How do I stop these if else statements from compounding? - c++

Question

I'm trying to write a c++ program that reads input from a text file and assigns grades using a ten point grading scale then prints the results onscreen.
I think my issue may be with the if else statements in the function deriveGrade, rather than incrementing the enum, they seem to be suming up the increments. Any help would be appreciated, thanks.
#include <iostream>
#include <string>
#include <iomanip>
#include <cmath>
#include <fstream>
using namespace std;
int deriveGrade(double avarage);
enum letter_grade { A, B, C, D, F };
namespace tenPoint
{
letter_grade deriveGrade(double avarage);
char grade;
}
using namespace tenPoint;
int main()
{
string name;
double average;
ifstream inData; // Is the variable for input data from the file.
inData.open("student_status.txt", ios::in);
while (!inData.eof())
{
getline(inData, name);
inData >> average;
inData.ignore();
grade = ::deriveGrade(average);
cout << name << " " << average << " " << char(grade) << endl;
}
inData.close();
return 0;
}
int deriveGrade(double average)
{
if (average >= 90)
{
grade = static_cast<letter_grade>(grade + 65);
}
else if (average >= 80 && average < 90)
{
grade = static_cast<letter_grade>(grade + 1);
}
else if (average >= 70 && average < 80)
{
grade = static_cast<letter_grade>(grade + 2);
}
else if (average >= 60 && average < 70)
{
grade = static_cast<letter_grade>(grade + 3);
}
else if (average <= 50)
{
grade = static_cast<letter_grade>(grade + 4);
}
else
{
cout << "Invalid entry." << endl;
}
return grade;
}
Input from file:
Doe, John K.
93.2
Andrews, Susan S.
84.7
Monroe, Marylin
75.1
Gaston, Arthur C.
62.8
Harpo, Joanie Y.
42.7
Ginger, Fred T.
95.8
Program output:
Doe, John K. 93.2 A
Andrews, Susan S. 84.7 B
Monroe, Marylin 75.1 D
Gaston, Arthur C. 62.8 G
Harpo, Joanie Y. 42.7 K
Ginger, Fred T. 95.8 î
Press any key to continue . . .

Answer 1

Logic of your program is quite strange, but some common remarks can be given without deepening into your task.
Pay attention, that while you use if... else statements one by one like
if (average >= 90)
{
grade = static_cast<letter_grade>(grade + 65);
}
else if (average >= 80 && average < 90)
{
grade = static_cast<letter_grade>(grade + 1);
}
...
there is no need to check average < 90 in the else branch after average >= 90 found false. So at least code can be shorter:
int deriveGrade(double average)
{
if (average >= 90)
{
grade = static_cast<letter_grade>(grade + 65);
}
else if (average >= 80)
{
grade = static_cast<letter_grade>(grade + 1);
}
else if (average >= 70)
{
grade = static_cast<letter_grade>(grade + 2);
}
else if (average >= 60)
{
grade = static_cast<letter_grade>(grade + 3);
}
else if (average <= 50)
{
grade = static_cast<letter_grade>(grade + 4);
}
else // check here! Invalid interval is for values between 50 and 60?
{
cout << "Invalid entry." << endl;
}
return grade;
}
But this is not significant improvement.... much better to make a formula and use single statement with assignment to grade = ...
UPDATE:
And one more comment. If you know the interval of unacceptable values, check it first (before all other calculations):
int deriveGrade(double average)
{
// check the correctness of argument first
if (average > 50 && average < 60)
{
cout << "Invalid entry." << endl; // notification
return grade; // previous value
// also consider returning special value for error case
}
// calculate value for grade
grade = ...
// return updated value
return grade;
}
section "calculate value for grade" is for you, and while writing this part of code keep in mind that:
ternary operation operation is useful for one special case, e.g. grade = (average >= 90)? 65 : floor(100 - average) / 10;
using global values (like grade) in a function is bad practice as well as making logic based on the assumption that initial value of global variable is correct

Answer 2

The reason is because you are adding to your grade variable without clearing it, so the result of previous operations are carried over in deriveGrade.
My advice is to remove the global char grade; in your namespace, use a local variable in your deriveGrade, and a different local variable in your main.
If you look at your function code, grade will only have 65 added to it (to make an ASCII 'A') if your grade is above 90. Every subsequent addition however, pretends that this addition has happened. If you instead make sure that each else if does not rely on previous if or else if code, then your code should be more correct.
char deriveGrade( double average )
if( average > 90.0 )
{
return 'A';
}
else if( average > 80.0 )
{
return 'B';
}
...
This solution removes even the need to use a grade variable in your deriveGrade
An even better alternative that uses the enum you so nicely created is:
enum letter_grade : char
{
A = 'A', B = 'B', C = 'C', D = 'D', F = 'F'
};
Which allows you through a (char)letter_grade to swap between the enum representation and a char (your deriveGrade would then return a letter_grade instead).