Assigning a Name

Learning Objectives:

A computer can associate a name with a value. It does this by creating a variable, which is space in computer memory that can represent a value. An example of a variable is a score in a computer game. The score usually starts at 0 and increases as you play the game. The score can change or vary during the game, which is why we call it a variable. You also associate a name with a value when you enter a new contact name and phone number in your cell phone. When you tell your phone to call “Alexa” it will look up the phone number associated with that name and call it.

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Figure 1: A pong game in Scratch with a score shown in the upper left.

Think of a variable as a box that has a label on it and you can store a value in the box. The value can be anything that can be represented on a computer and stored in a computer’s memory. A computer’s memory is only made up of numbers (really, just patterns of voltages, but we can think about them as numbers). Everything that a computer can remember in its memory is translated into these numbers – but don’t worry about how this works right now.

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Figure 2: Creating a variable and setting its value in memory.

In programming languages, setting a variable’s value is also called assignment. A statement like a = 4 means that the symbol a refers to space (in the computer’s memory) that is assigned the value 4. When we use the symbol a in a program the computer will substitute the value 4. If we later change the value stored at a, say by doing a = 7.2 then we say that the variable a now has the value 7.2 meaning that the value in the box (memory) associated with the name a is changed to 7.2.

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Figure 3: Changing the value of a variable in memory

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