how to add a line in a text file using Xcode

I am new to programming i wanted to add a string in between paragraphs in a text file , like using mutable string and nsfilemanager , i want to do it programmati开发者_如何转开发cally in " iOS " any help would be helpful.

I want to load the text file and put a line after each paragraph like find and replace .. find where \n is and replace it with \n\n so that it would seperate the para's with a line . i want to do it programatically just dont know how to do it?

[mutableString stringByReplacingOccurrencesOfString:@"\n" withString:@"\n\n"];