I was actually surprised how clean and white were the programer's pages.
My sketchbook's pages usually start with useful data: todo lists, algorithms or ideas. I'm not very tidy, but it makes sense to me...
As time goes by, and those pages spend more and more time next to me, doodles start invading them. Sometimes they actually mean something (level design, geometry collision, direction vectors, etc)... and sometimes they are just things my hand does while my brain is thinking...
In the end... if possible, i even start using darker pens to write over other things... there's a whole mess with barely any white on them... time to move to the next page...
I've found that having as sketchbook to doodle makes programming so much easier. To put something on a page (even though it's just a square representing a class or an object) organizes the code in my head.
The curious thing is that when i get into "artist mode" (character design, mainly), i am the opposite. I am very tidy and i don't like doodling on these pages. This is the same sketchbook, except showing the art pages.
In fact i think i'm TOO tidy. Too rigid. I've seen actual artists' sketchbook, their lines are all over the place, the re-do the same line several times, maybe trying to find something new that works better. Their shapes are not defined until later, they are actually sketches. And have like one hundred drawings of the same character with slight variations...
But i cannot do it, if i don't like a line I erase it... if i like a character, i'm afraid to draw it again because i don't know if i will manage to draw it the same. I have to pratice to loosen up... a lot.
What does your sketchbook look like?
As a programmer do yo have a sketchbook at all?
To be perfectly honest, I do the exact same thing. I also tend to use a black ball-pen for sketches and writing notes - it is kind of like "mental conditioning" for not allowing myself to make any mistakes and/or be tempted to erase something that could in retrospective be useful.
As a result, I tend to go through several iterations, sometimes writing the same things on several sheets of paper, until I think I got it right ; then when I have the patience, I will go through the process of scanning and/or writing out my note onto my computer.