Insertion sort algorithm flaw in Java code

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?

Note that in the first loop starting with int j = 0 in an imprecision. Think about it.. Also, I would refactor 'j' to 'index' and 'i' to 'previous' — Marsellus Wallace
– Marsellus Wallace, Commented Jan 14, 2012 at 15:04

JB Nizet · Accepted Answer · 2012-01-14 11:22:16Z

4

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)

answered Jan 14, 2012 at 11:22

JB Nizet

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Comments

x4u · Accepted Answer · 2012-01-14 11:21:46Z

2

You need >= here:

    while ( i >= 0 && a[i] > key ){

Without this change it never compares the following elements against the first one.

answered Jan 14, 2012 at 11:21

x4u

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Fahim Parkar · Accepted Answer · 2012-01-14 11:31:37Z

0

while ( i > 0 && a[i] > key ){ should be while ( i >= 0 && a[i] > key ){

Best luck...

answered Jan 14, 2012 at 11:31

Fahim Parkar

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Rajshri · Accepted Answer · 2012-05-15 06:22:26Z

0

In Insertion Sort we need to check each and every element from array and compare with other element getting more accurate result. The problem in your code in while loop we need line

while ( i >= 0 && a[i] > key )

answered May 15, 2012 at 6:22

Rajshri

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Comments

Frank · Accepted Answer · 2014-02-02 06:04:14Z

0

Here is another implementation of insertion sort in java

public class InsertionSort {

static int intArray[] = { 1000, 1, 100, 101, 15 };

public static void doSort() {
    for (int outer = 1; outer < intArray.length; outer++) {
        for(int inner=outer;inner>0; inner--){
            if(intArray[inner]<intArray[inner-1]){
                int temp=intArray[inner];
                intArray[inner]=intArray[inner-1];
                intArray[inner-1]=temp;
                continue;
            }
        }
    }
}

public static void printArray() {
    for (int i = 0; i < intArray.length; i++) {
        System.out.print("  " + intArray[i]);
    }
}

public static void main(String args[]) {
    System.out.print("Array Before Sorting->");
    printArray();
    doSort();
    System.out.print("\nArray After Sorting ->");
    printArray();
}

}

Detailed explanation of the code and/or the algortithm can be seen at - http://www.javabrahman.com/algorithms-in-java/insertion-sort-in-java/

edited Feb 2, 2014 at 6:04

answered Jan 30, 2014 at 9:53

Frank

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1 Comment

kleopatra Over a year ago

Note that link-only answers are discouraged, SO answers should be the end-point of a search for a solution (vs. yet another stopover of references, which tend to get stale over time). Please consider adding a stand-alone synopsis here, keeping the link as a reference.

tbodt · Accepted Answer · 2014-10-07 22:03:31Z

0

here is a very easy implementation of insertion sort

package insertion_sort;

public class insertion_sort {
    public static void main(String ar[]) {
        int[] a = {24, 13, 9, 64, 7, 23, 34, 47, 87, 9, 37, 1992};
        insertion(a,12);
    }

    public static void insertion(int[] a, int n) {
        for (int i = 1; i <= n - 1; i++) {
            int value = a[i];
            int hole = i;
            while (hole > 0 && a[hole - 1] > value) {
                a[hole] = a[hole - 1];
                hole = hole - 1;    
            }
            a[hole] = value;
        }
        print(a);    
    }

    public static void print(int[] A) {    
        for (int i = 0; i < A.length; i++) {    
            System.out.print(A[i] + " ");    
        }    
        System.out.println();
    }    
}

edited Oct 7, 2014 at 22:03

tbodt

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answered Aug 8, 2014 at 22:17

user3923927

9

1 Comment

witrin Over a year ago

That's not what the questioner asked for.

ROMANIA_engineer · Accepted Answer · 2014-10-07 22:25:13Z

0

Below is the pseudo code of Insertion sort(taken from CLR "Introduction to algorithms" book).

Insertion-Sort(A)

for (j=2 to A.length)
key = A[j]> 
i = j-1
while i>0 && A[i]>key
    A[i+1] = A[i]
    i = i-1
A[i+1] = key

Your code is almost similar to above pseudo code. All you need is to change initialization of int j=0 to int j=1 in for loop:

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

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

edited Oct 7, 2014 at 22:25

ROMANIA_engineer

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answered Jan 14, 2012 at 12:06

chandra mohan

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Comments

ROMANIA_engineer · Accepted Answer · 2014-10-07 23:03:25Z

0

Corrected 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;
    }
}

edited Oct 7, 2014 at 23:03

ROMANIA_engineer

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answered Jan 14, 2012 at 11:53

bchetty

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Comments

Stedy · Accepted Answer · 2015-02-14 05:41:46Z

0

public static int[] insrtSort(int array[]){

    for(int i=0 ; i<array.length-1;i++){
        int value=array[i+1],k=i;

        while(value<array[k] ){

            array[k+1]=array[k];
            if(k!=0)
            k--;
            else
                array[0]=value;
        }
        if(k!=0)
        array[k+1]=value;
    }
    return array;
    }

edited Feb 14, 2015 at 5:41

Stedy

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answered Feb 14, 2015 at 5:18

Tharaka Sandaruwan

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Comments

user3458225 · Accepted Answer · 2015-02-25 23:43:46Z

0

This is the same code but as a method that can be passed an array of ints to sort.

public static int[] insertionSort(int[] array) {
    int key; 
    int i;   


    for ( int j = 1; j < array.length; j++ ) {
        key = array[j];
        i = j;

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

answered Feb 25, 2015 at 23:43

user3458225

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