GPA Calculator, how do you translate a letter grade in Spinner into point value?

Hello, I have a question. How do you define the value of a selection? For example, I am creating a GPA calculator app, and I need to translate the letter grade in the selection into a grade point value such as A=4.0,B=3.0 etc. How do you do that?

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Look at Scott's GPA for part of the answer:  GPA calculator  

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Set up a list of lists.

(
(A,4.0)
(B,3.0)
...)

Then use the look up in list block to turn "A" into 4.0

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