removing duplicate values from an array in C

Question

I am trying to create a program in C that removes duplicate values in an integer array. My strategy is to first sort the array via a selectionsort function, and then call a function removedup that removes any consecutive, duplicate values in the array.

My code:

#include <stdio.h>
#include "simpio.h"

#define n 10

void GetArray(int a[]);
void SelectionSort(int a[]);
int FindMax(int a[], int high);
void swap(int a[], int p1, int p2);
int removedup(int a[]);
void printArray(int a[]);

main()
{
      int a[n];
      GetArray(a);
      SelectionSort(a);
      printf("The original, sorted array is\n");
      printArray(a);
      printf("The array with removed duplicates \n");
      printArray(removedup(a));
      getchar();
}


void GetArray(int a[])
{
     int i;
     for(i=0;i<n;i++)
     {
       printf("Enter integer# %d", i+1);
       a[i]=GetInteger();
     }
}

void SelectionSort(int a[])
{
     int i, max;
     for(i=0;i<n;i++)
     {
           max=FindMax(a,n-i-1);
           swap(a,max,n-i-1);
     }     
}

int FindMax(int a[], int high)
{
    int i, index;
    index=high;
    for(i=0;i<high;i++)
    {
       if(a[i]>a[index])
          index=i;
    }
    return index;
}
void swap(int a[], int p1, int p2)
{
     int temp;
     temp=a[p2];
     a[p2]=a[p1];
     a[p1]=temp;
}

int removedup(int a[])
{
     int i, count, OutArray[count], j;
     count=0;
     for(i=0;i<n-1;i++)
     {
                        if(a[i]==a[i+1])
                        {
                                        a[i+1]=a[i+2];
                                        count++;
                        }
     }
     count++;
     for(j=0;j<count;j++)
     {
                       OutArray[i]=a[i];
     }

     return OutArray;                            
}

I have two questions:

1) How do I fix the error the compiler in giving me in the main body when calling removedup inside the printarray function, saying "invalid conversion from int to int*"? (line 22)

2) How do I accurately define the size of OutArray[] in the removedup function? Currently I have it defined as the size variable, but the value of this variable isn't accurately defined until after the declaration of OutArray.

You can do it faster in O(n) by the way

BlackBear
– BlackBear

2012-03-24 18:19:58 +00:00
Commented Mar 24, 2012 at 18:19 — BlackBear
– BlackBear, Commented Mar 24, 2012 at 18:19
Read the comp.lang.c FAQ, starting with section 6.

pmg
– pmg

2012-03-24 18:26:10 +00:00
Commented Mar 24, 2012 at 18:26 — pmg
– pmg, Commented Mar 24, 2012 at 18:26

pmg · Accepted Answer · 2012-03-24 18:23:47Z

1

Notice your prototypes ...

int removedup(int a[]);
void printArray(int a[]);

And also notice you're calling printArray() with the result of removedup().

      printArray(removedup(a));

The result of removedup() is an int; printarray() requires a int[].
int and int[] are not compatible.

I suggest you remove duplicates and print array in two distinct statements.

answered Mar 24, 2012 at 18:23

pmg

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2 Comments

Joshpho Over a year ago

Okay, that makes sense. I am not familiar with arrays as a data type, however. Would I modify the prototype to read "int[] removedup(int a[])"?

pmg Over a year ago

No, basically you can't return arrays from functions. Make your removedup() function change the input array or send it an output array. Afterwards, in another statement, print the modified array.

Mig · Accepted Answer · 2012-03-25 02:02:46Z

1

You should be able to fix the compiling problems after reading comp.lang-c FAQ on arrays and pointers.

After you get your array sorted, you can use the following function to remove the duplicates:

int dedup(int arr[], int size) {
    int curr = 0, next = 0;
    while (next < size) {
        while (next < size && arr[next] == arr[curr])
            next++;
    if (next < size)
      arr[++curr] = arr[next++];
    }
    return size ? curr+1 : 0;
}

It takes two arguments, the array and its size. The duplicates are removed in-place, which means that the array is modified, without allocating a new array to store the unique elements.

Remember that the dedup function expects the elements to be sorted! I've noticed you are using your own implementation of selection sort, which makes me think this is homework. In that case, I feel a little reluctant on giving you a complete solution, although understanding it should be a good exercise anyway.

EDIT: I should've explained the last line of code.

return size ? curr+1 : 0; is equivalent to:

if (size) 
    return curr+1;
else
    return 0;

Just a shorter way of saying the same thing.

edited Mar 25, 2012 at 2:02

answered Mar 24, 2012 at 19:12

Mig

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2 Comments

Joshpho Over a year ago

Thanks for your help, and this makes sense for the most part -- although could you explain what return size ? curr+1 : 0; means? I'm not quite familiar with the notation.

Mig Over a year ago

I edited my answer because other people might have the same question in the future. Hope it helps. If it helped you, you should accept my answer =)

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