Introduction

• David plays a game called Oscartime that was the first Scratch program he created
• Scratch is a graphical programming language created by MIT’s Lifelong Kindergarten Group
• The language not only helps get kids excited about programming, but it’s also very instructive

Software

• Programing is ultimately about making software
  • Software is what runs on our hardware
    • Could run on a desktop, or phone, etc.

Finding Mike Smith

• Code is just a technical implementation of algorithms
  • Algorithms are step by step instructions for solving problems
• Consider a phonebook full of thousands of names and phone numbers
• How do we lookup someone like Mike Smith?
  • We could start at the first page, move to the next, and so on until we find him
    • This is a correct algorithm, as we will find Mike Smith eventually
    • However, it’s inefficient
  • We could start at the first page and count by 2s
    • I would find Mike Smith twice as quickly
    • However, this alone is not correct as we could miss Mike Smith if his name is sandwiched between two pages
    • We could fix this by checking the previous page if we go past where Mike Smith should be
• More likely, we’d probably go to the middle of the phonebook and find ourselves in the “M” section
  • As Smith is after M, he must be in the latter (right) half of the book
    • We can ignore the other half
  • After removing the other half, we are left with half of the book, representing the same problem we started with fundamentally
    • We can keep repeating this process until we’re down to one page with Mike’s number on it
  • This leverages the fact that the book is sorted alphabetically
  • We are deviding and conquering
    • 1000 pages → 500 pages → 250 pages → 125 pages…

Phonebook Algorithm

1 pick up phone book
2 open to middle of phone book
3 look at names
4 if Smith is among names
5     call Mike
6 else if Smith is earlier in book
7     open to middle of left half of book
8     go back to step 3
9 else if Smith is later in book
10    open to middle of right half of book
11    go back to step 3
12 else
13    quit

Pseudocode

• This example algorithm is code, not written in a programming language, but rather English
• This is called Pseudocode
  • Code-like syntax written in English
  • Numbered lines to maintain order and reference lines
  • pick up, open to, look at, call, open, and go back are functions
  • if, if else, and else are conditions
  • Smith is among names, Smith is earlier in book, and Smith is later in book are Boolean expressions
    • Can be either true or false
    • If these are true, the indented code below is executed
  • Both line 8 and 11 say to go back to step 3
    • This creates a loop
      • Doing the sane thing again and again

Programming Constructs

• These constructs of loops, Boolean expressions, functions, and conditions as well as others such as variables, threads, events, and more are common across all programming languages

C

• C is one of the oldest programming languages that someone might still write in

#include <stdio.h>

int main(void)
{
    printf("hello, world/n");
}

• Some of this syntax may look cryptic, but you can likely guess what it does
  • It prints “hello, world” to the screen
  • The other details can be learned
    • Just like with written human languages that are foreign, you just haven’t learned the patterns yet
• Many programming languages have similarities, so it becomes easier to learn new ones with knowledge under your belt
• Ultimately, programming is about writhing software to control hardware to solve a problem
• However, computers only understand binary (0’s and 1’s)
  • Source code what we humans write and it can be converted into machine code (0’s and 1’s)
    • This is achieved by using a program called a compiler
    • This allows a human to write the code and a machine to read and run it

C++

#include <iostream>

int main()
{
  std::out << "hello, world" << std::endl;
}

• This program written in C++ still prints “hello, world”
• Many programming languages do the same things differently
  • We can solve the same problem using any different number of languages
  • It could be easier to use one programming language for a specific problem
    • Different languages were invented to tackle different kinds of problems

Python

print("Hello, World!")

