ES 117. Formatted Input/Output Operations

Transcription

1 ES 117 Formatted Input/Output Operations

2 fpintf function writes formatted data in a user specified format to a file fid fprintf Function Format effects only the display of variables not their values through program execution fprintf(fid,format string,val1,val2,.) :the file id to which data will be written (no fid for printing on CW) format :the format string. % always marks the beginning of a format The structure of a format specifier %-12.5e Marker Modifier Field Width Precision Format Descriptor (Required) (Optional) (Optional) (Optional) (Required)

3 Common format specifier notation for fprintf Specifier Description %c Single character %d Decimal notation (signed) %e Exponential notation %E Exponential notation %f Fixed-point notation %g The more compact of %e and %f. Insignificant zeros do not print %s String of characters %u Decimal notation unsigned

4 Escape characters in Format strings symbol Description \n New line \t Horizontal tab \b backspace \\b Print an ordinary backslash (\) \ or Print an aposthrophe or single quote %% Print an ordinary percent symbol (%)

5 Example: Decimal (integer) data is displayed using %d format specifier. fprintf( %d\n,123) fprintf( %6d\n,123) fprintf( %6.4d\n,123) If a non decimal number is displayed with the %d specifier, the specifier will be ignored and the number will be displayed in exponential format fprintf( %6d\n,123.4) produces e+002

6 Example: floating point (real) data is displayed using %e, %f and %g format specifiers fprintf( %f\n,123.4) fprintf( %8.2f\n,123.4) (default is 6 chars after the decimal place) fprintf( %4.2f\n,123.4) fprintf( %10.2e\n,123.4) (6 chars wide field) 1.23e+002 fprintf( %10.2E\n,123.4) E+002 fprintf('%8.4f\n',123.4) fprintf('%8.4g\n',123.4)

7 Example: character data may be displayed with %c or %s format specifiers. fprintf( %c\n, s ) s fprintf( %s\n, string ) string fprintf( %8s\n, string ) string fprintf( %-8s\n, string ) String (left justified)

8 Data Import-Export Why is it important? Lab experiment results are: usually recorded in lab (even on paper) Put together and stored into a data file Analyzed using mathematical tools Text file (.txt) Microsoft Excel file (.xls) Binary file What do we want to export? Save everything in the workspace for post analysis. Save a selected number of results from the analysis in a text file (formatted or not)

9 Importing data Two common ways Direct input through the keyboard (using the input function) Good for individual input. Not so good for large amount of data. Use existing data stored in a file and import them into Matlab. Good for any data set size. Need to know the format of the data.

10 Different types of data files Importing data Text based files = formatted data for user usage. (.txt.dat ) Usually follows the American Standard Code for Information Interchange (ASCII) Binary files = Pre-converted data for computer usage. (.bin.mat) Software specific format = formatted data for software usage (.xls) Depending on the type of data file Different import function

11 Function load Syntax: load (filename) Importing data load filename loads all the variables from filename If filename has an extension other than.mat, load treats the file as ASCII data. If filename has no extension, load looks for file named filename or filename.mat and treats it as a binary MAT-file. test1.txt >> load test1.txt; >> whos Name Size Bytes Class test1 11x2 176 double array

12 Tips for loading data Use for loops to load entire series of files for j=1:n name1= test00'; name2=num2str(j); name3=.txt ; end NAME=[name1 name2 name3]; F=load(NAME); A(:,1)= F(:,1); A(:,j+1)= F(:,2); test001.txt test002.txt test003.txt

13 Importing data Function dlmread test2.txt 7.2;8.5;6.2; ;9.2;8.1;7.2 Syntax: A = dlmread('filename', 'delimiter'); test3.txt dlmread command works even if the contents of filename has spaces: 7.2; 8.5; 6.2; ; 9.2; 8.1; 7.2 >> A=dlmread('test2.txt',';') A =

14 Importing data Function textread Syntax: [A,B,C,...] = textread('filename','format') [A,B,C,...] = textread('filename','format',n) [A,B,C,...] = textread('filename','format') reads data from the file filename into the variables A,B,C, and so on, using the specified format, until the entire file is read. textread is useful for reading text files with known mixed formats. Both fixed and free format files can be handled. The format needs to be specified with a specifier like fprintf %s for string, %f for fix point notation %d for integers [A,B,C,...] = textread('filename','format',n) reads data from the file 'filename' N times.

15 Importing data test4.txt Ann Type Yes Joe Type No >> [A B C D E]=textread('test4.txt',' %s %s %f %f %s') A = 'Ann' 'Joe' B = 'Type1' 'Type2' C = D = E = 'Yes' 'No' >> [A B C D E]=textread('test4.txt',' %s %s %f %f %s',1) A = 'Ann' B = 'Type1' C = D = 45 E = 'Yes'

16 Importing data - Summary Data Format Sample File Extension Matlab function txt.dat or other load 1; 2; 3; 4; 5 6; 7; 8; 9; 10 or 1, 2, 3, 4, 5 6, 7, 8, 9, 10 Ann Type Yes Joe Type No Grade1 Grade2 Grade txt.dat.csv or other.txt.dat or other.txt.dat or other dlmread or csvread textread or fscanf textread or fscanf

17 Importing- Exporting data using fprintf or fscanf functions Three Steps: Functions fopen, fprintf/fscanf and fclose 1. fopen: opens a file or obtain information about open files Syntax: fid = fopen(filename,permission) Permission = 'r Open file for reading (default). 'w Open file, or create new file, for writing; discard existing contents, if any. 'a Open file, or create new file, for writing; append data to the end of the file.

