Read a text file for a particular thing and put it in your code as a string

Question

me 2015-11-6

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关闭： MATLAB Answer Bot 2021-8-20

I have a long list of GPS data that looks like this

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$GPGGA,191313.000,3237.0423,N,08526.2246,W,1,09,0.9,241.48,M,-29.4,M,,0000*54
$GPGSA,A,3,27,15,06,18,29,09,21,26,22,,,,1.7,0.9,1.4*33
$GPRMC,191313.000,A,3237.0423,N,08526.2246,W,5.05,196.66,080211,,,A*7B
$GPGGA,191314.000,3237.0408,N,08526.2250,W,1,08,1.2,241.28,M,-29.4,M,,0000*50
$GPGSA,A,3,27,15,06,18,09,21,26,22,,,,,1.9,1.2,1.5*3D
$GPRMC,191314.000,A,3237.0408,N,08526.2250,W,5.53,189.70,080211,,,A*78
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I only need the data for GSA, GGA, and RMC; i can just ignore the other stuff. How can I read a text document for only those rows. I think an if statement might work but I am new to matlab and unsure on how to implement it.

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Answer 1

Walter Roberson 2015-11-6

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https://ww2.mathworks.cn/matlabcentral/answers/253325-read-a-text-file-for-a-particular-thing-and-put-it-in-your-code-as-a-string#answer_198681

编辑：Walter Roberson 2015-11-6

在 MATLAB Online 中打开

filecontent = readfile('YourFileNameHere.txt');
wanted_lines = regexp(filecontent, '^\$GP(GSA|GGA|RMC).*$', 'match', 'dotexceptnewline', 'lineanchors');

Now wanted_lines is a cell array of strings that contains the lines you want.

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me 2015-11-6

what if i used text scan like this

fid = fopen('GPSUSER_933000122_20110208_140804.txt', 'r'); %Open and read the file

strg = textscan(fid, '%s'); %creat a string of each GPS sentence

fclose(fid);

how can I pick out the strings that are GSA, GGA, RMC

Walter Roberson 2015-11-6

在 MATLAB Online 中打开

You could use that textscan() provided that there is no chance at all that there will be a space anywhere in the file. In that special case the effect would be the same as

 filecontent = readfile('GPSUSER_933000122_20110208_140804.txt');
 strg{1} = strsplit(filecontent, '\n');

(provided that there are no carriage returns in the file)

 filecontent = readfile('GPSUSER_933000122_20110208_140804.txt');
 strg{1} = regexp(filecontent, '\r?\n', 'split');

(which allows for the possibility of carriage returns)

However, if there is even one space in the file, the effect would instead be the same as

 filecontent = readfile('GPSUSER_933000122_20110208_140804.txt');
 strg{1} = regexp(filecontent, '\s+', 'split');

Once you have your cell array strg that has one entry which is a cell array of strings, you can use

wanted_lines = regexp(strg{1}, '^\$GP(GSA|GGA|RMC).*$', 'match');

or

wanted_lines = regexp(strg{1}, '^\$GP(GSA|GGA|RMC).*$', 'match', 'dotexceptnewline', 'lineanchors');

In this particular case where each entry of the cell array is a string representing one line, dotexceptnewline and lineanchors are redundant as they have special meaning for strings that cross multiple lines.

You might at this point notice that you have not gained anything relative to directly using

 filecontent = readfile('GPSUSER_933000122_20110208_140804.txt');
 wanted_lines = regexp(filecontent, '^\$GP(GSA|GGA|RMC).*$', 'match', 'dotexceptnewline', 'lineanchors');

and you will find the textscan() version to be slower.