I have a selection sort implemented to sort an array of random integers. I would like the user to choose between ascending or descending order. The ascending sort works flawlessly, although descending does not. Here is what my selection sort looks like:
public String selection(int[] array,int num,String order) {
String output = "";
int min;
// This is the descending selection sort
if (order == "desc") {
for (int i = num - 1; i >= 0; i--) {
// Assume first element is min
min = i;
for (int j = i + 1; j < num; j++) {
if (array[j] < array[min]) {
min = j;
}
}
if (min != i) {
final int temp = array[i];
array[i] = array[min];
array[min] = temp;
}
output = output + Integer.toString(array[i]) + "\n";
}
} // This is the ascending selection sort
else {
for (int i = 0; i < num; i++) {
// Assume first element is min
min = i;
for (int j = i + 1; j < num; j++) {
if (array[j] < array[min]) {
min = j;
}
}
if (min != i) {
final int temp = array[i];
array[i] = array[min];
array[min] = temp;
}
output = output + Integer.toString(array[i]) + "\n";
}
}
return(output.trim());
}
I've seen a few questions similar to mine, although none of the questions I saw had their selection sort set up like this so I was unable to implement their solutions.
Firstly, the if
blocks should compare to minPosition
and maxPosition
, not i
. Secondly, if you are selecting both minimum
and maximum
, then your inner for loop should stop at a.length - i
, not a.length
(since the top i
elements are also sorted). Doing both gives you this as the ascending order algorithm.
public static void SortAscending(int[] a){
for(int i = 0; i < a.length; i++){
int maxPosition=i;
int minPosition=i;
for(int j = i+1; j < a.length - i; j++){
if(a[j] < a[minPosition]){
minPosition = j;
}
if(a[j] > a[maxPosition]){
maxPosition = j;
}
}
/*
if(i < a.length/2-1)
*/
swap(a,maxPosition,i);
swap(a,minPosition,a.length-i-1);
}
}
To switch to descending order, simply add one line.
public static void SortDescending(int[] a){
for(int i = 0; i < a.length; i++){
int maxPosition=i;
int minPosition=i;
for(int j = i+1; j < a.length - i; j++){
if(a[j] < a[minPosition]){
minPosition = j;
}
if(a[j] > a[maxPosition]){
maxPosition = j;
}
}
/*
if(i < a.length/2-1)
*/
swap(a,minPosition,maxPosition); // <-- this line
swap(a,maxPosition,i);
swap(a,minPosition,a.length-i-1);
}
}
Use swap
function https://www.geeksforgeeks.org/collections-swap-method-in-java-with-examples/
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