SelectionSort in c++ not sorting* - c++

Question

I am having a tough time trying to follow the logic here as to why it is not working correctly
expected output :
1 5 6 8
any help is greatly appreciated
Update: I got selection sort and insertion sort mixed up
OUTPUT:
unaltered array
5 8 1 6
1 -858993460 -858993460 6
#pragma once
#include <iostream>
using namespace std;
void swap(int &a,int &b)
{
int temp;
temp = b;
b = a;
a = temp;
}
void SelectionSort(int *arr,int n)
{
cout << "SelectionSORT1\n";
int i;
for (i = 0; i < n - 2; i++) //-1 ||-2//
{
int firstIndex;
firstIndex = arr[i];
int j;
for (j = i + 1;j < n - 1;j++)
{
if (arr[j] < arr[firstIndex])
{
firstIndex = j;
//cout << firstIndex;
}
swap(arr[i], arr[firstIndex]);
}
cout << "SelectionSORT2\n";
}
cout << "SelectionSORT3\n";
}
#include <iostream>
#include "SelectionSort.h"
using namespace std;
int main()
{
int array[] = { 5,8,1,6 };
int size = { sizeof(array) / sizeof(array[0]) };
cout << "unaltered array\n";
for (int i = 0; i < 4; i++)
{
cout << array[i] << " ";
}
cout << endl;
SelectionSort(array, size);
for (int i = 0; i < 4; i++)
{
cout << array[i] << " ";
}
cout << endl;
}
UPDATE
#pragma once
#include <iostream>
using namespace std;
void swap(int &a,int &b)
{
int temp;
temp = b;
b = a;
a = temp;
}
void SelectionSort(int *arr,int n)
{
cout << "Selection SORT1\n";
int I;
for (i = 0; i < n ; i++) //-1 ||-2//
{
int firstIndex;
firstIndex = i;
int j;
for (j = i + 1;j < n ;j++)
{
std::cerr << j << ' ' << firstIndex << '\n';
if (arr[j] < arr[firstIndex])
{
firstIndex = j;
}
swap(arr[i], arr[firstIndex]);
}
cout << " \n";
}
cout << " \n";
}
#include <iostream>
#include "BubbleSort.h"
#include "InsertionSort.h"
#include "SelectionSort.h"
using namespace std;
int main()
{
int array[] = { 5,8,1,6 };
int size = { sizeof(array) / sizeof(array[0]) };
cout << "unaltered array\n";
for (int i = 0; i < size; i++)
{
cout << array[I] << " ";
}
cout << endl;
SelectionSort(array, size);
for (int i = 0; i < size; i++)
{
cout << array[I] << " ";
}
cout << endl;
unaltered array
5 8 1 6
SelectionSORT1
1 0
2 0
3 2
2 1
3 2
3 2
5 6 1 8

Answer 1

You are using the selection sort method not the insertion sort method.
Bit in any case the function is incorrect
void InsertionSort(int *arr,int n)
{
cout << "INSERTION SORT1\n";
int i;
for (i = 0; i < n - 2; i++) //-1 ||-2//
{
int firstIndex;
firstIndex = arr[i];
int j;
for (j = i + 1;j < n - 1;j++)
{
if (arr[j] < arr[firstIndex])
{
firstIndex = j;
//cout << firstIndex;
}
swap(arr[i], arr[firstIndex]);
}
cout << "INSERTION SORT2\n";
}
cout << "INSERTION SORT3\n";
}
For starters it will not sort an array that has two elements due to this for loop
for (i = 0; i < n - 2; i++) //-1 ||-2//
Secondly, the variable firstIndex is not initialized by a value of the index i
firstIndex = arr[i];
So the condition in this if statement
if (arr[j] < arr[firstIndex])
does not make a sense.
Thirdly this inner for loop
for (j = i + 1;j < n - 1;j++)
ignores the last element of the array.
Fourth, this unconditional call of the function swap within the inner for loop
swap(arr[i], arr[firstIndex])
also does not make a sense.
The function can look the following way
void SelectionSort( int a[], size_t n )
{
for ( size_t i = 0; i < n; i++ )
{
size_t min = i;
for ( size_t j = i + 1; j < n; j++ )
{
if ( a[j] < a[min] )
{
min = j;
}
}
if ( min != i ) swap( a[i], a[min] );
}
}
And in main the variable size should have the type size_t - the type of the value of the expression with the operator sizeof
const size_t size = sizeof(array) / sizeof(array[0]);
And instead of the magic number 4 in for loops in main you should use the named constant size or you could use the range-based for loop as
for ( const auto &item : array )
{
cout << item << ' ';
}
cout << endl;
If you indeed mean the insertion sort method then a corresponding function can look for example the following way
void InsertionSort( int a[], size_t n )
{
for (size_t i = 1; i < n; i++)
{
if (a[i] < a[i - 1])
{
int tmp = a[i];
size_t j = i;
for ( ; j != 0 && tmp < a[j - 1]; --j )
{
a[j] = a[j - 1];
}
a[j] = tmp;
}
}
}

Answer 2

Thank you all for your help
I got it to work like this
#pragma once
#include <iostream>
using namespace std;
void swap(int &a,int &b)
{
int temp;
temp = b;
b = a;
a = temp;
}
void InsertionSort(int arr[],int n)
{
int i;
for (i = 0; i < n ; i++)
{
int firstIndex,j;
firstIndex = i;
for (j = i + 1;j < n ;j++)
{
if (arr[j] < arr[firstIndex])
{
firstIndex = j;
}
}
swap(arr[i], arr[firstIndex]);
}
}

Answer 3

The following is C++:
std::set<int> sorted_array({ 5,8,1,6 });
If you have duplicates and need to keep them, use:
std::multiset<int> sorted_array({ 5,8,1,6 });
Done. One line.