Question

So I am going through some of the common sorting algorithms and have written this:

Code:

public void insertionSort() {
    int key; 
    int i;   
    for ( int j = 1; j < this.a.length; j++ ) {
        key = a[j];
        i = j;

        while ( i > 0 && a[i-1] > key ) {
            a[i] = a[i-1];
            i = i - 1;
        }
        a[i] = key;
    }
}

Result:

jan@janspc:~/Dropbox/programming/java/algorithms$ javac sort.java
jan@janspc:~/Dropbox/programming/java/algorithms$ java Test 
49, 68, 60, 14, 70, 8, 83, 96, 29, 7, 92, 35, 17, 84, 31, 62, 48, 95, 16, 22, 31, 97, 72, 55, 88, 63, 1, 63, 96, 32, 74, 15, 92, 77, 50, 13, 12, 36, 90, 93, 20, 15, 67, 88, 23, 31, 95, 90, 86, 65, 35, 27, 60, 4, 90, 11, 22, 97, 65, 88, 23, 1, 25, 21, 9, 81, 87, 56, 2, 4, 63, 52, 55, 86, 62, 30, 55, 64, 19, 10, 45, 92, 87, 43, 39, 95, 20, 43, 3, 30, 74, 64, 4, 90, 91, 93, 94, 44, 87, 21, 

49, 1, 1, 2, 3, 4, 4, 4, 7, 8, 9, 10, 11, 12, 13, 14, 15, 15, 16, 17, 19, 20, 20, 21, 21, 22, 22, 23, 23, 25, 27, 29, 30, 30, 31, 31, 31, 32, 35, 35, 36, 39, 43, 43, 44, 45, 48, 50, 52, 55, 55, 55, 56, 60, 60, 62, 62, 63, 63, 63, 64, 64, 65, 65, 67, 68, 70, 72, 74, 74, 77, 81, 83, 84, 86, 86, 87, 87, 87, 88, 88, 88, 90, 90, 90, 90, 91, 92, 92, 92, 93, 93, 94, 95, 95, 95, 96, 96, 97, 97, 

Execution took: 110628 nano sek?

As you can see from testing, the first value is not affected by sort. What’s wrong with my algorithm?

Answer 1

In the first iteration, the while loop doesn’t execute, because i < 0. In the next iteration, the while loop doesn’t run because i == 0.

You should probably use while (i >= 0 && a[i] > key) (not tested though)