Printing only unique elements in an array - c

Question

Edit: I need to maintain the order in which the elements are present in the original array, so sorting won't work.
I have a 1-D array containing some elements and I am using printf() in C but I only want to print an element if and only if it has not already been printed before.
I am thinking of using nested loops to compare if the element I am about to print from the current position in the array was already present in a lower index of the array but it's not working. What am I missing? Or is my whole approach wrong?
So far, I have tried this, which is not working:
int arr[20];
After this I take user input for no. of elements in p and of course, p<20. Then, user enters the elements one by one. I use scanf() for this.
for(i=1;i<=p;i++)
{
for(j=i+1;j<=p;j++)
{
if(arr[i]!=arr[j])
{
printf("%d",arr[j]);
}
}
}

Answer 1

You need to check all previous items before you know if the item has already occurred.
#include <stdio.h>
#include <stdlib.h>
int compareFunc(const void *op1, const void *op2 )
{
int *a, *b;
a = (int*)op1;
b = (int*)op2;
return *a - *b;
}
void printUnique(int *array, int numElems)
{
int curPrintIndex, curCompareIndex;
char alreadySeen;
for (curPrintIndex=0; curPrintIndex<numElems; curPrintIndex++)
{
alreadySeen = 0;
for (curCompareIndex=0; curCompareIndex<curPrintIndex; curCompareIndex++)
{
if (array[curCompareIndex] == array[curPrintIndex])
{
alreadySeen = 1;
break;
}
}
if (alreadySeen == 0)
printf("%d\n", array[curPrintIndex]);
}
}
int main()
{
const int numItems = 100;
int *array, i, lastVal;
array = calloc(numItems, sizeof(int) );
for (i=0; i<numItems; i++)
array[i] = rand()%numItems;
printUnique(array, numItems);
free(array);
return 0;
/*
qsort(array, numItems, sizeof(int), compareFunc);
printf("%d\n", array[0]);
lastVal = array[0];
for (i=1; i<numItems; i++)
{
if (array[i] != lastVal)
{
lastVal = array[i];
printf("%d\n", array[i]);
}
}
*/
}

Answer 2

#include <stdio.h>
int main(void){
int arr[20] = { 4,8,4,2,4,8,1,3,2,7 };//the elements are positive integers.
int i, j, p = 10;
for(i=0;i<p-1;i++){
if(arr[i] < 0)
continue;
for(j=i+1;j<p;j++){
if(arr[j] > 0 && arr[i]==arr[j])
arr[j] *= -1;
}
}
for(i=0; i < p; ++i)
if(arr[i] > 0)
printf("%d ", arr[i]);
puts("");
return 0;
}