Computer Programming with MATLAB

Lesson 8: File Input/Output

by

Akos Ledeczi and Mike Fitzpatrick

File Input/Output

 File:

 File Input/Output (I/O)

 MATLAB can handle

Excel files

 Microsoft Excel® is a widely used data-analysis tool
 Many other programs support reading and writing
Excel files
 MATLAB does too with two built-in functions
◦ xlsread
◦ xlswrite

Reading Excel files

>> [num,txt,raw] = xlsread('Nashville_climate.xlsx');

Numerical

>> num
num =
46 28 3.98
51 31 3.7
61 39 4.88
70 47 3.94
78 57 5.08
85 65 4.09
NaN NaN NaN
NaN NaN NaN
89 69 3.78
88 68 3.27
82 61 3.58
71 49 2.87
59 40 4.45
49 31 4.53

Numerical

>> num
num =
46 28 3.98
51 31 3.7
61 39 4.88
70 47 3.94
78 57 5.08
85 65 4.09
NaN NaN NaN
NaN NaN NaN
89 69 3.78
88 68 3.27
82 61 3.58
71 49 2.87
59 40 4.45
49 31 4.53

Numerical

Text

>> txt =
txt =
[1x30 char] '' '' '' ''
[1x40 char] '' '' '' ''
'' '' '' '' ''
'' '' 'High temp (F)' 'Low temp (F)' 'Precip (in)'
'' 'Jan' '' '' ''
'' 'Feb' '' '' ''
'' 'March' '' '' ''
'' 'April' '' '' ''
'' 'May' '' '' ''
'' 'June' '' '' ''
'' '' '' '' ''
'' '' 'High temp (F)' 'Low temp (F)' 'Precip (in)'
'' 'July‘ '' '' ''
'' 'Aug' '' '' ''
'' 'Sep' '' '' ''
'' 'Oct' '' '' ''
'' 'Nov' '' '' ''
'' 'Dec' '' '' ''

All data: cell array

>> raw raw = [1x30 char] [ NaN] [ NaN] [ NaN] [ NaN]
[1x40 char] [ NaN] [ NaN] [ NaN] [ NaN]
[ NaN] [ NaN] [ NaN] [ NaN] [ NaN]
[ NaN] [ NaN] 'High temp (F)' 'Low temp (F)' 'Precip (in)'
[ NaN] 'Jan' [ 46] [ 28] [ 3.98]
[ NaN] 'Feb' [ 51] [ 31] [ 3.7]
[ NaN] 'March' [ 61] [ 39] [ 4.88]
[ NaN] 'April' [ 70] [ 47] [ 3.94]
[ NaN] 'May' [ 78] [ 57] [ 5.08]
[ NaN] 'June‘ [ 85] [ 65] [ 4.09]
[ NaN] [ NaN] [ NaN] [ NaN] [ NaN]
[ NaN] [ NaN] 'High temp (F)' 'Low temp (F)' 'Precip (in)'
[ NaN] 'July‘ [ 89] [ 69] [ 3.78]
[ NaN] 'Aug' [ 88] [ 68] [ 3.27]
[ NaN] 'Sep' [ 82] [ 61] [ 3.58]
[ NaN] 'Oct' [ 71] [ 49] [ 2.87]
[ NaN] 'Nov' [ 59] [ 40] [ 4.45]
[ NaN] 'Dec' [ 49] [ 31] [ 4.53]

Text files

 Text files contain characters
 They use an encoding scheme:
◦ ASCII or
◦ Any one of many other schemes
◦ MATLAB takes care of encoding and decoding
 Before using a text file, we need to open it
 Once done with the file, we need to close it

Opening text files

 Opening: fid = fopen(filename, permission)
 Closing: fclose(fid)
 fid: Unique file identifier for accessing file
 Permission: what we want to do with the file—
◦ read, write, overwrite, append, etc.

Reading text files

 One line at a time
 type prints a text file in the command window
 Let’s re-implement it:

function view_text_file(filename)
fid = fopen(filename,'rt');
if fid < 0
error('error opening file %s\n\n', filename);
end

% Read file as a set of strings, one string per line:
oneline = fgets(fid);
while ischar(oneline)
fprintf('%s',oneline) % display one line
oneline = fgets(fid);
end
fprintf('\n');
fclose(fid);

Reading text files

 Reading lines into string variables is easy
 Parsing these strings to get numerical data is
much harder
 Not covered
 Binary files are more suited for numerical data

Binary files

 Binary file = “not a text file”
 Many different ways to represent numbers
 All we need to know are their types.
 Binary files need to be
◦ Opened with fopen
◦ Closed with fclose

Writing binary files

 Data type is important
 Example: write a double array into a binary file

function write_array_bin(A,filename)
fid = fopen(filename, 'w+');
if fid < 0
error( 'error opening file %s\n‘, filename);
end

fwrite(fid,A, 'double');

fclose(fid);

Reading binary files

 Example: read a double array from a binary file

function A = read_bin_file(filename,data_type)
fid = fopen(filename,'r');
if fid < 0
error( 'error opening file %s\n',filename);
end

A = fread(fid,inf,data_type);

fclose(fid);