Inserting element in array - c++

Question

I'm trying to make this program which gets an element from user and inserts it into the made array by user (array must be sorted)
But it doesn't work properly
for example:
a[4]
a[0]=1
a[1]=3
a[2]=5
a[3]=7
then i insert>> 6
but then it goes like this
a[0]=1
a[1]=3
a[2]=5
a[3]=6
a[4]=3866936
the array is able to have upto 100 elements
:|
it's a practice from a book
#include<iostream>
#include<windows.h>
using namespace std;
void insert(int[], int&, int);
int main()
{
const int maxsize = 100;
int a[maxsize];
cout << "build ur array\n"; Sleep(650);
cout << "Enter the number of elements:";
int n;
cin >> n;
for (int i = 0; i < n; i++)
{
cout << "a[" << i << "]=";
cin >> a[i];
}
int k;
cout << "Enter a new element:";
cin >> k;
n++;
insert(a, n, k);
for (int i = 0; i < n; i++)
cout << "a[" << i << "]=" << a[i] << endl;
system("pause");
}
void insert(int a[], int& n, int x)
{
for (int i = n - 1; a[i]>x; i--)
a[i] = a[i + 1];
for (int i = n; i <= n; i--)
{
if (x >= a[i])
{
a[i + 1] = x;
break;
}
}

Answer 1

Assuming you know what you're doing ex. no one can enter more than 99, assuming you will check if the value is less than 100, you have only one error in insert:
a[i] = a[i+1];
should be
a[i+1] = a[i];
The full function should look like this to work with all possible cases:
void insert(int a[], int& n, int x)
{
for (int i = n - 2; a[i]>x && i >= 0; i--)
a[i+1] = a[i];
for (int i = n-2; i >= 0; i--)
{
if (x >= a[i])
{
a[i + 1] = x;
return;
}
}
a[0] = x;
}

Answer 2

You are experiencing undefined behavior due to the fact that you are reading from outside of the array bounds (0 - 3). You should use an std::vector for dynamically sized arrays:
std::vector<int> vec;
vec.push_back(1);
vec.push_back(3);
vec.push_back(5);
vec.push_back(7);
and then:
vec.push_back(6);
Later on, if you want to sort the array you can just do:
std::sort(vec.begin(), vec.end());

Answer 3

You just need to remember that index of array starts from 0. When you want to have 4 elements array int tab[4] you can can read/write elements value using indexes from 0 to 3 tab[0], tab[1], tab[2], tab[3]. Element tab[4] is not part of your array and when you want to read element from your such place, you will get value that is already there (you can think about it as unpredictable random value, but it is not true random value).
By the way, you question is about C language. If you using C++, read about containers like std::array, std::vector etc.

Answer 4

With some stuff from STL, you may use the following:
class SortedVector
{
public:
using const_iterator = std::vector<int>::const_iterator;
void insert(int value) {
auto it = std::lower_bound(v.begin(), v.end(), value);
v.insert(it, value);
}
const_iterator begin() const { return v.begin(); }
const_iterator end() const { return v.end(); }
private:
std::vector<int> v;
};
Live Example
std::vector is the container.
std::lower_bound finds the location where to insert (the way that insertion is still in order)
std::vector::insert does the insertion (which the shifting of the element).
if you don't want to use std::vector and us C-array: you may use:
void insert(int a[], int& n, int x)
{
auto it = std::lower_bound(a, a + n, x);
std::copy(it, a + n, it + 1);
*it = x;
++n;
}
Live example.

Answer 5

>
with help of prajmus
i solved the problem
:
#include<iostream>
#include<windows.h>
using namespace std;
void insert(int[], int&, int);
int main()
{
const int maxsize = 100;
int a[maxsize];
cout << "build ur array\n"; Sleep(650);
cout << "Enter the number of elements:";
int n;
cin >> n;
for (int i = 0; i < n; i++)
{
cout << "a[" << i << "]=";
cin >> a[i];
}
int k;
cout << "Enter a new element:";
cin >> k;
n++;
insert(a, n, k);
for (int i = 0; i < n; i++)
cout << "a[" << i << "]=" << a[i] << endl;
system("pause");
}
void insert(int a[], int& n, int x)
{
for (int i = n - 2; a[i]>x; i--)
a[i+1] = a[i];
for (int i = n-1; i <= n; i--)
{
if (x >= a[i])
{ a[i + 1] = x;
break;
}
}
}