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copied!<p>If you need to pass <code>+</code> or <code>-</code> as a <em>symbol</em> (not as a procedure), you have to quote it first:</p> <pre><code>'+ '- </code></pre> <p>For example:</p> <pre><code>((equal? gradeletter+- '+) .33) ((equal? gradeletter+- '-) -.33) </code></pre> <p>But from the context, I don't think the <code>gpa-allocator</code> procedure is correct. A grade can be <code>a</code> or <code>a+</code>, the conditions imply that <code>+</code> or <code>-</code> are actual grades, which is wrong.</p> <p>Maybe you should represent grades as strings and check (using <code>string-ref)</code> the first character in the string to determine if it's <code>#\a, #\b, #\c, #\d, #\e</code> and (if the string's length is greater than 1) test if the second character in the string is either <code>#\+</code> or <code>#\-</code>. Then you can determine the appropriate value of the grade by adding the two values. Alternatively, you could pass the grade as a symbol and convert it to string. This is what I mean:</p> <pre><code>(define (gpa-allocator gradeletter+-) (let ((grade (symbol->string gradeletter+-))) (+ (case (string-ref grade 0) ((#\a #\A) 4) ((#\b #\B) 3) ((#\c #\C) 2) ((#\d #\D) 1) ((#\e #\E) 0) (else 0)) (if (> (string-length grade) 1) (case (string-ref grade 1) ((#\+) 0.33) ((#\-) -0.33) (else 0)) 0)))) </code></pre> <p>Don't forget to test it:</p> <pre><code>(gpa-allocator 'A) => 4.0 (gpa-allocator 'A+) => 4.33 (gpa-allocator 'A-) => 3.67 </code></pre>

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