I am trying to implement a recursive mergesort but the method is writting only zeros in the sorted vector. Here is my code. At the main function i just read the values to a vector and pass it as argument to mergesort. Any ideias of what i am doing wrong? Thank you.
public static void mergesort(int [] v){
int mid=v.length/2;
if( v.length < 2)
return;
int l[]=new int [mid];
int r[] = new int [v.length-mid];
for(int i=0; i<mid-1; i++)
l[i]=v[i];
for(int i=mid; i<v.length-1; i++)
r[i-mid]=v[i];
mergesort(l);
mergesort(r);
mergesort(l,r,v);
}
public static void mergesort(int [] l, int [] r, int [] v){
int i=0, j=0, k=0;
while(i< l.length && j< r.length){
if(l[i] <= r[j]){
v[k]=l[i];
i++;
}
else{
v[k]=r[j];
j++;
}
k++;
}
while(i < l.length){
v[k]=l[i];
i++;
k++;
}
while(j< r.length){
v[k]=r[j];
j++;
k++;
}
}
There are two mistakes in the algorithm.
Mistake 1:
while (i < r.length && j < r.length) {
if (l[i] <= r[j]) {
v[k] = l[i];
i++;
} else {
v[k] = r[j];
j++;
}
k++;
}
Here the i must be less than l.length. This is the reason for ArrayIndexOutOfBoundsException if l.length != r.length.
Mistake 2:
for (int i = 0; i < mid - 1; i++)
l[i] = v[i];
for (int i = mid; i < v.length - 1; i++)
r[i - mid] = v[i];
The i's range must be 0 to (mid - 1). So the for loop should use <= operator in both places like this:
for (int i = 0; i <= mid - 1; i++)
l[i] = v[i];
for (int i = mid; i <= v.length - 1; i++)
r[i - mid] = v[i];
This is the reason for getting zeros.
The modified code is given below:
public static void mergesort(int [] v) {
int mid = v.length / 2;
if ( v.length < 2)
return;
int l[] = new int [mid];
int r[] = new int [v.length - mid];
for (int i = 0; i <= mid - 1; i++)
l[i] = v[i];
for (int i = mid; i <= v.length - 1; i++)
r[i - mid] = v[i];
mergesort(l);
mergesort(r);
mergesort(l, r, v);
}
public static void mergesort(int [] l, int [] r, int [] v) {
int i = 0, j = 0, k = 0;
while (i < l.length && j < r.length) {
if (l[i] <= r[j]) {
v[k] = l[i];
i++;
} else {
v[k] = r[j];
j++;
}
k++;
}
while (i < l.length) {
v[k] = l[i];
i++;
k++;
}
while (j < r.length) {
v[k] = r[j];
j++;
k++;
}
}
Suppose you input a vector of size 2; then according to your code
mid=1;
for(int i=0;i<mid-1;i++)
// here i < (1-1) which will never execute
l[i]=v[i];
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