Copy duplicates of an array in another array?

Question

I want to copy the duplicates of one array into another array. src[]={1, 3, 5, 3, 1} -> dst[]={1, 3}

This is my approach:

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

size_t copy_duplicates(int dst[], const int src[], size_t len) {

    size_t lengthdst = 0;

    for(size_t i =0; i < len -1; i++){

        for(size_t d = i +1; d < len; d++){

            if(src[i] == src[d]){
                dst[i]=src[i];
                lengthdst++;
            }
        }
    }
    return lengthdst;
}

int main(void){
    
    int i;
    int dst[?lenghtdst];
    const int scr[]={6, 4, 6, 4};
    copy_duplicates(dst, scr, 4);
    while(i < 2){
        printf("%d", dst [i]);
        i++;
    }
}

The first function works, but I don't know how I can get the length of dst in the main function. To get the length I already need dst. I think I have to change the return value. I've tried a few other returns but nothing works.

Is there a reason why you can't just type 100 and be done with it? Or use the same size as src? This is obviously just an academic exercise, so a few bytes allocated here or here doesn't matter the slightest. — Lundin
– Lundin, Commented Nov 25, 2022 at 13:52
Thank you for the answers! I also have another issue I have to copy the right number of duplicates. For example src[]={1, 3, 5, 3, 1} -> dst[]={1, 1, 3, 3}. Is there a way to get there with my approach or do I need to count the duplicates first and then copy them in the array? — Io12
– Io12, Commented Nov 26, 2022 at 0:06

Jonathan Leffler · Accepted Answer · 2022-11-25 15:21:11Z

2

The best way would be, to write the number of elements directly into dst (int dst[4];).

But if you want to do this dynamically, you can (only in the same function!) create the scr array and then write the length of the array as a value into dst:

const int scr[]={6, 4, 6, 4};
int dst[sizeof(scr)/sizeof(int)];

edited Nov 25, 2022 at 15:21

Jonathan Leffler

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answered Nov 25, 2022 at 13:55

Jonas

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3 Comments

Anthony Kelly Over a year ago

sizeof(src) returns the size of the pointer (8 bytes on a 64-bit system), not the size of the array. There is no way in C to find the size of an array. The programmer must manage this.

Jonas Over a year ago

Oh right, that's true. I change that.

Anthony Kelly Over a year ago

You learn something new everyday. Thanks @user3121023. There's a good description at stackoverflow.com/questions/37538/….

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