How to change a specific line in a text file?

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Hi
I have a text file, where different forms of content exist, data or strings or symbols, i need to change a specific line (it's only a number), i know its position (69 line)
Is there any way to do so?

Accepted Answer

ChristianW
ChristianW on 11 Feb 2013
Edited: ChristianW on 11 Feb 2013
  1. Loading txt into a cell.
  2. Then do whatever you want with the cell. In this example, line 69 will be 99.
  3. Then write the cell into a txt.
% Read txt into cell A
fid = fopen('test.txt','r');
i = 1;
tline = fgetl(fid);
A{i} = tline;
while ischar(tline)
i = i+1;
tline = fgetl(fid);
A{i} = tline;
end
fclose(fid);
% Change cell A
A{69} = sprintf('%d',99);
% Write cell A into txt
fid = fopen('test2.txt', 'w');
for i = 1:numel(A)
if A{i+1} == -1
fprintf(fid,'%s', A{i});
break
else
fprintf(fid,'%s\n', A{i});
end
end
  5 Comments
Thiago de Aquino Costa Sousa
Dear community,
I have many txt files that uses a tab delimiter. I have to change the content between the 17th and 18th tab delimiter of all my files. This is one the data inside the file:
_______
Data file generated on Tue Aug 30 12:40:29 2022
A B C D E F G H I J K L M N O P Q R S T U
1 2.00000 2.00000 0.00000 1.00000 28.00000 64.00000 88.00000 1.20000 275.00000 50.00000 30.00000 165.00000 850.00000 500.00000 0.00000 10.00000 10.00000 15.00000 0,0,0,0, 0,0,0,0, false
_______
So I have to change the value for the variable Q (in this case 10.0000) for a specific value. However, some of my files have a different pattern for numbers, instead of 10.00000 it is only 10, what makes the counting characters unfeasible. Furthermore, after some delimiters there are spaces. Then, I decided to count the tab delimeters, and change the content between the 17th and 18th delimiter to solve my problem, that I don't know if it will work. This is my code so far.
A = regexp(fileread(myfile), '\n', 'split'); %upload my file to a cell array
B = strfind(A{4}, sprintf('\t')); %takes the cell 4 in the array and finds where I have tab delimiters
C = B{4}(17)+1:B{4}(18)-1; %select the content between the 17th and 18th tab delimiters
Please, can somebody help me???

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More Answers (2)

Abdul
Abdul on 11 Aug 2015
Edited: Walter Roberson on 17 Jul 2018
I have written a function that scans for the specified SearchString in the InputFile and replaces it with the ReplaceString in the OutputFile. If the same file is specified as both the input and output file, the file is overwritten.
function [] = func_replace_string(InputFile, OutputFile, SearchString, ReplaceString)
%%change data [e.g. initial conditions] in model file
% InputFile - string
% OutputFile - string
% SearchString - string
% ReplaceString - string
% read whole model file data into cell array
fid = fopen(InputFile);
data = textscan(fid, '%s', 'Delimiter', '\n', 'CollectOutput', true);
fclose(fid);
% modify the cell array
% find the position where changes need to be applied and insert new data
for I = 1:length(data{1})
tf = strcmp(data{1}{I}, SearchString); % search for this string in the array
if tf == 1
data{1}{I} = ReplaceString; % replace with this string
end
end
% write the modified cell array into the text file
fid = fopen(OutputFile, 'w');
for I = 1:length(data{1})
fprintf(fid, '%s\n', char(data{1}{I}));
end
fclose(fid);
  7 Comments
Walter Roberson
Walter Roberson on 3 Mar 2022
data{1}{I} = regexprep(ReplaceString, '\\n', '\n'); % replace with this string

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Walter Roberson
Walter Roberson on 11 Feb 2013
sed. Or perl. Or python.
Or you could open the input file, open the output file, copy 68 lines from the input to the output, then read one line from input and write out the replacement output instead of the input line, then read the rest of the lines from input and copy them to output; finally, close both files.
There are fancier ways using (e.g.,) textscan() to read a bunch of lines at once.
Really, though, if you are on Linux or OS-X, I recommend using the system utility sed.
system('sed -e "69s/.*/31419/" < OLDFILE > NEWFILE')
here the 69 is the line you want changed and the 31419 is the new value you want put in
  1 Comment
Walter Roberson
Walter Roberson on 12 Mar 2021
These days, MATLAB also supports
readlines(filename)
which reads the file into a cell array of character vectors, one per line. You can then make whatever changes to the line (using {} indexing) and write the lines out again. For example,
S = readlines('test.txt');
S{69} = regexprep(S{69}, '-1', '31419'); %file says -1 change it to 31419
[fid, msg] = fopen('new_test.txt', 'w');
if fid < 1;
error('could not write output file because "%s"', msg);
end
fwrite(fid, strjoin(S, '\n'));
fclose(fid);

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