3:
Programming
CS105: Chapter Great Insights in Computer Science
The Vector
We can build (or at least imagine!) lots of circuits. We can even think about state machines that use circuits to do various things over time. Were headed towards using these ideas to create a programmable computer. Hillis has us take a detour to talk about programming rst.
Programming
Many different languages have been devised for programming. Each provides a way of writing a kind of script along with rules for the computer to interpret the script as instructions. They can all do pretty much the same things, but make it easier to say some things than others. How many languages can you name?
Scratch
Developed by the Lifelong Kindergarten group at MIT. Allows users to make media-rich programs by clicking together blocks. Share your creations a la YouTube. My miniNim program is an example.
Programming Window
Things
Performance area
! stage (background) ! sprites (objects) - scripts (behavior) - costumes (appearance) - sounds (available sounds)
More Things
Run/Stop Script inventory
! Motion ! Looks ! Sound ! Pen ! Control ! Sensing ! Numbers ! Variables
Shapes
Trigger Statement Ending statement Boolean value Numeric value
Motion Inventory
Looks Inventory
Sound Inventory
Pen Inventory
Control Inventory
Logo in Scratch
Logo is a language invented to help people (kids?) learn to program. Scratch is the language weve been using for examples and demos in this class. Its a descendent of Logo, in many ways. Hillis uses Logo for his examples in this chapter. Ill translate them into Scratch so it ts better with our other examples.
Drawing Commands
New Command
Do It!
Use It!
Command With I Variable
Afterwards, renamed it squareSize
Conditionals
Based on the random number, two possible outcomes. Executes only one block of code, depending on the Boolean condition.
If. If is good.
(Innite) Looping
(bug in book)
Never never never stops!
Kinds of Loops
Innite loop For loop: Repeat a set number of times.
! Flexible.
While loop: Until a condition holds.
! More Flexible.
Recursion: Repeat substructure.
! Most Flexible.
Just Four, Thanks
Sets a repeat count Stops after that number of repetitions
Unroll The Loop
Its as if the statements inside the loop are repeated four times.
More Power
For (repeat) loops are great if you know how many times you will be repeating. Sometimes need something more powerful. While (repeat-until) loops keep going until a condition becomes true. Can behave like for loops...
Four Square Countdown
Unroll, With Conditions
Even More Power
While (repeat-until) loops are great if repetitions are sequential. Sometimes need something more powerful. Recursion can allow control to proceed in multiple directions at once! But, can also behave like for loops...
Innite Recursion
Each pattern message spawns another. Ad innitum
Proper Recursion
Base case says what to do when the counter runs down. In this case, it stops when the counter reaches zero.
Not A Simple Loop
Loop proceeds in two directions at once. Cant do that with for or while. (In fact, cant do it at all in Scratch, but other it is commonplace in other programming languages.)
I Think That I Shall Never See...
Each tree is made up of a trunk and two smaller trees. Base case keeps the structure from growing innitely. Makes shapes known as fractals. Other examples?