C/C++ Remove Item From Struct Array - c++

Question

struct Student
{
char* name;
int balls;
};
void inputdata(Student **s, int *n)
{
int nn;
printf("%s\n", "Input amount of students");
scanf("%i", &nn);
Student* a = new Student[nn];
for (int i = 0; i < nn; ++i)
{
a[i].name = new char[4096];
scanf("%4095s", a[i].name);
scanf("%i", &a[i].balls);
}
*n = nn;
*s = a;
}
void print(Student *s, int n)
{
for (int i = 0; i < n; ++i)
{
printf("%s %i\n", s[i].name, s[i].balls);
}
}
void fixdata(Student *s, int *n)
{
int nn = *n;
for (int i = 0; i < nn; ++i)
{
if (s[i].balls > 100)
s[i].balls = 100;
else if (s[i].balls < 20)
{
for(int j = i; j < nn; ++j)
s[j] = s[j+1];
nn-=1;
}
}
*n = nn;
}
int main(int argc, char const *argv[])
{
Student* s;
int n;
inputdata(&s, &n);
print(s, n);
fixdata(s, &n);
print(s, n);
return 0;
}
I am trying to delete items where balls is less than 20. If so, I am supposed to shift items to the right, but at the same time remove items that are less than 20. I try, but it drastically shifts left 2 of the same records and is not doing the job properly.
UPDATE: Ok, I cleaned up the code a bit, now n decreases, but the problem when it stays at 1 record, that is supposed to be deleted too. I wonder why it does not get removed. Thankfully most of the trouble is fixed. Still, what is the problem deleting one item left when it is less than 20?

Answer 1

The function has a bug because after moving elements of a sub-array to the left in the next iteration of the for loop you will deal with another element instead of the required. Also the function should return the number of actual elements after removing some elements.
Also the function can produce memory leaks then one element of the array is assigned to other element of the array.
The function can look the following way
int fixdata( Student *s, int n )
{
int i = 0;
for ( int j = 0; j < n; ++j )
{
if ( not ( s[j].balls < 20 ) )
{
if ( i != j )
{
delete [] s[i].name;
s[i] = s[j];
s[j].name = nullptr;
}
if ( s[i].balls > 100 ) s[i].balls = 100;
++i;
}
}
return i;
}
And in main you can write
int m = fixdata(s, n);
print(s, m);
Also you should delete the allocated memory when the array is not needed any more.
for ( int i = 0; i < n; i++ )
{
delete [] s[i].name;
}
delete [] s;

Answer 2

In fixdata(), when "removing" an item, you are leaking that item's name, but more importantly your inner loop goes out of bounds of the array if the "last" item is removed.
Try this instead:
void fixdata(Student *s, int *n)
{
int nn = *n;
for (int i = 0; i < nn; ++i)
{
if (s[i].balls > 100)
{
s[i].balls = 100;
}
else if (s[i].balls < 20)
{
for(int j = i + 1; j < nn; ++j)
{
delete[] s[j-1].name;
s[j-1] = s[j];
s[j] = Student{};
}
--nn;
}
}
*n = nn;
}
That being said, your code is more C than C++. The C++ approach would be to use things like std::cin, std::string, and std::vector instead, eg:
#include <iostream>
#include <vector>
#include <string>
struct Student
{
std::string name;
int balls;
};
std::istream& operator>>(std::istream &in, Student &stud)
{
in >> s.name >> s.balls;
return in;
}
std::ostream& operator<<(std::ostream &out, const Student &stud)
{
out << stud.name << " " << stud.balls;
return out;
}
void inputdata(std:vector<Student> &s)
{
int nn;
std::cout << "Input amount of students\n";
std::cin >> nn;
std::vector<Student> a(nn);
for (auto &stud : s) {
std::cin >> stud;
}
s = std:move(a);
}
void print(const std::vector<Student> &s)
{
for (const auto &stud : s) {
std::cout << stud << "\n";
}
}
void fixdata(std::vector<Student> &s)
{
for (size_t i = 0; i < s.size();)
{
auto &stud = s[i];
if (stud.balls < 20) {
s.erase(s.begin()+i);
}
else {
if (stud.balls > 100) {
stud.balls = 100;
}
++i;
}
}
/* alternatively:
s.erase(
std::remove_if(s.begin(), s.end(),
[](const auto &stud){ return (stud.balls < 20); }
),
s.end()
);
std::for_each(s.begin(), s.end(),
[](auto &stud){ stud.balls = std::min(stud.balls, 100); }
);
*/
}
int main()
{
std::vector<Student> s;
inputdata(s);
print(s);
fixdata(s);
print(s);
return 0;
}