Transcript 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
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
Hard Disk
Memory (RAM)
Where files are stored
Where variables are stored
Permanent: Stays even after
computer is shut off
Temporary: Goes away
when computer is shut off
Slow to read/manipulate
Fast to read/manipulate
‘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
‘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)
‘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 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
Variable in RAM
File on Disk
1. Writing to a file
File on Disk
2. Reading from a file
Variable in RAM
• 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
3) Close the file
ppend to the file
Think about opening
files in a filing cabinet
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
• Warning! Opening with write permission (‘w’) will erase
the file if it exists!
• 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
• Typically, you will open and close files in a script or
function
• Should test to make sure there are no file opening errors
• fprintf can also be used to write
to files!
• Typically used inside a loop
• ‘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
• ‘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
• ‘textscan’ stores data in a cell array
Let’s experiment with
this script and the
resulting cell array
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…
MATLAB can
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
ASCII files in many formats
• Use fprintf inside a loop and print files line by line