[JackFr]

> I'm still retyping stuff from stack overflow instead of copying. I find it really effective to really think through the code you're borrowing from somewhere

Numerical Recipes in C. Had the hard copy but not the disk.

Often the code was just to obscure to work out as you typed, but there was some real value to typing it in. You got some real feel for it. Additionally it offered hard lessons in writing test cases.