Sort array using recursion - c++

Question

I have been trying to sort an array using recursion, but I am getting no output, can somebody help me with this, like why there is no output?
void sortarray(int arr[], int index, int key, int len){
if (index == len-2){
}
else{
if (key==len-1){
key=0;
sortarray(arr, index++, key, len );
}
else {
if (arr[key] > arr[key+1]){
swap(arr[key], arr[key+1]);
}
sortarray(arr, index, key++, len );
}
}
}
int main(){
int arr[5] = {5,6,4,3,2};
sortarray(arr, 0,0,5);
for(int i=0; i<5; i++){
cout << arr[i] << " ";
}
}

Answer 1

I fixed the segfault and simplified the code to make it more clear what it does. Namely, walks the array once and swap the key with the next if it's larger. This is not a valid sort algorithm:
#include <iostream>
using namespace std;
void sortarray(int arr[], int key, int len) {
// base case
if (key + 1 == len) return;
// recursive case
if (arr[key] > arr[key + 1]){
swap(arr[key], arr[key + 1]);
}
sortarray(arr, key + 1, len );
}
int main() {
int arr[] = {5,6,4,3,2};
int len = sizeof(arr) / sizeof(*arr);
sortarray(arr, 0, len);
for(int i = 0; i < len; i++) {
cout << arr[i] << " ";
}
return 0;
}
and the result is:
5 4 3 2 6

Answer 2

I made some edits to your code and it works now (don't mind the printArr function, it's there just because cycles in main are ugly):
#include <iostream>
using namespace std;
void printArr(int arr[], int size)
{
for (int i = 0; i < size; i++) {
cout << arr[i] << " ";
}
cout << endl;
}
void sortarray(int arr[], int index, int key, int len)
{
if (index < len)
{
if (key == len - 1) {
key = 0;
sortarray(arr, ++index, key, len);
}
else {
if (arr[key] > arr[key + 1]) {
swap(arr[key], arr[key + 1]);
}
sortarray(arr, index, ++key, len);
}
}
}
int main()
{
int arr[5] = { 5,6,4,3,2 };
cout << "Before sort:" << endl;
printArr(arr, sizeof(arr)/sizeof(*arr));
sortarray(arr, 0, 0, 5);
cout << "After sort:" << endl;
printArr(arr, sizeof(arr) / sizeof(*arr));
return 0;
}
So the first problem was missing iostream and std namespace.
Key changes in the algorithm itself were:
change argument++ to ++argument
change the first condition to (index < len)
As to why your code didn't work properly: the index missed the stopping condition and went over the value of len (thus the screen o' fives).
So this answers your question, but this algorithm you wrote breaks once there are two same values next to each other. There is a good reason we use cycles for sorting and unless recursion is the point, I would recommend abandoning it for this task.

Answer 3

This code works perfectly-:
void sortArr(int arr[],int index, int key, int len){
// static int index = 0;
// static int key = 0;
if (index == len-1){
}
else{
if (key==len-1){
key=0;
sortArr(arr,index+1,key,len );
}
else {
if (arr[key] > arr[key+1]){
swap(arr[key], arr[key+1]);
}
sortArr(arr, index, key+1 ,len );
}
}
}
int main(){
int arr[5] = {5,6,4,3,2};
sortarray(arr, 0, 5);
for(int i=0; i<5; i++) {
cout << arr[i] << " ";
}
return 0;
}
The output of the code is:
2 3 4 5 6