Member Avatar for ashboi

I'm trying to write a program that uses vector to store the grades for the number of students entered.But i seem to be getting some problems with my code.
I am able to enter the number of students, but when it asks for the grades, the program exits.

I hope someone can tellme whats wrong with my code

#include <cstdlib>
#include <iostream>
#include <vector>

using namespace std;

int main()
{
    vector<int> Grades;
    int i= 0;

    cout << "Enter number of students: ";
    cin >> i;

    for( i = 0; i < 0; i++)
    {
            cout << "Enter grade for student" << (i + 1) << ": ";
            cin >> Grades[i];
    }


    int Total = 0;
    for(int i = 0; i < 0; i++)
    {
            // Add to the total
            Total += Grades[i];
    }

    double Average = (double)Total / i;
    cout << "Average Mark: " << Average << "\r\n";
    system("PAUSE");
    return 0;
}
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  • Latest Post Latest Post by ashboi

Dani AI

Generated

Good catch by : the crash comes from indexing an empty vector. A quick rule of thumb: resize(n) sets size (safe for operator[]), while reserve(n) sets only capacity (still unsafe to index until you actually push elements). Either construct the vector with a size up front, or append as you read.

If you prefer to append, this pattern avoids out-of-range writes and keeps the code simple:

std::size_t n = 0;
std::cout << "How many students? ";
if (!(std::cin >> n) || n == 0) return 0;

std::vector<int> grades;
grades.reserve(n);  // capacity only

for (std::size_t k = 0; k < n; ++k) {
    int g = 0;
    std::cout << "Grade #" << (k + 1) << ": ";
    std::cin >> g;
    grades.push_back(g);   // now size() grows safely
}

To compute the average and also count passing and failing grades in one go, lean on the standard library:

#include <numeric>
#include <algorithm>

// average
double avg = std::accumulate(grades.begin(), grades.end(), 0.0) / grades.size();

// pass/fail counts (adjust threshold as needed)
int pass_mark = 50;
auto pass_count = std::count_if(grades.begin(), grades.end(),
                                [pass_mark](int g){ return g >= pass_mark; });
auto fail_count = static_cast<int>(grades.size()) - pass_count;

std::cout << "Avg: " << avg << "\n"
          << "Pass: " << pass_count << ", Fail: " << fail_count << "\n";

Small tips that prevent bugs: use std::size_t for indices, avoid reusing the same i across scopes, and guard against n == 0 before dividing. If you want to use operator[] instead of push_back, pre-size the vector to n first so each index is valid.

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All 6 Replies

Member Avatar for scudzilla
  1. You are storing the number of students into i, which gets reassigned a new value. Store the number of students into another integer variable (num, perhaps?).
  2. At lines 15 and 23, you use i for the counter, starts at 0, but your condition is that it will end once i is greater than or equal to 0 which means your for loops do nothing at all. You are supposed to use another integer variable (num) at the condition. for(i = 0; i < num; i++);
Edited by scudzilla
Member Avatar for ashboi

ah.. thanks for pointing that out.

I've changed my code, however i'm getting a debug assertion failed! error after entering the no. of students.

#include <cstdlib>
#include <iostream>
#include <vector>

using namespace std;

int main()
{
    vector<int> Grades;
    int num;
    int i;

    cout << "Enter number of students: ";
    cin >> num;

    for( i = 0; i < num; i++)
    {
            cout << "Enter grade for student " << (i + 1) << ": ";
            cin >> Grades[i];
    }

    int Total = 0;
    for(int i = 0; i < num; i++)
    {
            // Add to the total
            Total += Grades[i];
    }
    double Average = (double)Total / num;
    cout << "Average Mark: " << Average << "\r\n";
    system("PAUSE");
    return 0;
}
Edited by ashboi
Member Avatar for scudzilla

That happens because you are trying to access the elements of an empty vector. You could do two things: first would be to resize the vector after you input the number of students (syntax: Grades.resize(num); or, initialize the vector entirely after the number of students is entered (aka move line 9 to line 15, and include num as the size)

Edited by scudzilla
Member Avatar for ashboi

thanks, that solved my problems.
If I would like to count the number of passing & failing grades from the grades that are stored in the vector. How I am supposed to write a code for it?

Member Avatar for scudzilla

Just use an if-else statement and a pair of counters within one of the for loops. (preferably the 2nd for loop)

Edited by scudzilla
Member Avatar for ashboi

ah, thanks for the tip. totally solved my problems.

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