How can I see what is inside a vector? - c++

Question

When I check the values inside of the homework vector using cout it seems to only be returning the value inside homework[0]. Can somebody look over my code and let me know where I'm going wrong?
int main()
{
cout << "Please enter midterm and final: " << endl;
double midterm, gfinal;
cin >> midterm >> gfinal;
cout << "Enter all your homework grades, " << endl;
double x;
cin >> x;
// initing a vector object named homework
vector<double> homework;
// show debug on the vector
while (homework.size() != 3)
homework.push_back(x);
if (homework.size() == 0)
{
cout << endl << "You need to enter at least one number";
return 1;
}
// vector before sorting
// since cout << homework did not seem to work I could always just write a debug function to iterate over structures and print them to the console
cout << homework[0] << endl;
cout << homework[1] << endl;
cout << homework[2] << endl;
// sort the vector here
sort(homework.begin(), homework.end());
// vector after sorting
//cout << homework;
cout << homework[0] << endl;
cout << homework[1] << endl;
cout << homework[2] << endl;
int mid = homework.size() / 2;
cout << "The below is mid" << endl;
cout << mid << endl;
double median;
if (homework.size() % 2 == 0)
median = (homework[mid - 1] + homework[mid]) / 2;
else
median = homework[mid];
//streamsize prec = cout.precision(3);
cout << "Your course grade is "
<< 0.2 * midterm + 0.4 * gfinal + 0.4 * median << endl;
//cout.precision(prec);
return 0;
}
This is the specific code that is causing me confusion:
// vector before sorting
// since cout << homework did not seem to work I could always just write a debug function to iterate over structures and print them to the console
cout << homework[0] << endl;
cout << homework[1] << endl;
cout << homework[2] << endl;
// sort the vector here
sort(homework.begin(), homework.end());
// vector after sorting
//cout << homework;
cout << homework[0] << endl;
cout << homework[1] << endl;
cout << homework[2] << endl;
When the program starts it asks for 2 values, so I plugged in 100 100.
Then it asks for 3 values so I used 80 90 100 the cout of all the homework positions is showing 80 when I am expecting 80 90 100.
The actual program works as the final cout returns 92 as expected.

Answer 1

In your code:
double x;
cin >> x; // <-- reading from cin only once
// initing a vector object named homework
vector<double> homework;
// show debug on the vector
while (homework.size() != 3)
homework.push_back(x); // <- inserting same value three times
you are reading from cin only once, i.e., you are reading a single value. Then, you are inserting that read value three times into the vector homework. Therefore, homework[0], homework[1] and homework[2] contain the same value.
Consider placing cin >> x inside the while loop to read three times from cin instead of just once, i.e., read from cin on each iteration of the loop:
vector<double> homework;
while (homework.size() < 3) {
double x;
cin >> x;
homework.push_back(x);
}

Answer 2

In addition to the bug that El Profesor pointed, for iterating a vector in modern C++, all you have to do is:
#include <vector>
#include <iostream>
int main()
{
std::vector<int> homework{ 70,80,90 }; // For the example
// "See what is inside the vector":
for (auto grade : homework)
std::cout << grade << '\n';
}

Answer 3

So, the bugs have been fixed already. I will add a "more-C++"-style solution using STL algorithms.
For filling the vector, I will use std::copy_n. Meaning, read n values from std::cin and insert them into the target vector.
And for your question
How can I see what is inside a vector?
the solution is, iterate over the elements in the vector and copy the vectors values to ctd::cout. For this we will use the std::ostream_iterator
Please note: I always use qualifed names lke std::cout. Please consider. And I rarly use std::endl, becuase this always call flush, which is in most cases not needed. Also: All variables should always be initialized. Always.
And then, I added many comments. Please also consider. This improves the code readability and quality drastically.
Please see:
#include <iostream>
#include <vector>
#include <algorithm>
#include <iterator>
constexpr size_t NumberOfHomeworkGrades = 3U;
int main()
{
// Print title and ask use to enter data
std::cout << "\nCalculation of course grade\n\nPlease enter midterm and final: " << '\n';
double midterm{ 0.0 };
double gfinal{ 0.0 };
std::cin >> midterm >> gfinal;
// Ask the use to enter the howmwork grades
std::cout << "Please Enter "<< NumberOfHomeworkGrades << " homework grades\n";
// Get the data from the user and put it into the vector
std::vector<double> homeworkGrades{};
std::copy_n(
std::istream_iterator<double>(std::cin), // We will iterate over std::cin and read data
NumberOfHomeworkGrades, // Over all we read data NumberOfHomeworkGrades times
std::back_inserter(homeworkGrades) // And we psuh the data into our homeworkGrades-vector
);
// Show the vector before sorting. Simply copy all data in the vector to std::cout using the ostream_iterator
std::cout << "\n\nEntered grades before sort\n";
std::copy(homeworkGrades.begin(), homeworkGrades.end(), std::ostream_iterator<double>(std::cout, "\n"));
// Sort the vector here
std::sort(homeworkGrades.begin(), homeworkGrades.end());
// Show the vector adter sorting. Simply copy all data in the vector to std::cout using the ostream_iterator
std::cout << "\n\nSorted grades\n";
std::copy(homeworkGrades.begin(), homeworkGrades.end(), std::ostream_iterator<double>(std::cout, "\n"));
// Calculate the median
double median{ 0 };
// First calculate the mid, to do the calculation only one time and to show the result
size_t mid{ homeworkGrades.size() / 2 };
if (!homeworkGrades.empty())
if (homeworkGrades.size() % 2 == 0)
median = (homeworkGrades[mid - 1] + homeworkGrades[mid]) / 2;
else
median = homeworkGrades[mid];
// Show the result to the user
std::cout << "\n\nThe mid value is (maybe truncated): " << mid
<< "\nThe median value is: " << median
<< "\n\nYour course grade is: " << 0.2 * midterm + 0.4 * gfinal + 0.4 * median << '\n';
return 0;
}