Introduction to Scratch

Introduction to Scratch Computer Programming Modeling a Passive Solar Home

What is Scratch? • Scratch is a visual computer language • Uses graphical blocks to create a program as opposed to typewritten commands. • Output is primarily visual • Made to be simple to use, but it contains all the elements of a programming language

Overview • The output is displayed on a stage, which can have several different backgrounds • Sprites arecharacters or objects that are moved about the stage and which also can change appearance • The inputs and results of calculations are also displayed on the stage • Scripts, or programs, are used to change the background and to move or change the appearance of sprites

Sprites • Characters or pictures in the animation that you can manipulate • Move, Glide, Shrink, Grow, Alter, appearance • The first Sprite that shows up is the Cat • In the lower right hand part of the screen are where the Sprites are loaded, created, or altered • Sprites do not have to be characters. They can also be objects that you want to change or move, such as a lamp that can be on or off or a ball moving across the screen

Stage • The white area in the upper right that has a background and on which the sprites move. • The stage can have multiple backgrounds which can be changed by the script • Selecting Stage, just to the left of the Sprites, brings up three tabs for the stage in the center of the window: Scripts, Backgrounds and Sounds • Under the Background tab, you can load in pictures that will serve as the background behind your Sprites. • You can paint a background, import a background that Scratch provides (in the Backgrounds folder), or import in your own background picture

Scripts • Scripts are the computer programs that govern the movement and appearance of each Sprite or Background • They are created by dragging commands from the left column into the center column • Commands are arranged in categories shown in the upper right of the window

Calculations and Values • Variables hold numeric values • Create them under the “variables” commands • Can set or change a variable using “set” and “change” commands • Operators perform calculations • Addition, subtraction, multiplication • Operands include existing variables and numbers • Operators give results that are used in “set”, “change”, “move to”, and other commands

Controlling the Program • Start – starts the script via flag, message, or clicking • Stop – stops the script or all of the scripts • Loops let you repeat parts of a script • Other control blocks allow a script to change its behavior if certain conditions are met

A Simple Calculator • Follow the steps below to create a program to add two numbers together. • Steps • Open scratch and click on the “stage” in the lower right. • Select backgrounds in the middle panel, then “edit” • Use the line tool to draw a “+” and a line, as one might do for addition with a pencil and paper. • Move the Cat sprite out of the way (we won’t use the sprite in this example)

Create Variables • Select “Variables” in the upper left • Select “Make a Variable” and create three variables. Suggested names are “number 1”, “number 2”, and “sum”. These appear on the stage. • Move them so theymake sense, withthe sum below theline and the numbersabove

Write the script • Click on the “Script” tab in the center panel • Click on “Control” button on the upper left and drag the “When stage clicked” into the center panel • Click on the “variables” button and drag the “set” block to the center panel, just under theexisting piece. A white line will appear that will tell you when the piece is fitting correctly. • Use the small arrow in the “set” block to change the variable from “number 1” to “sum”

Write the script (continued) • Click on “Operators” in the upper left • Drag the top operator (with the plus) so that it falls into place on the “set” block where the 1 used to be • Go back to the “variable” area and drag the “number 1” and “number 2” variables into the “+” operator. The placeholders will be highlighted when the fit is correct. • Go back to the “control” area and drag the “stop script” block to theend of the script

Run the Script • On the stage, double-click on the “number 1” and “number 2” variables. Both will have a small slider beneath them. • Double-click twice on the “sum” variable. It will turn into a larger number. • Use the sliders to changevalues of“number1” and “number 2” • Single-click anywhere onthe stage and the value of “sum” will change to be the sum of the other two variables.

Activity • Use scratch to calculate the change in temperature in a house due to heat loss

Introduction to Scratch