• Straightforwardly, this prints “hello, world”

• Python is a different type of language as you don’t type source code and manually convert it into machine code

  • A special program called an interpreter converts the source code into an intermediate language called byte code

    • Which is not machine code (0’s and 1’s)

      2     0 LOAD_GLOBAL         0 (print)
          3 LOAD_CONST          1 ('hello, world')
          6 CALL_FUNCTION       1 (1 positional, 0 keyword pair)
          9 POP_TOP
          10 LOAD_CONST         0 (None)
          13 RETURN_VALUE
      

  • The interpreter reads this one line at a time

• The takeaway from this is that there different ways of doing the same thing in many different languages
  • And languages get used in different ways!
  • Sometimes you need to compile code, sometimes you need to interpret code
• At the end of the day, the consumer interacts with the software in the same way
  • They don’t need to know what language it’s written in as long is it run on their computer
    • Programs are often packaged differently for different operating systems

Other Programming Language

• Java

class Hello
{
    public static void main(String [] args)
    {
        System.out.println("hello, world");
    }
}

• Ruby

put "hello, world"

• Lisp

put "hello, world"

• JavaScript

console.log("hello, world")

Introducing Scratch

• Each of these languages have a lot more features, but they have commonalities
  • Functions, loops, conditions, Boolean expressions, variables, threads, etc.
• To focus on these ideas in a graphical manner, we’ll explore Scratch
• In the earlier Oscartime example, trash was moving down the screen
  • A screen is just a grid of pixels
  • To make animation, we move an image on it slightly
    • If done quickly, it looks like movement
• To make the trash stop at the bottom of the screen, we use some Boolean expression and condition
  • “If you’re touching the bottom of the screen, stop moving”
  • Or “Only if you’re not touching the bottom of the screen, keep moving”
• When trash is put in the can, the lid lifts and Oscar counts the pieces of trash disposed of
  • The sprite for Oscar is using a variable
    • Initialized (set initially) to 0
    • Increments (adds 1 to the variable) for each piece of trash
  • A condition is also used here
    • “If a piece of trash is added, then increment the variable for trash pieces”
• The music was playing in some kind of loop
• Even though this program is complex enough to take 8 hours to make, it’s ultimately built with the same fundamental building blocks of conditions, loops, etc.

hello, world in Scratch

• This is how we say “hello, world” in Scratch

  • By default, a graphical cat will preform this code

    • Can change the cat into other things

    

• This is the function for say

  

• This says “hello, world” forever

  

• This says “hello, world” 50 times

  

• This is an example of how to specify things conditionally

• Scratch allow you to programing by piecing together puzzle pieces with shapes that imply what to do
• We can put an if else inside another if else
• The green blocks are Boolean Expressions

Scratch Interface

• Scratch is not only a language but a programming environment as well

• On the left is Scratch the cat in a 2D world with height and width
  • Can change background and more sprites to this world
• In the middle are palettes containing scripts
  • Blue are motion blocks
  • In the costumes tab we can change aesthetics
  • The sounds tab can introduces sounds and multimedia
• The blank slate on the right is where we can drag and drop the puzzle pieces and connect them in order to instruct Scratch to do things
• when green flag clicked is equivalent to the start of your program
  • The green flag button starts, the red stop sign button ends
• When we drag blocks together, the edge of the block glows white to signify they connect

• The hello, world Scratch program won’t stop until we click the red stop sign as we never told Scratch to stop in the script

Sounds

• We can also add sounds

Loops

• If we want Scratch to do something repeatedly, we can use loops

  • Can move the sound into a repeat block

    • The containing block will grow to fit

      

      • This seems to only ply the meow once

      • The sound repeats so quickly they overlap

        

      • This one plays the sound until done before the next cycle in the loop

• This processes was an example of a common and frustrating experience when programming: bugs

Animation

• I want the cat to move back and forth forever

• This moves the Scratch the cat forward (to the right) until he hits the edge

  • If we drag the cat back, he’ll keep moving forward

    

• Scratch will now rotate 180° if touching the edge of the screen

  • But scratch is flipping upside down (literally rotating 180°)
    • Another bug!
• We can record custom sounds under the sounds tab and add it

Breaking Down Problems

• Its much easier to write complex programs if you start out by breaking them down into their component parts
  • Consider individual milestones for yourself
• Even companies like MS didn’t create Word in a day
  • Software developers make one small feature at a time
  • Eventually, this becomes millions of lines of code

pet the cat

• Reading and understanding code is another side of software development

  • Teams need to do this to collaborate

    

