Removing duplicate items from an array using C language

Question

Removing duplicate items from a sorted array

#include <stdio.h>
#include <stdlib.h>

int main() {
    int arr[12] = { 1, 2, 2, 3, 3, 3, 4, 4, 4, 4, 5, 5 };
    int temp[12];
    int i, j, k, n = 12;

    for (i = 0; i < n - 1; i++) {
        if (arr[i] != arr[i + 1]) {
            temp[j] = arr[i];
            j++;
        }
    }

    temp[j] = arr[n - 1];

    for (k = 0; k <= j; k++) {
        printf("%d\n", temp[k]);
    }

    return 0;
}

Output:

6356652
1955753237
1956070172
6356716
1955750536
8
1955687363
1955687354
1
2
3
4
5

Process returned 0 (0x0)   execution time : 0.014 s
Press any key to continue.

I do not want these numbers to be printed out:

You don't initialize j or temp[12]

Tormund Giantsbane
– Tormund Giantsbane

2019-06-17 13:37:58 +00:00
Commented Jun 17, 2019 at 13:37 — Tormund Giantsbane
– Tormund Giantsbane, Commented Jun 17, 2019 at 13:37

Vlad from Moscow · Accepted Answer · 2019-06-17 15:08:14Z

4

You did not initialize the variable j. So using the uninitialized variable invokes undefined behavior.

It is better to form the array without duplicates in one loop instead of splitting the loop into a loop and one more statement after the loop.

You can write a separate function for example the following way as it is shown in the demonstrative program.

#include <stdio.h>

size_t remove_copy( const int a[], int b[], size_t n )
{
    size_t j = 0;

    for ( size_t i = 0; i < n; i++ )
    {
        if ( i == 0 || a[i] != b[j-1] ) b[j++] = a[i];
    }

    return j;
}

int main(void) 
{
    int a[] = { 1, 2, 2, 3, 3, 3, 4, 4, 4, 4, 5, 5 };
    int b[sizeof( a ) / sizeof( *a )];
    const size_t N = sizeof( a ) / sizeof( *a );

    for ( size_t i = 0; i < N; i++ ) printf( "%d ", a[i] );
    putchar( '\n' );

    size_t n = remove_copy( a, b, N );

    for ( size_t i = 0; i < n; i++ ) printf( "%d ", b[i] );
    putchar( '\n' );

    return 0;
}

The program output is

1 2 2 3 3 3 4 4 4 4 5 5 
1 2 3 4 5

edited Jun 17, 2019 at 15:08

answered Jun 17, 2019 at 14:04

Vlad from Moscow

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3 Comments

user3629249 Over a year ago

Weither this works (or not) depends on the data in the 'a[]' being is assending order. This dependency is a serious short coming of the algorithm in the answer

Vlad from Moscow Over a year ago

@user3629249 You are wrong. The algorithm works also for data sorted in the descending order.

chqrlie Over a year ago

@user3629249: the algorithm works for any sorting order: ascending, descending, alphabetical, pixel density... the only requirement is that all duplicates be adjacent, which is the case for all regular sorting orders.

chqrlie · Accepted Answer · 2019-06-27 10:59:43Z

1

Your code and logic is correct. But you did a mistake by not intializing j = 0. Because of that you were getting some gibberish values. So, I corrected and executed it and got correct answer.

#include <stdio.h>

int main() {
    int arr[12] = { 1, 2, 2, 3, 3, 3, 4, 4, 4, 4, 5, 5 };
    int temp[12];
    int i, j = 0, k, n = 12;

    for (i = 0; i < n - 1; i++) {
        if (arr[i] != arr[i + 1]) {
            temp[j] = arr[i];
            j++;
        }
    }

    temp[j] = arr[n - 1];

    for (k = 0; k <= j; k++) {
        printf("%d\n", temp[k]);
    }

    return 0;
}

edited Jun 27, 2019 at 10:59

chqrlie

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answered Jun 27, 2019 at 10:36

ashish agarwal

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