How to show message when negative number is entered? - c++

Question

I am coming to a problem where when negative numbers are entered that it shows an error message. libc++abi.dylib: terminating with uncaught exception of type std::bad_alloc: std::bad_alloc. is there a way to detect that error when negative numbers and entered with a message: Enter 0 or greater. So, can anyone help me solve this. Thanks for the help.
I tried by doing this:
int students = 0;
while (students <= 0) {
cout << "How many students? ";
cin >> students;
if (students <= 0) {
cout << "Enter 0 or greater";
}
}
int testsScore = 0;
while (testsScore <= 0) {
cout << "How many tests per student? ";
cin >> testsScore;
if (testsScore <= 0) {
cout << "Enter 0 or greater";
}
}
Main Code:
#include <iostream> // input out stream library.
#include <iomanip> // parametic manipulators library for our table.
using namespace std;
//MARK: Structure to use the following data given in the instructions.
struct student_information {
//MARK: Student name.
string studentName;
//MARK: Student ID number.
int id;
//MARK: Pointer to an array of test scores.
int *tests;
//MARK:Average test score.
double average;
//MARK: Course grade.
char grade;
};
//MARK: The main() method should keep a list of test scores for group of students.
int main() {
// MARK: Declare variables.
int numberOfStudents;
int testScores;
//MARK: So, in our logic it should ask the user how many test scores there are to be,
// and how many students there are.
// It should then dynamically allocate an array of structures. However,
// each structure tests member should point to a dynamically allocated array
// that will hold the test scores according to our instructions.
cout << "How many students?";
cin >> numberOfStudents;
cout << endl;
cout << "How many tests per student? ";
cin >> testScores;
cout << endl;
student_information *pointerToArray = new student_information[numberOfStudents];
for (int i = 0; i < numberOfStudents; i++) {
pointerToArray[i].tests = new int[testScores];
}
//MARK: So, this is when the array has been allocated,
//the program should ask for the student ID number and all the scores for each
// every student.
for (int i = 0; i < numberOfStudents; i++) {
cout << "Student Name: ";
cin >> pointerToArray[i].studentName;
cout << endl;
cout << "ID Number: ";
cin >> pointerToArray[i].id;
cout << endl;
for (int v = 0; v < testScores; v++) {
cout << " Test # " << (v + 1) << " : ";
//MARK: Checking to see all the data is entered for each student / member.
cin >> pointerToArray[i].tests[v];
cout << endl;
// MARK: Checking if negative numbers are entered for any test scores.
while(pointerToArray[i].tests[v] < 0 || pointerToArray[i].tests[v] > 100) {
cout << "Enter 0 or greater";
cin >> pointerToArray[i].tests[v];
cout << endl;
}
}
}
//MARK: The average test score should be calculated and stored in the average member of each structure.
//The course grade should be computed on the basis of the following grading scale:
// Average Test Grade Course Grade
// 91-100 A
// 81-90 B
// 71-80 C
// 61-70 D
// 60 or below F
for (int i = 0; i < numberOfStudents; i++) {
double add = 0;
for(int x=0; x < testScores; x++)
add = add + pointerToArray[i].tests[x];
pointerToArray[i].average = add / testScores;
//MARK: The course grade will be stored in the grade member structure.
//After the data entry has been calculated.
if(pointerToArray[i].average >= 91 && pointerToArray[i].average <= 100)
pointerToArray[i].grade = 'A';
else if(pointerToArray[i].average >= 81 && pointerToArray[i].average <= 90)
pointerToArray[i].grade = 'B';
else if(pointerToArray[i].average >= 71 && pointerToArray[i].average <= 80)
pointerToArray[i].grade = 'C';
else if(pointerToArray[i].average >= 61 && pointerToArray[i].average <= 70)
pointerToArray[i].grade = 'D';
else if(pointerToArray[i].average < 60)
pointerToArray[i].grade = 'F';
//MARK: Displaying the table to list each students name, ID number,
//average test score and the course grade.
for(int i=0; i < numberOfStudents; i++)
{
cout << "Student Name: " << setw(19) << left << pointerToArray[i].studentName <<
" ID Number: " << setw(19) << pointerToArray[i].id <<
" Average test score: " << setw(15) << pointerToArray[i].average <<
" Grade: " << setw(15) << pointerToArray[i].grade << endl;
}
return 0;
}
}

Answer 1

If negative numbers are entered for numberOfStudents or testsScore, then
student_information *pointerToArray = new student_information[numberOfStudents];
or
pointerToArray[i].tests = new int[testScores];
try to allocate memory with huge amount. When this fails std::bad_alloc is thrown and OP doesn't catch it. Hence, it is catched outside OPs main().
The array size in new[] expects a size_t → an unsigned value and silently converts the provided signed. E.g. -1 becomes 0xffffffff (32 bit) or even bigger.
#include <iostream>
#include <iomanip>
int main(int argc, char* argv[])
{
int value = -1;
size_t size = value;
std::cout << "size: " << size << " = 0x" << std::hex << size << '\n';
try {
int *mem = new int[size];
} catch (const std::exception &error) {
std::cerr << error.what() << '\n';
}
return 0;
}
Output:
size: 18446744073709551615 = 0xffffffffffffffff
std::bad_array_new_length
Live Demo on coliru
OP should add a check that input numbers are > 0.
It puzzled me a bit that my sample throws std::bad_array_new_length (instead of the std::bad_alloc as reported by OP). Investigating a bit into this I found
it's precisely documented this way (e.g. in cppreference.com)
Otherwise, the new-expression does not call the allocation function, and instead throws an exception of type std::bad_array_new_length or derived from it
std::bad_array_new_length is derived from std::bad_alloc.

Answer 2

Assuming you're trying to input negative numbers to the students and test scores, you just need to add a similar while loop to test them as you do bellow for the grades...
cout << "How many students?";
cin >> numberOfStudents;
while (numberOfStudents <= 0) {
cout << "Number of students should be greater than 0";
cin >> numberOfStudents;
cout << endl;
}
cout << endl;
As noted in the previous comments, new[] expects size_t, meaning an unsigned number https://en.cppreference.com/w/cpp/types/size_t