Applying selection sort on an array of integers - c++

Question

int arr[] = {7,4,10,8,3,1};
int size = sizeof(arr) / sizeof(arr[0]);
for(int i = 0; i<size-1; i++){
int temp = arr[i];
for(int j = i+1; j < size; j++){
if(arr[j] < temp){
temp = arr[j];
}
}
swap(temp, arr[i]);
}
I am trying to apply the selection sort algorithm on the given array, but the output I am getting is only [1,1,1,1,1,1], I am finding the minimum element through the inner loop, Ican't figure out what is going wrong?

Answer 1

Slightly modified your code;
You need to pass reference(address) to both elements to take place of swapping contents
int arr[] = { 7, 1, 10, 8, 3, 11, 0, 12, 5, 8 };
int size = sizeof(arr) / sizeof(arr[0]);
for(int i = 0; i < size; i++)
{
auto temp = std::min_element( arr + i, arr + size );
std::swap( arr[i], *temp );
}
You have to add algorithm header to use std::min_element

Answer 2

int arr[] = {7,4,10,8,3,1};
int size = sizeof(arr) / sizeof(arr[0]);
for(int i = 0; i<size-1; i++){
int temp = arr[i];
int pos = i;
for(int j = i+1; j < size; j++){
if(arr[j] < temp){
temp = arr[j];
pos = j;
}
}
if(pos != i)
std::swap(arr[pos], arr[i]);
}
This should work.

Answer 3

It is suggested not to use using namespace std;. There is a plethora of reasons why you should not; that I will not mention.
By the way I tried to keep some of your variables the same but to be honest, I didn't. It is better to create variable names that explain what the code is doing. It makes your code a lot more legible and readable.
So opt out of one letter variables. It is fine in for loops, however this is a special case.
Now, here is another alternative suggested by #user4581301 & #Swift -Friday Pie. This method is using std::size using c++17.
For example:
#include <iostream>
#include <utility> // to use the swap() function.
#include <iterator> // to use std::size() function.
int main()
{
int arr[] = { 7,4,10,8,3,1 };
// This --> int size = sizeof(arr) / sizeof(arr[0]); is archaic.
const int length = static_cast<int>(std::size(arr)); // Call this something other than "size"; you can run into issues.
// We use static_cast<int> as a implicit conversion, and the obvious std::size(arr)).
// Going through the elements
for (int StartOfIndex = 0; StartOfIndex < length - 1; ++StartOfIndex)
{
// smallest is the index of the smallest element we’ve encountered this iteration
int smallest = StartOfIndex;
// Looking for a smaller element..
for (int current = StartOfIndex + 1; current < length; ++current)
{
// if we found an element smaller than our last; take note.
if (arr[current] < arr[smallest])
smallest = current;
}
// swap StartOfIndex with smallest.
std::swap(arr[StartOfIndex], arr[smallest]);
}
//Prints array.
for (int index = 0; index < length; ++index)
std::cout << arr[index] << " ";
std::cout << "\n";
return 0;
}
Output: 1 3 4 7 8 10

Answer 4

The first mistake you made in writing for loop's condition, don't use swap(temp, array[i]); yet try to get the basics first.
#include <iostream>
using namespace std;
int findsmall(int arr[], int i, int size){
int s, pos, j;
s = arr[i];
pos = i;
for(j = i+1; j < size; j++){
if(arr[j] < s){
s = arr[j];
pos = j;
}
}
return pos;
}
int main() {
int arr[] = {7,4,10,8,3,1};
int size = sizeof(arr) / sizeof(arr[0]);
int smallnum;
int temp;
int count = 0;
cout << "Original array: ";
for(int i = 0; i < size; i++){
if(i < size - 1){
cout << arr[i] << ", ";}
else{
cout << arr[i];
}
}
cout << endl;
for(int i = 0; i < size; i++){
smallnum = findsmall(arr,i, size);
temp = arr[i];
arr[i] = arr[smallnum];
arr[smallnum] = temp;
count++;
}
cout << "Sorted array: ";
for(int i = 0; i < size; i++){
if(i < size - 1){
cout << arr[i] << ", ";}
else{
cout << arr[i];
}
}
cout << endl;
return 0;
}

Answer 5

void swap(int *xp, int *yp)
{
int temp = *xp;
*xp = *yp;
*yp = temp;
}
void selectionSort(int arr[], int n)
{
int i, j, min_idx;
// One by one move boundary of unsorted subarray
for (i = 0; i < n-1; i++)
{
// Find the minimum element in unsorted array
min_idx = i;
for (j = i+1; j < n; j++)
if (arr[j] < arr[min_idx])
min_idx = j;
// Swap the found minimum element with the first element
swap(&arr[min_idx], &arr[i]);
}
}
selectionSort(arr,size);
This should work.