree 10 fid = fopen('temperature.txt', 'w'); 11 for i = 1 : length(str) 12 fprintf(fid, '%s\n', str{i}); 13 end 14 fclose(fid); 15 % main 16 fid = fopen('temperature.txt', 'r'); 17 [A, count] = fscanf(fid, ['%d' char(176) 'C']) 18 fclose(fid); Zheng-Liang Lu 397 / 403

22 1 >> 2 3 A = count = Zheng-Liang Lu 398 / 403

23 Binary Files fread(fid, size, precision) interprets values in the file according to the form and size described by precision. fwrite(fid, A, precision) translates the values of A according to the form and size described by precision. Zheng-Liang Lu 399 / 403

24 Valid entries for size are: N: read N elements into a column vector. inf : read to the end of the file. [M, N]: read elements to fill an M-by-N matrix, in column order. Valid entries for precision are: uchar : unsigned integer, 8 bits. int64 : integer, 64 bits. uint64 : unsigned integer, 64 bits. float64 : floating point, 64 bits. Note that 64 can be replaced by 8, 16, and 32. Zheng-Liang Lu 400 / 403

25 Example Create a binary file containing a 3-by-3 magic square, whose element is stored as 4-byte integers. 1 clear all; 2 clc 3 % main 4 A=magic(3) 5 fid = fopen('magic3.txt', 'w'); 6 fwrite(fid, A, 'int32'); 7 fclose(fid); 8 fid = fopen('magic3.txt', 'r'); 9 fread(fid, [3 3], 'int32'); % try [3 1]? Zheng-Liang Lu 401 / 403

26 Access to Internet urlread(url, Name, Value) returns the contents of a URL as a string. 1 contents =... urlread(' 2 f = fopen('matlab.html', 'w'); 3 fprintf(fid, '%s', contents); 4 fclose all; 5 dos('start matlab.html'); Try sendmail, ftp. Zheng-Liang Lu 402 / 403

27 Yahoo Finance API Current market and historical data from the Yahoo! data server Blog: 研究雅虎股票 API (Yahoo finance stock API) Google: yahoo-finance-managed Historical Stock Data downloader by Josiah Renfree (2008) Zheng-Liang Lu 403 / 403