What's a Function Again?
In order to understand the vertical line test, it's important that you understand what a function is. Make sure to check out the intro to functions lesson if you're new to the idea. You can think of a function like a vending machine. A function takes an input (like the code you enter on the vending machine) and gives you an output (like the candy or pop you get out of the vending machine).
An important rule that all functions must follow is that they can only assign ONE output to an input. For example, if you enter the code D4 in the vending machine and it drops a bag of pretzels, it must drop a bag of pretzels every time you enter the code D4. It can't give you a bag of pretzels sometimes and other times a bag of chips. Every input must be matched with exactly one output. It's ok if there's another input that gives the same output. For example, there might be two slots in the vending machine that both hold pretzels. So you may be able to enter the code D4 or D5 to get pretzels. It's ok if two different inputs give the same output. It's not ok if you enter the same input and get a different result.
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