CS50x 2026 - Lecture 4 - Memory

All right, this is CS 50, and this is week 4, the week in which we take off the proverbial training wheels that have been the CS 50 library and reveal to you all the more what's going on underneath the hood of a computer in terms of its memory. We'll also talk about files and how you can

actually persist information for a long time whether. A file you've downloaded or today that you've created yourself, but first I just wanted to share some artwork that two of your classmates, Avery and Marie, kindly made before class, which is a picture made out of Post-it notes, uh, some green, some purple, which collectively from where you are looks like what?

Yeah, so indeed it's a cat that they made using only zeros and ones or green and purple pieces. And in fact, even though this is fairly low resolution in that it only has a few pixels this way and a few pixels this way, it's actually representative of how computers do actually store images

underneath the hood. So let's actually start there. In fact, we've had this bowl of stress balls for some time here on the lectern, and if we take a beautiful photo of it, they look a little something like this. Of course, this too is a finite resolution. And by resolution I just mean how many dots

go horizontally and how many dots go vertically, multiply those two together and you get some number of bytes, maybe in kilobytes, megabytes, or heck, if it's a massive image, it could be even bigger than that. But it is in fact finite. And if we zoom in on this image, you start to see a little more detail.

But at the same time, if you keep zooming in, you start to see indeed that there's only finite detail. And when we go really zoomed in, you start to see actual dots or pixels as they're called. In fact, on almost any screen, any. The image you look at, if you look close enough by pulling your phone up