Quick sort implementation

Coding conventions

1. Have at least one empty line after all #includes.
2. Have one space character before and after every binary operator. For example, a + 1 instead of a+1. That makes your code easier to read.
3. Have one space after each for keyword: not for(i=1;i<10;i++), but rather
   for (i = 1; i < 10; i++). (Same applies to if.)
4. Fix your indentation:

for(j=low;j<high;j++)
  {
      if(a[j]<a[high])
      {
         i++;
         swap1(&a;[i],&a;[j]);
      }
  }

to

for(j=low;j<high;j++)
{
    if(a[j]<a[high])
    {
        i++;
        swap1(&a;[i], &a;[j]);
    }
}

5. Instead of writing &a[index] you can write simply a + index.
6. Whenever printing a simple (non-formated) standard output, use puts instead of printf. (Also, note that puts automatically adds the new line character after the argument string.)

All the above points are relevant to readability of your source code.

Summa summarum

All in all, I had this in mind:

#include<stdio.h>

void quicksort(int*, int, int);
int partition(int*, int, int);
void swap(int*, int*);

int main()
{
    int low = 0;
    int length;
    int i;
    int a[100];
    int high;

    puts("Enter the length of array:");
    scanf("%d", &length);

    high = length - 1;

    puts("Enter the array elements:");

    for (i = 0; i <= high; i++)
    {
        scanf("%d", &a[i]);
    }

    quicksort(a, low, high);

    puts("The sorted array:");

    for (i = low; i <= high; i++)
    {
        printf("%d ",a[i]);
    }

    puts("");
    return 0;
}

void quicksort(int a[],int low,int high)
{
    int p;

    if (low < high)
    {
        p = partition(a, low, high);
        quicksort(a, low, p - 1);
        quicksort(a, p + 1, high);
    }
}


int partition(int* a, int low, int high)
{
    int pivot = a[high];
    int i = low;
    int j;

    for (j = low; j < high; ++j)
    {
        if (a[j] <= pivot)
        {
            swap(a + i++, a + j);
        }
    }

    swap(a + i, a + high);
    return i;
}

void swap(int* a, int* b)
{
    int temp = *a;
    *a = *b;
    *b = temp;
}

Hope that helps.