Variables Local and Global Computer Science I --- Haas Variables are one of the central concepts in computer science and one of the most powerful tools in a programmer's toolbox. They will pop up steadily throughout the rest of the semester. Let’s begin with script variables, also called local variables To create a variable use You can click on the orange "a" to change the name. The script below will draw a regular polygon with that number of sides. The value of sides is used twice, first in the repeat block to say how many times the move-and-turn combination should be done, and again in computing the angle through which to turn for each side. Try running the script (by clicking the green flag) several times to see what shapes it draws. MAKE SURE YOU UNDERSTAND EXACTLY WHAT IS HAPPENING IN THIS EXAMPLE. Another place where script variables can be useful is in a program that interacts with the user. Get both of the above scripts running, show Haas the results and explain what is happening. Now let’s look at Global variables Sometimes it's not good enough to remember a value inside one script. Instead, you need the value available everywhere in your project. The classic example is the score in a video game; even when no script is running, the score should be remembered. There was a time, in the early history of programming languages, when all variables were global. This led to a lot of bugs, because different parts of the program would use the same name for different purposes and erase each other’s saved values. So don't use global variables as your first choice; think whether a script variable would work instead. To make a global variable, you must go to the Variables palette and click on the Make a variable button (not a block -- you can't put it into a script!): Click on the variables tab and click "Make a variable" In the box that pops up - type the name of our variable "speed" (don't type the quotes). Now you'll have blocks to use your variable. Now create the scripts on the following page. Write the scripts below to move an object around the screen. Notice the variable speed is used in more than 1 script. Speed is a global variable. Complete the scripts below. MAKE SURE YOU UNDERSTAND EXACTLY WHAT IS HAPPENING, then show Haas the results and explain.