To sort an array in required manner - c

Question

I need to sort the elements in the odd positions in the descending order and elements in the even position in ascending order. Here is my code, I'm unable to break the first loop.
#include<stdio.h>
int main()
{
int n, t;
printf("Enter the size of the array\n");
scanf("%d", &n);
int i, a[n];
if ((n > 20) || (n <= 0))
printf("Invalid Size");
else
{
printf("Enter the values\n");
for (i = 0; i < n; i++)
{
scanf("%d", &a[i]);
}
for (i = 0; i < n; i + 2)
{
if (a[i] > a[i + 2])
{
t = a[i];
a[i] = a[i + 2];
a[i + 2] = t;
}
}
for (i = 1; i < n; i + 2)
{
if (a[i] < a[i + 2])
{
t = a[i];
a[i] = a[i + 2];
a[i + 2] = t;
}
}
for (i = 0; i < n; i++)
{
printf("%d\n", a[i]);
}
}
}

Answer 1

For starters according to the C Standard the function main without parameters shall be declared like
int main( void )
There is no great sense to declare the variable n as having the type int that after that to check whether its value is less than zero. It is much better to declare it as having the type size_t.
And the array should be declared after the check
if ((n > 20) || (n <= 0))
printf("Invalid Size");
else
{
int a[n];
//...
In loops like this
for (i = 0; i < n; i + 2)
the variable i is not increased. It is obvious that you mean i += 2.
And the loops only moves the first minimum even and the first maximum odd elements to the end of the array. You need additional loops that will do the same operation for other elements of the array. That is the implementation of the bubble sort algorithm is incorrect.
Here is a demonstrative program that shows how the array can be sorted according to the requirements for even and odd elements of the array.
#include <stdio.h>
#define N 20
int main(void)
{
int a[N] = { 18, 1, 16, 3, 14, 5, 12, 7, 10, 9, 8, 11, 6, 13, 4, 15, 2, 17, 0, 19 };
for ( size_t i = 0; i < N; i++ )
{
printf( "%d ", a[i] );
}
putchar( '\n' );
for ( size_t n = N, last; !( n < 3 ); n = last )
{
for ( size_t i = last = 2; i < n; i++ )
{
if ( ( i % 2 == 0 && a[i] < a[i - 2] ) ||
( i % 2 == 1 && a[i - 2] < a[i] ) )
{
int tmp = a[i];
a[i] = a[i - 2];
a[i - 2] = tmp;
last = i;
}
}
}
for ( size_t i = 0; i < N; i++ )
{
printf( "%d ", a[i] );
}
putchar( '\n' );
return 0;
}
The program output is
18 1 16 3 14 5 12 7 10 9 8 11 6 13 4 15 2 17 0 19
0 19 2 17 4 15 6 13 8 11 10 9 12 7 14 5 16 3 18 1

Answer 2

The most obvious problem is that your for never ends because i is never actually updated. The i+2 in for (i = 0; i < n; i + 2) does not update i which keeps its initia value forever.
Try something like for (i = 0; i < n; i=i+2) instead.
A second problem is that you are not really performing a sorting.
I guess that you are trying to implement some sort of bubble sort.
It sorts using comparison. It is impossible to sort an array using less than n logn operation (when sorting using comparison). You are sorting the array in linear time and this should look as a red flag to you.
Try adding another for as follows:
for (i = 0; i < n; i+= 2)
for (j = i+2; j < n; j+= 2)
if (a[i] > a[j])
{
t = a[i];
a[i] = a[j];
a[j] = t;
}
and most importantly then read about why you need it.
And if you feel brave you can swap the intgers without using an intermediate variable t as follows (read more on the topic here: XOR swap):
if (a[i] > a[j])
{
a[i] = a[i]^a[j];
a[j] = a[j]^a[i];
a[i] = a[i]^a[j];
}
Hope it helps.