Using files. Computer Programming for Engineers (2014 Spring)

Transcription

1 Computer Programming for Engineers (2014 Spring) Using files Hyoungshick Kim Department of Computer Science and Engineering College of Information and Communication Engineering Sungkyunkwan University

2 Two goals How to read data from files? How to write data to files?

3 Reading file data There are many options to get data from files. load for numeric data only importdata for data with headers fopen/fscanf for complex data (low-level input functions) xlsread for Excel worksheets fread for binary data imread for graphics file

4 load load opens and imports data from a standard ASCII (American Standard Code for Information Interchange) file into a matlab variable Usage: var_name = load( filename ) Restrictions Data must be constantly sized Data must be ASCII No other characters

5 Example using load Example: >>edit inputnum.txt Write following and save it >>load inputnum.txt %it will load this file in workspace with a variable If we want to load the data in 'inputnum.txt' to a variable, >>test=load('inputnum.txt') %now data 'inputnum.txt' will store in variable 'test'

6 importdata importdata opens and imports data from a standard ASCII file with both data values and headers. jane joe jimmy headers values To import data with headers from files, use importdata >>a=importdata('testinput.txt'); a is a struct with data, textdata, and colheaders fields >>a=importdata('testinputcomma.txt', ','); a = data: [3x3 double] textdata: {'jane' 'joe' 'jimmy'} colheaders: {'jane' 'joe' 'jimmy'}

7 fopen Opens a file object in MATLAB that points to the file of interest fid = fopen('filepath', 'permission code') fid is an integer that represents the file fid is a file identifier of an open file

8 Permission code for fopen

9 fclose Closes an open file object in MATLAB fclose(fid) When you are done with a file, it is a good idea to close it especially if you are opening many files

10 fscanf fscanf(fid, format, size) format You have to tell MATLAB the type of data it should be expecting in the text file so that it can convert it %d, %f, %c size You can specify how to organize the imported data [m,n] import the data as m by n, n can be infinite Be careful because MATLAB will mangle your data and not tell you

11 Example of fscanf Read the data of the file exp.txt. Create a file having following code and rut it. fid = fopen('exp.txt'); A = fscanf(fid, '%g %g', [2 inf]); %A is 2 row x inf column matrix A = A' %transpose to get the original matrix fclose(fid);

12 Reading Excel files To read from an Excel file, use xlsread >>[num,txt,raw]=xlsread('randomnumbers.xls') Reads the first sheet num contains numbers, txt contains strings, raw is the entire cell array containing everything >>[num,txt,raw]=xlsread('randomnumbers.xls', Sheet1') Reads the Sheet1 sheet >>[num,txt,raw]=xlsread('randomnumbers.xls', -1) Opens the file in an Excel window and lets you click on the data you want!

13 Reading binary data The function fread performs all binary data reading in MATLAB. A = fread(fid) A = fread(fid, count) A = fread(fid, count, precision) [A, count] = fread(...)

14 Reading image from graphics file The function imread performs the image reading from a graphics file in MATLAB. A = imread(filename, format) Example of imread to load a JPEG file: A = imread('hyoung.jpg'); image(a) See further details:

15 Writing file data There are many options to put data to files. save for workspace variables fopen/fprintf for complex data (low-level output functions) xlswrite for Excel worksheets

16 save save stores variables/data from workspace to the disk file save %save all workspace data in matlab.mat file save('filename', 'variables') %save complete workspace data in specified file name Note: 1. No extension with file name means.mat file 2. If file name has extension other than (.mat) it will be treated as ASCII data file 3. To save variables to a directory other than current directory, full pathname of the file has to be specified

17 fprintf fprintf(fid, format, data) Example: create a file having following code and run it. x = 0:.1:1; y = [x; exp(x)]; fid = fopen('exp.txt','w'); fprintf(fid,'%6.2f %12.8f\n', y); fclose(fid); Note: Here %6.2f 6 = total width of field 2 = number of digits after decimal f = fixed point

18 Writing Excel files To write a matrix to an Excel file, use xlswrite >>[s,m]=xlswrite('randomnumbers', rand(10,4), 'Sheet1'); % we specify the sheet name You can also write a cell array if you have mixed data: >>C={'hello','goodbye';10,-2;-3,4}; >>[s,m]=xlswrite('randomnumbers', C, 'mixeddata'); s and m contain the 'success' and 'message' output of the write command

19 Students scores TASK: Create a m-file function to read the students Math, English, and Science scores from scores.txt and to write the mean and standard deviation of each course into a file. output.txt e e e e e e+00 mean std Math English Science Hint: use mean and std to calculate the mean and standard deviation, respectively.

20 Questions?