Advanced File I/O Chapter 9

Above: An early computer input/output device on the IBM 7030 (STRETCH)http://computer-history.info/Page4.dir/pages/IBM.7030.Stretch.dir/

Io? I/O?IoOne of the moon’s of Jupiter(A Galilean satellite)

File I/OShorthand 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

File I/OHard Disk Memory (RAM)

File Input

File Output

Where variables are stored

Temporary: Goes away when computer is shut off

Fast to read/manipulate

Where files are stored

Permanent: Stays even after computer is shut off

Slow to read/manipulate

File Output: A ReviewFile Output: ‘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

File Output: A ReviewFile Output: ‘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)

File Input: A ReviewFile Input: ‘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”)

What’s Next: Lower-Level File I/OWhat 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

2. Reading from a file

File on DiskVariable in RAM

File Output

Variable in RAMFile on Disk

File Input

Lower-Level File I/O• 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/O1) Open the file2) Read, Write, or Append to the file3) Close the file Think about opening

files in a filing cabinet

Opening and Closing Filesfopen: 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

Opening and Closing Files• Warning! Opening with write permission (‘w’) will erase

the file if it exists!

Opening and Closing Files• 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

Opening and Closing Files• Typically, you will open and close files in a script or

function• Should test to make sure there are no file opening errors

Writing to Files: fprintf• fprintf can also be used to write

to files!• Typically used inside a loop

Reading Files with Mixed Content• ‘load’ can’t read files with numbers and characters/stringsFor 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

Reading Files with Mixed Content• ‘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

Reading Files with Mixed Content• ‘textscan’ stores data in a cell array

Let’s experiment with this script and the resulting cell array

FUN!!

Reading Inconsistently Formatted Files

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…

Final ThoughtsMATLAB can read in ASCII files in many formats• If ‘load’ works, use it!• 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 write ASCII files in many formats• Use fprintf inside a loop and print files line by line