Questions & Answers

There are two main alternatives for tracking and testing which valid numbers have already been added:

• search the array itself
• maintain and use an external data structure for tracking the stored values

You have implemented a slightly buggy and inelegant form of the first. Another answer, now deleted, demonstrated a correct and more elegant variation on this theme, using a nested loop to iterate over the array elements already assigned.

In the speed for space tradeoff category, however, there are solutions of the second type. You might, for example, use a binary search tree to record the values added so far, and then search that instead of the main array. For so few total values, however, and such a small range of valid ones, a pretty good alternative would be to maintain a simple lookup table of which values had been recorded so far. For example:

#include <stdio.h>

#define SIZE 20
#define MIN_VALID 10
#define MAX_VALID 100

int main(void) {
    int a[SIZE];
    _Bool seen[MAX_VALID + 1] = { 0 };

    for (int next_position = 0; next_position < SIZE; ) {
        printf("Please insert %d numbers (whole)\n", SIZE - next_position);
        scanf("%d", &a[next_position]);

        if (a[next_position] < MIN_VALID || a[next_position] > MAX_VALID){
            printf("%d is not a valid value!\n", a[next_position]);
        } else if (seen[a[next_position]]) {
            printf("You already inserted %d!\n", a[next_position]);
        } else {
            printf("You have inserted: %d\n", a[next_position]);
            seen[a[next_position]] = 1;
            next_position += 1;
        }
    }

    return 0;
}

Given that the range of numbers is limited to 91 options, it is reasonable to create an array of bool initialized to false to store flags to determine if a number has been entered.

On each number being added, if the corresponding flag is false, accept the number and change that flag to true.

A simple implementation of this idea might look like:

#include <stdio.h>
#include <stdbool.h>

#define N 20
#define LOWER 10
#define UPPER 100

int main(void) {
    bool flags[UPPER - LOWER + 1] = { false };
    int input_numbers[N];

    for (size_t i = 0; i < N; i++) {
        while (true) {
            int n;

            if (scanf("%d", &n) != 1) {
                printf("Please enter a number.\n");
                while (getchar() != '\n');
                continue;
            }
       
            if (n >= LOWER && n <= UPPER && !flags[n-LOWER]) {
                input_numbers[i] = n;
                flags[n-LOWER] = true;
                break;
            }
            printf("Invalid input.\n");
        }
    }

    return 0;
}