I want to write data at the end of every nth line of a text file

I have a long text file which contains several thousands of the following type of data

a0 =

333222

323312

310132

a1=

33222

323312

310132

a2=

133222

323312

310132

a3=

233222

323312

310132

a4=

303222

323312

310132

a5=

313222

323312

310132

a6=

323222

323312

310132

:

:

a2776=...

And I want to add at the end of evry 4th line a letter c and save back that modified file in txt format. After modification it should look like that

a0 =

333222

323312

310132c

a1=

33222

323312

310132c

a2=

133222

323312

310132c

a3=

233222

323312

310132c

a4=

303222

323312

310132c

a5=

313222

323312

310132c

a6=

323222

323312

310132c

:

:

:

a2776=...

Answer 1

[Voss]

str = readlines(filename);
str(4:4:end) = str(4:4:end) + "c";
writelines(str,filename)

where "filename" is the absolute or relative path to your text file, e.g., filename = 'C:\work\files\my_file.txt'.

[Radu Mihail]

Thank you Voss!

[Voss]

You're welcome!