by Spencer Tiberi


  • 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


  • 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


  • 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 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


#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


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

    (print "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

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

Scratch Function

  • This is the function for say

Scratch Forever Loop

  • This says “hello, world” forever

Scratch Repeat Loop

  • This says “hello, world” 50 times

Scratch If Else

  • 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

Scratch Interface

  • 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

Scratch 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


  • We can also add sounds

Scratch Sounds


  • 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 Scratch Meow 1
        • This seems to only ply the meow once
        • The sound repeats so quickly they overlap Scratch Meow 2
        • 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


  • I want the cat to move back and forth forever

Scratch Move 1

  • 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 Move 2

  • 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

Scratch Move 3

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

Pet the Cat

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

don’t pet the cat

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

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


  • 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


  • We can improve this with loops


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


  • 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


  • 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: 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: 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!


  • 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

Events 1

  • 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

Events 2

  • 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
    • 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

It’s Raining Men

  • David closes it all with another Scratch project