Borrowing GPL code

[December]

I want to use some code from another GPL project in my own (which is also GPL).

I want to use about 15 lines almost as they are (seems no better way to do it then it is there).

I was wondering where to credit the original author (problem is there are 3 listed in the C++ file I am copying from, no idea which one wrote this bit of code). Should I put it a the top, or in a comment just around the bits I copied?

[wysota]

Put it in the header of the cpp file that uses the code and in the documentation of your program.

[Michiel]

I sometimes wonder how much code you have to copy before you are obligated to give credit.

In other words / similarly: where's the line between using techniques you have learned from other peoples code and copying other peoples code?

[wysota]

I sometimes wonder how much code you have to copy before you are obligated to give credit.

Anything, I guess.

[Michiel]

Really? So if I just copy the word "if", I have to give credit?

Ok, ok. If I copy a swap(a, b) function?

What if I learn someone elses sorting algorithm by heart and then type it in myself, changing the variable-names?

And what if I very carefully read someones entire program, get to understand the principles behind it and start from scratch using what I've learned?

I just think about that stuff sometimes.