What is wrong with this insertion sort in C? - c

Question

#include <stdio.h>
#include <string.h>
#include <math.h>
#include <stdlib.h>
#include <assert.h>
#include <stddef.h>
void insertionSort(int ar_size, int* ar) {
int i, j;
int temp = 0;
for (i = 1; i < ar_size; i++) {
j = i;
while (j > 0 && ar[i - 1] > ar[i]) {
temp = ar[i - 1];
ar[i - 1] = ar[i];
ar[i] = temp;
j--;
}
}
for (j = 0; j < ar_size; j++) {
printf("%d", ar[j]);
printf(" ");
}
}
int main(void) {
int _ar_size;
scanf("%d", &_ar_size);
int _ar[_ar_size], _ar_i;
for (_ar_i = 0; _ar_i < _ar_size; _ar_i++) {
scanf("%d", &_ar[_ar_i]);
}
insertionSort(_ar_size, _ar);
return 0;
}
I have been trying to look for the error. I cannot see any. What is wrong with this code?
For input as 6 and 4 1 3 5 6 2 , it gives output as 1 3 4 5 2 6. There is one less iteration of the loop but I cannot see why? Please help. Thanks.

Answer 1

You are using index i instead of index j in the internal loop of the function.
while(j>0 && ar[i-1]>ar[i])
{
temp = ar[i-1];
ar[i-1] = ar[i];
ar[i] = temp;
j--;
}
Here everywhere index j has to be used.
Also it is a bad idea that the function also outputs the sorted array. It should do only sorting.
The other bad idea is to use identifiers that start with undescores.
It is better when the first parameter is an array and the second parameter is the size of the array.
The code could look the following way
#include <stdio.h>
void InsertionSort( int *a, int n )
{
int i;
for ( i = 1; i < n; i++ )
{
int j = i;
while ( j > 0 && a[j-1] > a[j] )
{
int tmp = a[j-1];
a[j-1] = a[j];
a[j] = tmp;
--j;
}
}
}
int main(void)
{
int size;
scanf( "%d", &size );
int a[size];
int i;
for ( i = 0; i < size; i++ ) scanf( "%d", &a[i] );
for ( i = 0; i < size; i++ )
{
printf( "%d ", a[i] );
}
puts( "" );
InsertionSort( a, size );
for ( i = 0; i < size; i++ )
{
printf( "%d ", a[i] );
}
puts( "" );
return 0;
}
If the input is
10
2 7 5 4 9 1 4 8 3 5
then output is
2 7 5 4 9 1 4 8 3 5
1 2 3 4 4 5 5 7 8 9

Answer 2

You seem to be using the wrong iterator when pushing the value to the front.
while(j>0 && ar[j-1]>ar[j]) {
temp = ar[j-1];
ar[j-1] = ar[j];
ar[j] = temp;
j--;
}

Answer 3

Try performing a dry run on a piece of paper, it's easy to spot the problem.
for (i = 1; i < ar_size; i++) {
j = i;
while (j > 0 && ar[i-1] > ar[i]) { // problem begins here
temp = ar[i-1];
ar[i-1] = ar[i];
ar[i] = temp;
j--;
}
}
i does not change in the inner loop.
At some point your program swaps 6 with 2 and your sample input list becomes 1 3 4 5 2 6, a[i-1] is 2 and a[i] is 6 and because of ar[i-1] > ar[i] condition the program's flow does not go into the inner loop.
Try this fix:
for (i = 1; i < ar_size; i++) {
j = i;
while (j > 0 && ar[j-1] > ar[j]) {
temp = ar[j-1];
ar[j-1] = ar[j];
ar[j] = temp;
j--;
}
}

Answer 4

The problem with your insertion-sort is you have your j indexes replaced with i indexes in the following code:
while(j>0 && ar[j-1]>ar[j])
{
temp = ar[j];
ar[j] = ar[j-1];
ar[j-1] = temp;
j--;
}
Just a note, when requesting input, it is good practice to print a brief statement describing the input expected. (yes, quick a dirty testing is an exception) It makes if much easier to avoid mistakes with something like:
printf ("enter array size: ");
scanf ("%d", &_ar_size);
int _ar[_ar_size], _ar_i;
for (_ar_i = 0; _ar_i < _ar_size; _ar_i++) {
printf ("enter array element[%d] : ", _ar_i);
scanf ("%d", &_ar[_ar_i]);
}