CMU CS Academy 2.2.4 and 2.2.5

CMU CS Academy 2.2.4 and 2.2.5
Variables and Mouse Release Events

By Maria Cruz Farooqi

​Learning Target:
I can identify and utilize global and local variables to modify the properties of a shape in a graphical program.
I can use Python to change shapes when I release the mouse and explain what happens when I press and release the mouse in different ways.

Drag and Drop

What we want is to use only ​

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circle. And for that, we need a way in our ​

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function to refer to that circle. And for that, we need a ​

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.

Drag these tiles and drop them in the correct blank above

one

onMousePress

global variable

Multiple Choice

How can we be sure that there are 2 circles after we click the mouse in the previous example?

• We cannot. There is no way to tell.

• 
  The Inspector shows one point with two colors.

• Both a red and a blue circle are clearly visible.

• 
  The circle is purple, because it combines blue and red.

Dropdown

What's happening here? Look at line 1, and you will see that instead of just calling Circle, we have ​

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. When we create a shape, we can ​

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. Here, we used the ​

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c. After that, we can use c to get and ​

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of that circle. For example, c.fill is the ​

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. So the line c.fill = 'blue' sets the fill color of the circle to blue.

Dropdown

Run the code and press the mouse a few times.

See how the circle ​

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to where you press the mouse. Now, look at lines 4 and 5. See how we use ​

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to set the x and y ​ locations of the center of the circle.

Dropdown

The ​

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c is not a function parameter. It is defined ​

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of any function. We call a variable defined outside of any function a ​

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variable. So c is a global variable.

Unlike local variables, global variables can be used anywhere, inside or outside any function. That is why we can use c inside ​

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Multiple Choice

Which variables are local, and which are global, in the previous example?

• x and y are local, and c is global.

• 
  They are all local.

• 
  

  They are all global.

• c is local, and x and y are global.

Drag and Drop

onMouseRelease

When you press the mouse down, eventually you stop ​

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. At that time, we say that you ​

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the mouse. Then, Python calls ​

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, if you wrote it.

Drag these tiles and drop them in the correct blank above

holding it down

release

onMouseRelease

Dropdown

Run the code. Press the mouse, move it around, then ​

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. See how you get a ​

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each time you press the mouse, and a ​

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each time you release it.

Multiple Choice

In the previous example, what happens if you press the mouse and immediately release it (without moving it)? Try it first before answering.

• We get two circles -- a navy circle drawn directly over a gold circle.

• The code crashes if we do that.

• We get only one circle -- the gold circle becomes navy.

• There is no way to tell if we have one or two circles.

Dropdown

Run the code. Press the mouse, ​

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for a while, and ​

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it. See how the circle ​

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when you press the mouse, and stays blue until you release it, when it ​

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again. Nice!

​Your Turn: Go to CMU CS Academy and finish the lessons, Checkpoints, and Exercises.