How to use a sentinel to trigger end of vector? - c++

Question

I'm new to programming (in general) and C++ (in particular). I'm learning vectors and am trying to write a simple program that:
allows the user to enter a vector of students' test scores
when the user types the sentinel (-1 in this case), the vector terminates
outputs a tally of the student's grades
Here's my code:
#include "stdafx.h"
#include <iostream>
#include <vector>
using namespace std;
const int SENTINEL = -1;
vector<int> studentGrades = { 0 };
int myInput;
int main()
{
do
{
cout << "Please enter a student's grade: ";
cin >> myInput;
if (myInput < 1000)
{
studentGrades[myInput]++;
}
studentGrades.push_back(myInput);
} while (myInput != SENTINEL);
cout << "\n";
for (int i = 0; i < 1000; i++)
cout << i << " grade(s) of " << studentGrades[i] << endl;
return 0;
}
Two questions:
1) Can anyone provide guidance on why this code is only allowing me to enter one student's grade?
2) Is the for loop that compute the "tally" correct?
Thanks in advance for taking a look,
Ryan
* REVISED CODE *
# JCx - this is the revised code:
#include "stdafx.h"
#include <iostream>
#include <vector>
using namespace std;
const int SENTINEL = -1;
vector<int> studentGrades = { 0 };
int myInput;
int main()
{
do
{
cout << "Please enter a student's grade (or -1 to QUIT): ";
cin >> myInput;
if (myInput < 1000)
{
studentGrades.at(myInput)++;
}
studentGrades.push_back(myInput);
} while (myInput != SENTINEL);
cout << "\n";
for (int i = 0; i < 1000; i++)
cout << i << " grade(s) of " << studentGrades.at(myInput) << endl;
return 0;
}
and, I'm seeing this error:
Unhandled exception at 0x7707C42D
Microsoft C++ exception: std::out_of_range at memory location 0x0035F890

Answer 1

There's more than one problem. The attempt to access studentGrades[-1] when the users enters your sentinel value, and the fact that the default vector only contains an entry for 0 and the use of push_back.
Let's just walk through some of the problems:
User runs program. User enters 100. studentGrades[100] is out of range. Undefined behaviour occurs as the vector only has one element.
User runs program, enters -1 studentGrades[-1] is out of range.
User runs program, enters 0. studentGrades[0] is in range, incremented to 1. studentGrades.push_back(1) adds an element to the vector studentGrades[1] is now also equal to 1.
As a great starting point, if you swap your subscript vector references for the vector at method as I've shown below you will get out-of-range errors which will help (a lot). The code below still needs work but at least you'll have run-time errors instead of odd behaviour.
int main()
{
do
{
cout << "Please enter a student's grade: ";
cin >> myInput;
if (myInput < 1000)
{
studentGrades.at(myInput)++;
}
studentGrades.push_back(myInput);
} while (myInput != SENTINEL);
cout << "\n";
for (int i = 0; i < 1000; i++)
cout << i << " grade(s) of " << studentGrades.at(myInput) << endl;
return 0;
}
I think if I was implementing this I'd be using std::map instead of a vector. It would let you have a studentGrade[1000] without having to allocate memory for studentGrade[0] to [999] first.
However as you are learning about std::vector check out vector::resize to set the vector big enough for the required elements, std::vector::size to find out whether you need to increase the size. You could then ditch the push_back.
References
vector::at http://www.cplusplus.com/reference/vector/vector/at/
vector::size http://www.cplusplus.com/reference/vector/vector/size/