MATLAB: Overwriting Data in Text File with Content from Matlab Table, sample code attached!

data exportexportfile outputMATLAB

Hi Community, the following problem occured to me:
I have a .txt file with looks like:
Name: Drifter #92 
Note: No notes
Serial Number#: 1
Address: 92
Accumulation of samples: 1
The measurement period: 10
Latitude:  0.000000
Longitude: 0.000000
Time UTC                Water      Air        Chip       
15.07.2015 12:24:47     +107.117    +106.925    +17.179 ...
15.07.2015 12:24:52     +107.116    +106.926    +17.124 ...
15.07.2015 12:24:57     +107.115    +106.935    +17.167 ...
15.07.2015 12:25:02     +107.116    +106.931    +17.131 ...
15.07.2015 12:25:07     +107.115    +106.924    +17.125 ...
15.07.2015 12:25:12     +107.113    +106.930    +17.107 ...
                                                    .
                                                    .
                                                    .
Due to speed issues I load only the first three columns into Matlab Workspace as a table, like this:
   15.07.2015 12:24:47     +107.117    +106.925
   15.07.2015 12:24:52     +107.116    +106.926
   15.07.2015 12:24:57     +107.115    +106.935
   15.07.2015 12:25:02     +107.116    +106.931
   15.07.2015 12:25:07     +107.115    +106.924
   15.07.2015 12:25:12     +107.113    +106.930
Now I calculate a little and change the date format:
    2015-07-15 12:24:47    17.70681925195          17.0387490668112
    2015-07-15 12:24:47    17.7042602411612  17.041305748443
    2015-07-15 12:24:47    17.7017012303724  17.064315883129
    2015-07-15 12:24:47    17.7042602411612  17.0540891566019
    2015-07-15 12:24:47    17.7017012303724  17.0361923851795
    2015-07-15 12:24:47    17.6965832087948  17.0515324749701
This data is saved in a cell array, where the first cell content is of type "string" and the others of type "double.
Finally I would like to replace the top three columns in the original textfile by the newly calculated ones, without overwriting the whole textfile.
I tried a lot with pointers to get to the right line in the textfile before wrting to it, but I don't come to a smart solution.
An exemplary .txt file, running sample code plus an import function generated by Matlab are attached.
Thanks for your help! With best regards, Helge.

Best Answer

With the example you've shown what you're asking is not possible. The only way you could do what you want is if the replacement text has exactly the same number of characters as the one you replace. If it has less, you can add spaces to make it equal, but if it has more, like in your example, it's not physically possible as the rest of the file needs to be moved to make room for the extra characters. Hence you've got to rewrite everything after the replacement.
If you're willing to keep the formatting of the water and air column as it is originally, and assuming the header is always the same number of lines, then:
fid = fopen('somefile.txt', 'rt+')
for headerline = 1:10
   fgetl(fid); %skip header lines
end
for row = 1:size(newdata, 1) %assuming your cell array is newdata
   startline = ftell(fid); %get current position
   fseek(fid, 0, 'cof'); %required because switching from reading to writing
   fprintf(fid, '%s\t% +7.2f\t%+7.2f', newdata{row, 1:3}); %the format string assumes no value is above 999.99
   fseek(fid, startline, 'bof'); %return to beginning of line
   fgetl(fid); %read whole line to position file pointer to beginning of next line
end
Code is untested, there may be bugs in the formatting string, number of header lines, etc.
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