How to modify text file

I'm trying to overwrite data in the following text file:

"L_Chamber"=7.748

"D_Chamber"=6.757

"L_Nozzle"=8.545

"D_Nozzle"=8.509

"D_Throat"=2.941

"Theta_N"=31.278

using the following code:

fileID = fopen('equations.txt');
fprintf(fileID,'"L_Chamber"= %8.3f\n',L_Chamber);

I'm trying to update the numerical value only. I don't get any errors, but the text file doesn't change no matter what I do.

Answer 1

[nick12391]

I wasn't giving fopen a permission. Should have written:

fileID = fopen('equations.txt','w');
fprintf(fileID,'"L_Chamber"= %8.3f\n',L_Chamber);

Answer 2

[KSSV]

fid = fopen('data.txt') ; 
S = textscan(fid,'%s') ; 
fclose(fid) ; 
S = S{1} ; 
% Find the string you want to change 
idx = contains(S,'L_Chamber') ; 
% Replace 
S(idx) =  {'"L_Chamber"=8.34'} ; 
% Write to text file 
fid = fopen('new.txt','w');
fprintf(fid,'%s\n',S{:});
fclose(fid);