C++ Project: Grade Calculator

What you will build

A grade calculator takes numeric scores, maps each one to a letter, then prints a report with an average. The letter boundaries live in one function so the table and the average never disagree about what 89 means. Scores sit in a vector<int>.

Demo scores are hardcoded. Use Try it in C++ and /cpp/try. The Python and HTML try pages will not compile this.

Numeric score to letter

Walk the scale from the top. 90 and up is A, 80 is B, 70 is C, 60 is D, anything below is F. A score outside 0 through 100 still gets a letter here; you can clamp or reject those in practice.

#include <iostream>
using namespace std;

char letterFrom(int score) {
  if (score >= 90) {
    return 'A';
  } else if (score >= 80) {
    return 'B';
  } else if (score >= 70) {
    return 'C';
  } else if (score >= 60) {
    return 'D';
  }
  return 'F';
}

int main() {
  cout << 91 << " " << letterFrom(91) << endl;
  cout << 84 << " " << letterFrom(84) << endl;
  cout << 59 << " " << letterFrom(59) << endl;
  return 0;
}

Output is A, then B, then F. 90 is the first A; 89 is still a B.

Average of a vector

Sum the scores as a double, then divide by size(). Integer division of the sum would drop the fraction. An empty vector has no average; return 0 and let the caller print a message.

#include <iostream>
#include <vector>
using namespace std;

double average(const vector<int>& scores) {
  if (scores.empty()) {
    return 0.0;
  }
  double sum = 0.0;
  for (int s : scores) {
    sum = sum + s;
  }
  return sum / scores.size();
}

int main() {
  vector<int> scores;
  scores.push_back(91);
  scores.push_back(84);
  scores.push_back(76);
  cout << average(scores) << endl;
  return 0;
}

GPA-style points

A GPA-style average maps letters to 4.0, 3.0, 2.0, 1.0, and 0.0, then averages those points. That is not the same as averaging the raw percents. Both numbers are useful; this project prints each.

#include <iostream>
using namespace std;

char letterFrom(int score) {
  if (score >= 90) return 'A';
  if (score >= 80) return 'B';
  if (score >= 70) return 'C';
  if (score >= 60) return 'D';
  return 'F';
}

double pointsFrom(char letter) {
  if (letter == 'A') return 4.0;
  if (letter == 'B') return 3.0;
  if (letter == 'C') return 2.0;
  if (letter == 'D') return 1.0;
  return 0.0;
}

int main() {
  cout << pointsFrom(letterFrom(91)) << endl;
  cout << pointsFrom(letterFrom(84)) << endl;
  return 0;
}

91 becomes A and 4.0. 84 becomes B and 3.0.

Complete grade report

Print every score with its letter, then the percent average and the GPA-style average, both with two decimal places. Names are optional here; the vector is scores only so the arithmetic stays obvious.

#include <iostream>
#include <iomanip>
#include <vector>
using namespace std;

char letterFrom(int score) {
  if (score >= 90) return 'A';
  if (score >= 80) return 'B';
  if (score >= 70) return 'C';
  if (score >= 60) return 'D';
  return 'F';
}

double pointsFrom(char letter) {
  if (letter == 'A') return 4.0;
  if (letter == 'B') return 3.0;
  if (letter == 'C') return 2.0;
  if (letter == 'D') return 1.0;
  return 0.0;
}

int main() {
  vector<int> scores;
  scores.push_back(91);
  scores.push_back(84);
  scores.push_back(76);
  scores.push_back(68);
  scores.push_back(95);

  cout << "Score  Letter" << endl;
  double sum = 0.0;
  double pointSum = 0.0;
  for (int s : scores) {
    char L = letterFrom(s);
    cout << setw(5) << s << "  " << L << endl;
    sum = sum + s;
    pointSum = pointSum + pointsFrom(L);
  }

  cout << fixed << setprecision(2);
  cout << "Average percent: " << (sum / scores.size()) << endl;
  cout << "GPA-style: " << (pointSum / scores.size()) << endl;
  return 0;
}

Five rows print first. The percent average is 82.80. The GPA-style average uses A, B, C, D, A which is (4+3+2+1+4) / 5 = 2.80.

Common mistakes

• Testing F first with score < 60 and then using if instead ofelse if for the rest. A 95 is less than nothing in a broken chain and can fall through. Start from A and go down, or use exclusive ranges.
• Averaging with int sum and sum / scores.size(). Both sides integer-like means you lose the fraction before you print.
• Calling letterFrom in the report and a second copy of the scale in the GPA function. One function for the letter; one function from letter to points.
• Dividing by size() without checking for an empty vector. Guard that case.

Practice

1. Treat 90–100 as A, and print invalid for a score below 0 or above 100 instead of a letter.
2. Count how many A grades appear and print that count under the report.
3. Add plus/minus letters: 87–89 is B+, 83–86 is B, 80–82 is B−, and similar bands for A and C.

Last project: a shopping cart with a receipt and a money total.