How to remove duplicate elements from random array I generated?

Question

So I have this code:

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

int main() {

    char a[200], b = 0;
    int x;
    x = 100 + rand() % 200;
    for (int i = 0; i < x; i++) {
        a[i] = 'a' + rand() % 26;
    }

    for (int i = 0; i < x; i++) {
        if (b % 10 == 0) { 
            printf("\n%c ", a[i]); 
            b = 0;
        }
        else {
            printf("%c ", a[i]);
        }
        b++;
    }
    printf("\n");

    return 0;

}

Purpose is that I should generate random array of letters from 'a' to 'z' (which I've managed to do) and after that print new array without elements that repeat in first array. I've tried implementing code from here for removing duplicate elements but it didn't worked in my code.

Use shuffle to generate an array that contains numbers from 1 to 200 in random order. — , Apr 19 '17 at 13:59
@Sven B The program has undefined behavior because the value of the variable x set like x = 100 + rand() % 200; can exceed the size of the array. — Vlad from Moscow, Apr 19 '17 at 14:01

score 1 · Accepted Answer · answered Apr 19 '17 at 14:00

1

A simple solution is to loop over the array and copy each element to a new array, but first check that the value doesn't already exist in the new array.

answered Apr 19 '17 at 14:00

Some programmer dude

400,186
35
402
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1

Simple and square – Apr 19 '17 at 14:01
1

Simple and "squared" – Miket25 Apr 19 '17 at 14:38
It's simple but not very effective. – Some programmer dude Apr 19 '17 at 14:44

score 0 · Answer 2 · answered Apr 19 '17 at 14:14

An O(n) solution, assuming your array contains only letters a to z, consists of creating another small array of 26 integers initialized to zeroes, exist[26], then for each letter from the the main array,

if exist[letter - 'a'] > 0 don't print it
otherwise print it and increment exist[letter - 'a']

for instance,

int exist[26] = { 0 };

for(int i=0 ; i<x ; i++) {
   if (exist[a[i] - 'a'] == 0) {
      exist[a[i] - 'a']++;
      printf("%c ", a[i]);
   }
}

score 0 · Answer 3 · answered Apr 19 '17 at 14:31

For starters the program has undefined behavior due to the statement

x = 100 + rand() % 200;

because the calculated value of the variable x can exceed the size of the array.

I think you mean

x = 1 + rand() % 200;
   ^^^

Also it is desirable to call the standard function srand to get different random sequences when the program runs.

The program can look the following way.

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

#define N   200

int main(void) 
{
    char s[N];
    size_t n;

    srand( ( unsigned int )time( NULL ) );

    n = 1 + rand() % N;

    for ( size_t i = 0; i < n; i++ )
    {
        s[i] = 'a' + rand() % ( 'z' - 'a'  + 1 ); 
    }

    for ( size_t i = 0; i < n; i++ )
    {
        putchar( s[i] );
        if ( ( i + 1 ) % 10 == 0 || i + 1 == n ) putchar( '\n' );
    }

    putchar( '\n');

    size_t m = 0;

    for ( size_t i = 0; i < n; i++ )
    {
        size_t j = 0;
        while ( j < m && s[j] != s[i] ) j++;

        if ( j == m )
        {
            if ( m != i ) s[m] = s[i];
            ++m;
        }
    }

    n = m;

    for ( size_t i = 0; i < n; i++ )
    {
        putchar( s[i] );
        if ( ( i + 1 ) % 10 == 0 || i + 1 == n ) putchar( '\n' );
    }

    putchar( '\n');

    return 0;
}

Its output might be like

bimiwgnkew
tphzfidwmn
yqoyoxbbxd
kalxljfyvj
upzdoglrez
edsubgsfjr
kvrvscgadb
lxsmdhuoaz

bimwgnketp
hzfdyqoxal
jvursc

How to remove duplicate elements from random array I generated?

3 Answers3