• When this program starts, nothing happens until the mouse pointer touches the cat, in which the cat meows

don’t pet the cat

• This script has an if else
• Will play a lion’s roar if the mouse pointer touches the cat, but will meow and wait 2 seconds if not

counting sheep

• This first sets a variable called counter to 0
• It will forever say counter for 1 second, wait one second, then increment the counter
• Ultimately, this will count forever

cough0

• We can create our own puzzle pieces

  • We can do this in most programming languages
    • Where we create functions

  • In scratch we can utilize the functionality of existing puzzle pieces

    

• There is an opportunity for better design here

  • IT looks like we’ve copied and pasted puzzle pieces

cough1

• We can improve this with loops

• Better design as we can change what the cat is saying or the wait time in one place

cough2

• What if I just want a puzzle piece to make any sprite cough?

  • Gain the ability to share the functionality to use elsewhere

    

  • We’ve defined a new block called cough

    • We repeat cough 3 times, abstracting away the complexity

    

• We can go even further by passing in a value to your custom block

  • This value is called an argument or parameter
    • For example, the say block takes in an argument of “hello, world” or some other phrase
• Whatever the user passed into cough will replace n!
• The evolution of this program is an example of what it’s like to program and solve problems
  • There were opportunities to improve from a correct yet poor design
  • To be good at programming is to be able to notice opportunities like this

Threads

• In Scratch, we can have multiple sprites, each with their own scripts

  • Two things will happen simultaneously, called threads

    

• This program has a cat chasing a bird

• Here’s what guides the bird:

  

  • Location in the world can be addressed with coordinates
  • Will keep moving around if not touching the cat

• Here’s what guides the cat:

  

  • The cat will point in a random direction
  • Forever checks if touching the bird and moves towards the bird
    • If touching the bird, a lion’s roar will play and the script will stop
• If we increase the movement speed of the bird to 6 steps, it still gets caught
• If we increase the movement speed of the cat to 10 steps, the bird stands no chance!

Events

• A computer can do multiple things at a time due to multithreading
  • Now that computers have multiple cores, they can literally do two things at once
  • However, computers are so fast that even if two things are technically not happening at the same time, we can’t notice the difference
  • These threads can also intercommunicate in Scratch with events

• This sprite (an orange puppet) will forever check for the spacebar being pressed

  • If this happens, the sprite will say “Marco!” for 2 seconds and broadcast event

    • Events are messages only the computer can hear

      • If another sprite is configured to listed for event it can respond

      

• This sprite will say “Polo!” for 2 seconds if it hears event

• When the green flag is clicked, the orange puppet will wait for the spacebar and then tell the other sprite when to say “Polo!”
• This idea allows two sprites to interact in such a way that one sprite does something only if the other does something first

Closing Thoughts

• Programmers in the real world don’t typically program by dragging and dropping code blocks
  • They write textural lines of code (C, Java, Python, etc.)
  • However, the ideas are fundamentally identical
    • Scratch gets rid of the syntactical distractions
• Understanding functions, loops, conditions, variables, etc. provides a fundamental understanding of what it’s like to program
• We focused on imperative or procedural programming, but other types of programming exist as well
  • Object oriented programming
  • Functional programming
• Even in all these different ways of programming, we are still utilizing the same basic building blocks we’ve explored in Scratch
  • We can assemble these building blocks to solve problems
• Oscartime was a complex game
  • Zooming in, we see these basic concepts
    • Forever loops make the trash fall, an if conditions to raise the lid of the trash, etc.
• There are many more languages out there
  • https://en.wikipedia.org/wiki/List_of_programming_languages
  • There tend to be trends in the industry
  • A programmer typically has one or a few languages that the reach for to tackle a problem
  • Good to introduce yourself to new languages
    • They are easier to learn than spoken or written languages as the ideas persist