Chapter 9. Above: An early computer input/output device on the IBM 7030 (STRETCH)

Transcription

1 Chapter 9 Above: An early computer input/output device on the IBM 7030 (STRETCH)

2 Io One of the moon s of Jupiter (A Galilean satellite) File I/O Shorthand for file input and output. Refers to the process of reading data from files and writing data to files We have already covered basic file I/O! We need to cover more advanced low-level file I/O

3 Hard Disk Memory (RAM) Where files are stored Permanent: Stays even after computer is shut off Slow to read/manipulate Where variables are stored Temporary: Goes away when computer is shut off Fast to read/manipulate

4 save : saves a matrix as an ASCII or.mat file All values printed with the same formatting and delimiter File will have consistent number of columns

5 dlmwrite : saves a matrix as an ASCII file Can specify the delimiter (must be a single character) File will have consistent number of columns (can be formatted)

6 load : loads in an ASCII file or.mat file File must have consistent number of columns All data must be numeric; no formatting options (loaded as class double )

7 What if you want to write/read a file that has Different formatting in each column Different numbers of columns in some rows Has both numbers and words/characters in a single row 1. Writing to a file Variable in RAM File on Disk 2. Reading from a file File on Disk Variable in RAM

8 The save, dlmwrite, and load commands do a lot of things automatically that we never are aware of Lower-level file I/O requires a more step by step process General steps for lower-level file I/O 1) Open the file 2) ead, rite, or ppend to the file 3) Close the file Think about opening files in a filing cabinet

9 fopen: opens a file r open for reading (default) w open for writing Will erase/overwrite the file if it already exists a open and append to file Will add to the end of an existing file fopen returns a file identifier Class: double (the value is an integer) Used to identify the file later on in your code -1 signifies an error in opening the file

10 Warning! Opening with write permission ( w ) will erase the file if it exists!

11 After you are finished with a file, you should close it Upon closing MATLAB, all open files are closed Closing files in your code, prevents errors and unwanted behavior fclose Returns 0 if close was successful Returns 1 if there was an error in closing Can test in an if statement

12 Typically, you will open and close files in a script or function Should test to make sure there are no file opening errors

13 fprintf can also be used to write to files! Typically used inside a loop

14 load can t read files with numbers and characters/strings For all of these examples, the file must be first opened with fopen and closed with fclose after you are finished fscanf can read files with mixed content Data is stored in a matrix Automatically converts characters to ASCII values Format of file is given in fprintf style Automatically converts characters to ASCII values textscan can read files with mixed content Data is stored in a cell array Each cell can have different types of variables (strings, doubles, cells, etc ) fgetl reads a file line by line Each line is stored as a string Use string processing with strtok to split up each line into variables

15 textscan can read files with mixed content Data is stored in a cell array Each cell can contain any type of variable (char, double, cell) File must already be opened with fopen Must close file with fclose when finished

16 textscan stores data in a cell array Let s experiment with this script and the resulting cell array

17 textscan reads all lines Includes the header line Will do this even if the header starts with % For more details, refer to in-class demo

18 MATLAB can If load works, use it! in ASCII files in many formats If file has a mix of numbers and letters use textscan textscan is quite flexible. Use fprintf style control characters some files are too inconsistent to be read accurately by textscan Alternative? Read in a file line by line using fgetl and strtok I won t test you on fgetl and strtok MATLAB can ASCII files in many formats Use fprintf inside a loop and print files line by line