How to change a specific line in a text file?

Question

Sebastian 2013-2-11

1
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https://ww2.mathworks.cn/matlabcentral/answers/62986-how-to-change-a-specific-line-in-a-text-file

评论： Thiago de Aquino Costa Sousa 2022-10-2

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?

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Answer 1

ChristianW 2013-2-11

15
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https://ww2.mathworks.cn/matlabcentral/answers/62986-how-to-change-a-specific-line-in-a-text-file#answer_74543

编辑：ChristianW 2013-2-11

在 MATLAB Online 中打开

Loading txt into a cell.
Then do whatever you want with the cell. In this example, line 69 will be 99.
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

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Ali Mohammadzadeh 2018-7-16

Thanks, it works perfectly.

Thiago de Aquino Costa Sousa 2022-10-2

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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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Answer 2

Abdul 2015-8-11

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https://ww2.mathworks.cn/matlabcentral/answers/62986-how-to-change-a-specific-line-in-a-text-file#answer_188900

编辑：Walter Roberson 2018-7-17

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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);

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Marcos dos Santos 2021-3-12

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Hey Bhaskar. I did it by defining SearchString and ReplaceString as cells. SearchString = {str1,str2, etc.}; ReplaceString = {str1,str2, etc.}. I created for loop that calls the function.

SearchString = {str1,str2, etc.};
ReplaceString = {str1,str2, etc.}
for ii=1:length(SearchString)
    
    func_replace_string(InputFile, OutputFile, SearchString{ii}, ReplaceString{ii})
 
end
 
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);
end

I hope this helps.

Walter Roberson 2022-3-3

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data{1}{I} = regexprep(ReplaceString, '\\n', '\n'); % replace with this string

Allan 2022-9-22

good stuff! thank you ;)

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Answer 3

Walter Roberson 2013-2-11

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https://ww2.mathworks.cn/matlabcentral/answers/62986-how-to-change-a-specific-line-in-a-text-file#answer_74537

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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

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Walter Roberson 2021-3-12

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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);