Starting from Scratch

What is programming?

Programming = solving a problem with code

Problem => Algorithm => Code

Examples of Problems:

- Cooking boiled eggs
- Integer division of two numbers
- Missile Pursuit (or animal chasing a prey)
- Play a game like chess
- Recover 3D information from stereoscopic images

It is important to realize the different nature of the two tasks:

  1. finding an algorithm to solve a problem

  2. translating an algorithm into a code (coding)

Things to be aware of

  • Because computers are stupid, that is, lack a theory of mind and cannot guess your desires and intentions, you must provide very detailed and exhaustive instructions.

  • Learning to code takes time because it requires to master not only the semantic but also the syntax (=grammar) of the language.

  • Just as learning to play an instrument, or speak a second language, programming is a skill that can only be acquire with lots of practice.

Starting with Scratch

To begin our journey into programming, we will use Scratch, a system developed the MIT media lab to teach how to program to kids.

A great advantage of Scratch is that programs are created using a graphical interface, preventing syntactic errors. Thus, you can learn the language without having to learn its grammar!

One can either work online at or offline, by downloading Scratch at

It may be a good idea to run the tutorial “Getting Started with Scratch” on the website

First steps

Program 001

In the motion group, take the instruction turn 15 degrees and drag it onto the Scripts panel.


Double-Click repeatedly on the block turn 15 degrees, you should see the cat (sprite 1) rotate.

In Scratch, when one double-clicks an instruction in the Scripts panel, the computer executes it.

Program 002

Drag the instruction move 10 steps from the motion group, and add it to the bottom of the instruction turn 15 degrees. Change the value 10 into 50.


You have just created a block of instructions, that is, your first program or script, Bravo!

  • Double-Click on the block and see the sprite moving.

  • Note that inside a block, instructions are exectuted sequentially, one after the other. Can you prove it?

  • Experiment with changing the argument of the instruction move (Tip: to clear the drawing area, move the instruction pen/clear to the script window and execute it)

Program 003

Click on the pen group, and add pen down at the top of the block.


Run it.

Program 004

Construct the following scripts and play with them until you are sure to understand the behavior of the computer..


Concepts learned so far

  • Instruction

  • Argument of an instruction (change 10 in move 10 steps)

  • Block of instructions and sequential execution


repeat (for loop)

Computers are good at doing tasks repeatedly (as they do not get tired).

Click on the “Control” group, and try to construct the following script:


  • Clicking on the green flag will execute the block of instructions

  • The Repeat instruction executes the inner block of instruction a number of times specified as an argument. This is called a loop

  • Adjust the parameter of the Repeat instruction so that the sprite draws a full circle when you click once on the green flag.

  • Replace the repeat instruction by forever.

Repeat until

Modify the script as follows:


Tip: the condition key space pressed? is in the Sensing group.

This illustrates a repeat…until loop: the inner block is executed until the condition is satisfied.

Two sprites

Add a new sprite, and duplicate the script from sprite1. Click on the green flag. You should see the two sprites running in circles.


Remark that the scripts associated to the two sprites run in parallel (rather than sequentially).

Conditional execution or branching

Create a new scratch project, and change the costume of the sprite into a ball.

Then write and execute the following script.


You should see the ball bounce on the edges.

First series of exercices

  1. With Scratch, use the instructions “pen down” and “move” and “turn” to (a) make the cat draw a square (with sides measuring 100 steps) (b) draw an hexagon (c) draw a circle

  2. Using the Control/Forever, make the cat turn continuously along a circle.

  3. Bouncing ball

  • Delete the cat. Using new sprite/open, add a ball.

  • Make the ball move automatically horizontally from left to right and bounce when it touches an edge (tip: use Control/forever)

  • Make the ball follow the mouse.

  • Add a second ball that follows the first.

  1. Create a script that asks for your name and then displays “Hello !”. Tip: use the instructions sensing/ask, looks/say and operator/join and the variable sensing/answer.



Using the group variable, we are going to create a variable a and make it display continuously the x-coordinate of the ball.


The concept of variable is very important. You can think of it as a name for a object that can change (here the object is a number).

Now study the following script:


The loop is executed 100 times. Each time, the value of the variable a is incremented by 1, and is used to compute new x and y coordinates where to sprite is instructed to moved to.


Second series of exercices


“Multiply by adding”: Write a program that reads in two integer numbers and displays their sum.


Guess a number

“Guess a number”. Make Scratch pick up a random number in the interval [1,100], and loop asking you for a guess and reply either too low, too high, or you win! depending on your answer.


Estimate PI

  1. We are going to estimate the number PI by a Monte Carlo method.:

  • Repeatly (e.g. 2000 times) picks up two random numbers on the interval [-1, 1]. This corresponds to a dot inside a square of size 2x2.

  • Count how many times the dot falls within the circle of radius 1 centered on the origin (Pythagore helps you here: the dot is within the circle iff (x * x + y * y) < 1.)

  • The proportion of dots falling within the circle, multiplied by four (the area of the square), is an estimate of teh area of the disk, that is, the number pi.