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save append and read data from a file using Matlab

Problem

Create a 4x4 random matrix and save it in a file. Now create a 4x4 matrix and append the new matrix to the file. Finally read the data from the file and display on the command window.

Solution

In this program we will create a matix using the rand function and then save it to the file. Again we will create a matrix and append the matrix to the file. Finally we will read the data from the file. In this program we will only use the command prompt window to accomplish the task. First of all head on to the program section to get insight into what is happening, then you can refer to the explanation section to get detailed explanation of the program section.
Some of the concepts used to solve this post are as follows:

Program

>> mat = rand(4,4)
mat =
    0.7094    0.6551    0.9597    0.7513
    0.7547    0.1626    0.3404    0.2551
    0.2760    0.1190    0.5853    0.5060
    0.6797    0.4984    0.2238    0.6991
>> save file.dat mat -ascii
>> mat_2 = rand(4,4)
mat_2 =
    0.8909    0.1493    0.8143    0.1966
    0.9593    0.2575    0.2435    0.2511
    0.5472    0.8407    0.9293    0.6160
    0.1386    0.2543    0.3500    0.4733
>> save file.dat mat_2 -ascii -append
>> load file.dat
>> file
file =
    0.7094    0.6551    0.9597    0.7513
    0.7547    0.1626    0.3404    0.2551
    0.2760    0.1190    0.5853    0.5060
    0.6797    0.4984    0.2238    0.6991
    0.8909    0.1493    0.8143    0.1966
    0.9593    0.2575    0.2435    0.2511
    0.5472    0.8407    0.9293    0.6160
    0.1386    0.2543    0.3500    0.4733
>>

Explanation

First I have created a matrix using the rand function and then stored it in a variable called mat. Next I have saved the data of the mat variable to the file named file.dat using 'save' command. Remember that if you don't have a file named file.dat(or the name of the file you are giving), then the file is created automatically and the data is saved. If the file exists and there is data then using 'save file.dat mat -ascii' rewrites the file as if a new fresh file is created. If you don't want to delete the data then you should append to the file. I will discuss about -ascii after the explanation of the whole program. Next I have created a mat_2 variable and stored a randomly generated values using the rand function in the variable 'mat_2'. Now I have saved the data of mat_2 to the file.dat and added a -append at the end of the conventional write of the data to the file. If I hadn't added the -append at the end then all the data that I had previously stored in the file will be lost and a new file with the new data is created. Finally to load(or read) the data I have used load file.dat to read the file. The data after reading form the file gets stored in a variable of the file name. Finally I have displayed the whole data using the 'file(Which was the name of the file with .dat removed)'. There are a few more important things to discuss:
Why have I added the -ascii to the save command?
You can also save the file without using -ascii at the end but what happens is, when you try to read the data using the load command then you will encounter the following error:


>> mat = rand(4,4)
mat =
    0.3517    0.9172    0.3804    0.5308
    0.8308    0.2858    0.5678    0.7792
    0.5853    0.7572    0.0759    0.9340
    0.5497    0.7537    0.0540    0.1299
>> save file.dat mat
>> load file.dat
??? Error using ==> load
Unknown text on line number 1 of ASCII file C:\Users\SEKHAR
PEESAPATI\Documents\MATLAB\radiusofcircle\file.dat
"MATLAB".
 
>>
 Which says that it can't read the text as the file is not in the ascii format. ASCII is American Standard Code for Information Interchange. Now lets say you have saved a very large matrix to a file so that your friend will interchange the information with you. Now if he/she tries to read the file with some other programming language and it fails because you have not followed a standard to write the data to the file. That is why you are asking the matlab to store the data to the file in ascii format. You can read more about ascii from here: Wikipedia - Ascii

Final Note 

I have tried to explain everything in this post in such a way that it is easy for everyone to understand. If you haven't understood anything or have a doubt then please do contact me. You can contact me from here: Contact Me
Practice the save, load, save  -append command with more data and play with them.
Keywords: save,load, append, read, write, matlab, file,ascii
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