Question 2 of 8: Programming

Question 2: Programming

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Given. A positive integer, read as a sequence of decimal digits. Find. Whether, for every digit position $p$ (0-indexed from the left), the digit stored there equals the count of digit $p$ occurring anywhere in the number.

Approach. Read the number as a string (or repeatedly divide/mod by 10 into an array), then for every position count occurrences of that position's own index value across the whole array and compare.

#define MAXLEN 16

int is_self_describing(int digits[], int len) {
    for (int pos = 0; pos < len; pos++) {
        int count = 0;
        for (int i = 0; i < len; i++)
            if (digits[i] == pos) count++;
        if (count != digits[pos]) return 0;   /* fails at this position */
    }
    return 1;
}

int main(void) {
    char buf[MAXLEN + 1];
    int digits[MAXLEN], len;

    printf("Enter a positive integer: ");
    scanf("%s", buf);
    len = 0;
    for (int i = 0; buf[i] != '\0'; i++)
        digits[len++] = buf[i] - '0';

    if (is_self_describing(digits, len))
        printf("%s is self-describing.\n", buf);
    else
        printf("%s is NOT self-describing.\n", buf);
    return 0;
}