C++成绩计算器问题

我需要帮助找出我的成绩计算器的问题。该程序在用户插入字母等级时工作，例如"A或A"，输出为4.0。但我遇到的问题是，当用户插入一个"a+或a-"时，"+或-在字母等级上加0.3或-0.3"不会增加PLus或减号的值。

这是我所拥有的：

const double Plus = 0.3;
const double Minus = 0.3;

/**
 * Letter grades are A, B, C, D, and F, possibly followed by + or –. 
 * Their numeric values are 4, 3, 2, 1, and 0. In our scheme, there is no F+ or F–, 
 * or A+. Adding a + to the grade increases the numeric value by 0.3, a – decreases it 
 * by 0.3.
 *
 * Input: The letter of Grade with + or -.
 * Output: The value of inserted grade.
 *
 * @param cin the standard input stream.
 * @param cout the standard output stream.
 * @return 0 for success.
 */
int run(istream& cin, ostream& cout)
{
    // Enter your letter grade: C+
    //Grade value is[2.3]
    char list [26] = { "A, B, C, D, F, a, b, c, f" };
    char grade;
    double value;
    cout << "Enter your letter grade: ";
    cin >> grade;
    cout << fixed << setprecision(1);
    if ((grade == 'A' || grade == 'a')){
        value = 4.0;
    }
    else if ((grade == 'A+') || (grade == 'a+')){
        value = 4.0 + 0.3;
    }
    else if (grade == 'A-' || grade == 'a-'){
        value = (4.0 - Minus);
    }
    else if (grade == 'B' || grade == 'b'){
        value = 3.0;
        if (grade == 'B+' || grade == 'b+'){
            value = (3.0 + Plus);
        }
        else if (grade == 'B-' || grade == 'b-'){
            value = (3.0 - Minus);
        }
    }
    else if (grade == 'C' || grade == 'c'){
        value = 2.0;
        if (grade == 'C+' || grade == 'c+'){
            value = (2.0 + Plus);
        }
        else if (grade == 'C-' || grade == 'c-'){
            value = (2.0 - Minus);
        }
    }
    else if (grade == 'D' || grade == 'd'){
        value = 1.0;
        if (grade == 'D+' || grade == 'd+'){
            value = (1.0 + Plus);
        }
        else if (grade == 'D-' || grade == 'd-'){
            value = (1.0 - Minus);
        }
    }
    else if (grade == 'F' || grade == 'f'){
        value = 0.0;
        if (grade == 'F+' || grade == 'f+'){
            value = (0.0 + Plus);
        }
        else if (grade == 'F-' || grade == 'f-'){
            value = (0.0 - Minus);
        }
    }
    else if (!(grade == list[26])){
        cout << "INVALID LETTER GRADE" << endl;
        value = 0;
    }
    cout << "Grade value is [" << value << "]" << endl;

    return 0;
}