How to change specific number in a specific line in text file?

I want to change a specific number in line number (91) from a text file. Line 91 is: a=c+d*1.5. I need to change 1.5 with some other number. But the problem here is 1.5 is not constant, it keeps on changing. So, I want a general code that changes any number instead of 1.5 as well.

Answer 1

[Walter Roberson]

in_filename = 'YourInputFileNameGoesHere.txt';
out_filename = 'YourOutputFileNameGoesHere.txt';

S = regexp(fileread(in_filename), '\r?\n', 'split');
if isempty(S{end}); S(end) = []; end   %regexp often introduces an empty trailing field
S{91} = sprintf('a=c+d*%g', NewValue);

fid = fopen(out_filename, 'wt');   %or 't' instead of 'wt' if you are not using windows
fprintf(fid, '%s\n', S{:});
fclose(fid);

This code might alter the detail of whether the file ends in a newline, or if instead it just ends (either of which are valid for text files.)

It is not recommended that you write to the same output file as your input file: if something were to go wrong then you would have lost the input file. (But if you are sure you can recover from that...)

Answer 2

[ss]

Text in line 91 is a=c(d)*1.5 ; i.e. c(d) implies that c as a function of d. I need to keep on changing 1.5 only for multiple times with different values.

[Walter Roberson]

So where the c+d* appears in my code use c(d)* instead.

If this is code for a function you should consider making it an input and parameterize the function calls if you need to.

[ss]

Is there a way to change the text (1.5) by indicating row and column number. Here the row is 91 and column for 1.5 is 7?

[Walter Roberson]

Where I had

S{91} = sprintf('A=c+d*%g', NewValue);

put

S{row} = [S{row}(1:column-1), sprintf('%g', NewValue)];

This assumes that you want to change to the end of line. If you want to change N characters, then:

S{row} = [S{row}(1:column-1), sprintf('%g', NewValue), S{row}(column+N-1:end)];