[after edit 1] I have seen the other answers here in SO, and from looking at my code, I find that I am adhering to the principle of Java's pass-by-value-of-reference. But still my array is not getting sorted. Please can someone point out any errors I am making in my code?
[after edit 2] Found the problem. It was nothing to do with array passing. In my merge method, it should be if (end-start<=0) not the other way round!
I am trying to implement mergesort. However, I am unable to display values of my array and am unsure of how to pass my arrays in Java such that the original array can be modified.
How can I modify my current code to display the values of the sorted array?
I understand java passes the copy of the array's reference around, but doesnt this mean that the original array gets modified?
CODE:
I am calling mergesort method from my main method.
public static void main(String[] args) {
int[] qn = {10,22,33,4,5,6,1};
qn= mergesort(qn,0,qn.length-1);
for (int i=0;i<qn.length;i++){ //print to see values if sorted/not
System.out.print(qn[i]+ " ");
}
}
public static int[] mergesort(int[] arr,int start,int end){
int mid = (end+start)/2;
if (end - start<=0){
return arr;
}
else if (end-start>=1){
arr=mergesort(arr,start,mid);
arr=mergesort(arr,mid+1,end);
}
arr=merge(arr,start,end);
for (int i=0;i<arr.length;i++){
System.out.print(arr[i]+ " ");
}
System.out.println();
return arr;
}
public static int[] merge(int[] arr,int start,int end){
int mid= (start+end)/2;
if(start-end<=0){
return arr;
}
int a= start; int b = mid+1;
while (a<=mid && b <=end){
if (arr[b]<arr[a]){
int tmp = arr[b++];
for(int i=++mid;i>a;i--){
arr[i]=arr[i-1];
}
arr[a++]=tmp;
}
else if (arr[b]>arr[a]){
a++;
}
else{ //arr[b]=arr[a]
if(a==mid && b == end){
break; //all between mid and end will be equal too
}
int tmp= arr[b++];
a++;
for (int i=++mid;i>a;i--){
arr[i]=arr[i-1];
}
arr[a++]=tmp;
//a++;
//b++;
}
}
return arr;
}
}
modified my answer based on Manu's below. but it's still not working. Attaching code below:
public static void mergesort(int[] arr,int start,int end){
int mid = (end+start)/2;
if (end - start<=0){
return;
}
else if (end-start>=1){
mergesort(arr,start,mid);
mergesort(arr,mid+1,end);
}
merge(arr,start,end);
for (int i=0;i<arr.length;i++){
System.out.print(arr[i]+ " ");
}
System.out.println();
return;
}
public static void merge(int[] arr,int start,int end){
int mid= (start+end)/2;
if(start-end<=0){
return;
}
int a= start; int b = mid+1;
while (a<=mid && b <=end){
if (arr[b]<arr[a]){
int tmp = arr[b++];
for(int i=++mid;i>a;i--){
arr[i]=arr[i-1];
}
arr[a++]=tmp;
}
else if (arr[b]>arr[a]){
a++;
}
else{ //arr[b]=arr[a]
if(a==mid && b == end){
break; //all between mid and end will be equal too (pearl)
}
int tmp= arr[b++];
a++;
for (int i=++mid;i>a;i--){
arr[i]=arr[i-1];
}
arr[a++]=tmp;
//a++;
//b++;
}
}
return;
}
}