18 Importing - Exporting data fopen file permissions r Open an existing file for reading only r+ Open an existing file for reading and writing w w+ a a+ Delete the contents of an existing file (or create a new file) and open it for writing only. Delete the contents of an existing file (or create a new file) and open it for reading and writing Open an existing file (or create a new file) and open it for writing only, appending to the end of file. Open an existing file (or create a new file) and open it for reading and writing, appending to the end of file.

19 Exporting data 2 fprintf: Used the same way as display function, except that this time, the formatting of the data is retained in the data file. Syntax: fprintf(fid,format,a,...) Formats the data in the real part of matrix A (and in any additional matrix arguments) under control of the specified format string, and writes it to the file associated with file identifier fid.

20 Importing data fscanf : Read formatted data from file Syntax: A = fscanf(fid, format) [A,count] = fscanf(fid, format, size) A = fscanf(fid, format) reads data from the file specified by fid, converts it according to the specified format string, and returns it in matrix A. Argument fid is an integer file identifier obtained from fopen. format is a string specifying the format of the data to be read. [A,count] = fscanf(fid, format, size) reads the amount of data specified by size, converts it according to the specified format string, and returns it along with a count of values successfully read. size is an argument that determines how much data is read. Options for size n Read at most n numbers, characters, or strings. inf Read to the end of the file. [m,n] Read at most (m*n) numbers, characters, or strings. Fill a matrix of at most m rows in column order. n can be inf, but m cannot.

21 Importing - Exporting data Functions fopen, fscanf/fprintf and fclose 3 fclose: close one or more open files Syntax: fclose(fid)

22 Exporting data Functions fopen, fprintf and fclose x = 0:0.1:1; Y = [x; exp(x)]; fid = fopen('exp.txt','w'); fprintf(fid,'%6.2f %12.8f \n',y); fclose(fid) File created in current directory Exp.txt

23 Importing data Example 1: fid = fopen('exp.txt', 'r'); a = fscanf(fid, '%f %f', [2 inf]) a = a'; fclose(fid) xdata.txt Example 2: clc;clear; fileid=fopen( xdata.txt', 'r' ); [a,nvals]=fscanf( fileid,'%d ',inf); fclose( fileid); a nvals % It has two rows now. a = nvals =

24 Importing data (more ) Function xlsread reads MS Excel files Syntax: num = xlsread(filename) num = xlsread(filename, -1) num = xlsread(filename, sheet, 'range ) num = xlsread(filename) returns numeric data in double array num from the first sheet in the Microsoft Excel spreadsheet file named filename. The filename argument is a string enclosed in single quotes. num = xlsread(filename, -1) opens the file filename in an Excel window, enabling you to interactively select the worksheet to be read and the range of data on that worksheet to import. num = xlsread(filename, sheet, 'range') reads data from a specific rectangular region (range) of the worksheet specified by sheet.

25 Importing data (more ) Example: xlsread A = xlsread('testdata1.xls') A = testdata1.xls

26 Importing data (more ) Example: xlsread A = xlsread('testdata1.xls,-1) A = testdata1.xls

27 Importing data (more ) Example: xlsread A = xlsread('testdata1.xls',1, 'A4:B5') A = testdata1.xls

28 Exporting data (more ) Function xlswrite Syntax: xlswrite(filename, M) xlswrite(filename, M, sheet, 'range') xlswrite(filename, M) writes matrix M to the Excel file filename. The filename input is a string enclosed in single quotes. The input matrix M is an m-by-n numeric, character, or cell array, where m < and n < 256. The matrix data is written to the first worksheet in the file, starting at cell A1. xlswrite(filename, M, sheet, 'range') writes matrix M to a rectangular region specified by range in worksheet sheet of the file filename.

29 Exporting data (more ) Example: xlswrite xlswrite('testdata', [ ]) d = {'Time', 'Temp'; 12 98; 13 99; 14 97}; s = xlswrite('tempdata.xls', d, 'Temperatures', 'E1') tempdata.xls Time Temp

30 The Import Wizard To import ASCII data, you must know how the data in the file is formatted. For example, many ASCII data files use a fixed (or uniform) format of rows and columns (continued )

31 The Import Wizard (continued) For these files, you should know the following. How many data items are in each row? Are the data items numeric, text strings, or a mixture of both types? Does each row or column have a descriptive text header? What character is used as the delimiter, that is, the character used to separate the data items in each row? The delimiter is also called the column separator (continued )

32 The Import Wizard (continued) You can use the Import Wizard to import many types of ASCII data formats, including data on the clipboard. When you use the Import Wizard to create a variable in the MATLAB workspace, it overwrites any existing variable in the workspace with the same name without issuing a warning. The Import Wizard presents a series of dialog boxes in which you: 1. Specify the name of the file you want to import, 2. Specify the delimiter used in the file, and 3. Select the variables that you want to import. 3-86

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