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I am trying to assign a number to a letter grade that a user inputs. The user will input a letter such as A, B, or C and then based on what they enter a value is stored in an integer.

I figured the easiest way to do this was setup an array such as:

char[] grade = char[] grade = {'A','B','C','D','F'};
grade[0] = 4;
grade[1] = 3;
// ... as so on

So, whenever a user inputs 'A' for their grade, I use the 4 when I need to.

I am trying to figure out how to read an input (JOptionPane) and read the letter they enter to the corresponding value I have assigned it. How do I go about parsing the letter input based on my array?

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2 Answers 2

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I'm not sure, whether I understood you right:

int grade (char input) 
{
    return 5 - (input - 'A');
}

Think of it as a graph. In computer encoding, Ascii or UTF8, the characters A-F are sequentially encoded, with A being the lowest, but not 0 or 1, but 65 or something, which we don't remember exactly. 

  5  |               *
  4  |                 *   
  3  |                   * 
  2  |                     * 
  1  |                       *
  0  +-- ... ------------------*----->
                     A B C D E F 
                    65 6 7 8 9 70

Drawing this graph, I mentioned that you jump form D to F - is that intentionally? If not:

If we subtract from 5 the difference from input and 'A', we get 5 - 0 for 'A', and 5 - 1 for 'B' and so on. Since we don't want to look up the number for 'A', we use 'A' directly, which is fine, since we can perform arithmetics on characters.

We could as well write 

return 70 - input;

or 

return 'F' - input;

The standard form of a linear equation is y = mx + n, where n is the cut through the y-axis (70), and m = -1, the gradient, negative in our case.

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2 Comments

No need. You wouldn't need an array using this method. What happens is that the arithmethic causes the char to be cast to an int, allowing it to be subtracted from input. Basically, you calculate the "difference" between input and A. For A, this is 0. For B, this is 1. For C, this is 2. And so on. Then this is subtracted from 4, meaning A will eventually yield 4, B will yield 3, C will yield 2 and so on. To make this safe to use, do some bounds checking in your input to make sure people don't supply a Z or something.
In case my above comment is slightly confusing, think of your input being a number from 65 to 69. We first want to "normalize" this as 0 to 5. So we subtract 65 from it. Only in the form of 'A'. We're counting the offset of the input character from 'A'. Then we subtract that from 4 to get your result. Btw, your original post might have to say E instead of F. Unless you want some non-linear grading system. In that case you'll need a different approach, like a map.
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It might be easier to just cast the character to an int. A char basically has an int value. Doing this:

int i = (char)'A';

will yield 65. For a lower case a it would be 97. You could cast the char to int, then use that value to do bounds checking and some arithmetic. Sequential letters will yield sequential integers. This is safe since you're running on a JVM and don't have to take bizarro character set orders for different platforms into account.

Apart from that, seeing how you have limited allowed inputs, a map could work well too:

Map<Character, Integer> grades = new HashMap<Character, Integer>();
grades.put('A', 4); //optionally also: grades.put('a' 4);
...

Type params and auto-boxing and unboxing makes this a lot more convenient these days.

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2 Comments

I basically am just trying to ask a user what grade they got in a class and then based on that grade, use a number assigned which in this example are used as the amount of credits they received for a class. So, if they enter that they got an A, then they received 4 credit hours for that class.
We gathered that, and even so the exact context isn't important since your question is clear. Both my and user unknown's approach will do what you want.

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