Printing unique values of the array in C

Question

I wrote a function creating a dynamic array of random values and another function creating a new array consisting of unique values of the previous array. The algorithm used counts unique values correctly. However, I faced a problem in printing all values. In the example below the program printed 7 2 12714320 4 5 instead of 7 2 4 5 6 .

This is the program which can be tested:

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

int *delduplicate(int *v, int size_old, int *size_new);

main()
{
    int n;
    int *norepeat;
    float *results;
    int dim, size_norepeat, i;
    
    int a[7] = {7,2,2,4,5,6,7};
    
    norepeat = delduplicate(a, 7, &size_norepeat);
    
    for (int i = 0; i < size_norepeat; i++)
        printf("%d ", norepeat[i]);
}



// delduplicate function
int *delduplicate(int *v, int size_old, int *size_new)
{
    int i, j, k = 1, uniques = 1, repeats, *new_v, temp;
    
    // count the number of unique elements
    for (i = 1; i < size_old; i++)
    {
        int is_unique = 1;
        
        for (j = 0; is_unique && j < i; j++)
        {
            if (v[i] == v[j])
                is_unique = 0;          
        }
        
        if (is_unique)
            uniques++;
    }
    
    *size_new = uniques;
    
    // create new array of unique elements
    new_v = (int*) malloc(*size_new * sizeof(int));
    
    // fill new array with unique elements
    new_v[0] = v[0];
    
    for (i = 1; i < size_old; i++)
    {
        int is_unique = 1;
            
        for (j = 0; j < i; j++)
        {
            if (v[i] == v[j])
                is_unique = 0;
        }
    
        if (is_unique)
            new_v[k] = v[i];
            k++;
            
    }
    return new_v;
}

The problem should be happening here:

// fill new array with unique elements
new_v[0] = v[0];

for (i = 1; i < size_old; i++)
{
    int is_unique = 1;
        
    for (j = 0; j < i; j++)
    {
        if (v[i] == v[j])
            is_unique = 0;
    }

    if (is_unique)
        new_v[k] = v[i];
        k++;       
}

when the posted code is run through a compiler, a LONG string of warnings is output. Starting with: untitled1.c:7:1: warning: return type defaults to ‘int’ [-Wimplicit-int] When compiling, always enable the warnings, then fix those warnigs. ( for gcc, at a minimum use: -Wall -Wextra -Wconversion -pedantic -std=gnu11 ) Note: other compilers use different options to produce the same results. — user3629249
– user3629249, Commented Jun 21, 2020 at 4:31
Note: the last line of the compiler output may be: Compilation finished successfully. but that only means the compiler applied some workaround to the problems, not that the correct code was produced. — user3629249
– user3629249, Commented Jun 21, 2020 at 4:32
OT: for ease of readability and understanding: 1) please follow the axiom: only one statement per line and (at most) one variable declaration per statement. — user3629249
– user3629249, Commented Jun 21, 2020 at 4:35
OT: regarding: new_v = (int*) malloc(*size_new * sizeof(int)); 1) the function: malloc() expects a parameter of type: size_t but size_new is an integer. 2) in C, the returned type is void* which can be assigned to any pointer. Casting just clutters the code and is error prone. 3) always check (!=NULL) the returned value to assure the operation was successful. — user3629249
– user3629249, Commented Jun 21, 2020 at 4:39

Abhishek Bhagate · Accepted Answer · 2020-06-20 16:23:27Z

3

Your problem is probably occurring in the following section -

if (is_unique)
    new_v[k] = v[i];
    k++;

Here you are incrementing k at each iteration. However, you only want to increment it whenever you have found a unique element. if() without brackets only considers the first statement. So change it to this -

if (is_unique){
    new_v[k] = v[i];
    k++;
}

This change should make your program run fine.

Side Note : If you do not want to use brackets for an if() , for() , etc, you can separate the statements by commas and use without having the brackets. Like this -

if (is_unique)
    new_v[k] = v[i],
    k++;

edited Jun 20, 2020 at 16:23

answered Jun 20, 2020 at 16:19

Abhishek Bhagate

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1 Comment

Abhishek Bhagate Over a year ago

You are welcome. You could also do it like new_v[k++] = v[i].

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