How to read specific lines from text file?

Hello, I have a little question. I have a text file that consists of four columns, and several lines, columns also have headers. And i woder how to read a specific line, i just want to read first line(with headers) and the third one, and export it to another file.

I'm lost and I don't know how to do this.

I'm sure that at the beginning I have to start with:

fid=fopen('filename.txt');

[dpb]

Unless the file is truly huge, just read the whole thing (readtable or one of his friends) and just keep the lines of interest will be faster and simpler to code.

[Cat Koval]

No, no, the file is not huge, okay, but I have a problem because I don't know how to specify which line I want to keep.

When I use fgetl, it takes the first line from the file, and i want for example the third one

or I want to read all the lines where the value of one column is for example '645'

(I'm so sorry if I ask simple things, but I'm not skilled in matlab and I'm very VERY confused about this program)

[Jan]

How do you determine, which lines you want to read? If you search for a specific value, you have to read all lines, of course. Otherwise you cannot check the contents.

[Cat Koval]

So, I have a file 'cor.txt':

Nr      X             Y           Z    
1    323.12   121.22    12.11
2   111.10   133.29    0
3    233.12   645.00    13.12
4    325.12   645.00   23.33
9    553.13   128.22    13.77

And I want to create new file with the first line (with headers) and the forth and fifth line where the Y=645.00

And I made up something like this

clc, clear 

% open the file
 fid2=fopen('cor.txt', 'r'); 
%reading all lines
line =fgetl(fid2)
line1 =fgetl(fid2);
line2 =fgetl(fid2);
line3 =fgetl(fid2)
line4 =fgetl(fid2)

%creating a new file with specific lines
fid=fopen('ikd.txt','w');

fprintf(fid, '%.19s\n ',line);
fprintf(fid, '%.19s\n ',line4);
fprintf(fid, '%.19s\n ',line5);
fclose (fid)

Okay.... its not advanced but it works, but now i wonder what if I'm in a situation where there is a lot more lines in file, can use a loop or sth, to find all lines with Y=645.00? And if i can, how to do it :(

Answer 1

[Scott MacKenzie]

No need for a loop. Since your data are rectangular and the data in each column are of the same type, you're best option is to use readtable combined with a logical expression to identify the rows of interest:

% read your data into a MATLAB table (includes header line)
T = readtable('corr.txt');

% identify rows of interest using logical expression
rowLogical = T{:,3} == 645;

% build new table containing header and only the rows of interest
Tnew = T(rowLogical,:);

% write new data in file
writetable(Tnew, 'idk.txt');

You can get fancy with logical expressions. For example, if you want to save all rows were the value in the Y column is > 600, then change the logical expression to

rowLogical = T{:,3} > 600;

For more examples on using logical expressions, see Find Array Elements That Meet a Condition

[dpb]

And, even more convenient with the table, you get the variable names for free along with the data so you can write logical expressions like

T = readtable('corr.txt');
WantedY = 645;              % use variables, don't bury "magic" numbers inline in code
outfile='idk.txt';          % ditto, now can get new file by adding an input statement, not change code
Tn=T(T.Y==WantedY,:);       % Scott's logic except use variable name(*) instead of position
writetable(T,outfile)

(*) And, you can write variable names as character variables, you don't have to hardcode them, necessarily, either.

Read the documenation on the table data class and the excellent tutorial material provided on how to reference data in tables..."let me count the ways!"