Finding repeated characters in arguments to function with variable-length argument list

Question

The function has to read from the characters provided in the argument list, then stop as soon as it finds two the same, or when it reaches a null byte argument. I believe I've got that part coded. But I can't understand how to account for the first character 'a'; it skips that character, and I can't figure out how to compare the two.

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

char fancyMostFrequentChar(char c, ...);

int main(void)
{
    char c = fancyMostFrequentChar('a', 'b', 'c', 'd', 'd', 'b', 'a', '\0');

    if (c == 'd')
        printf("Hooray!\n");
    else
        printf("fail whale :(\n");

    return 0;
}

char fancyMostFrequentChar(char c, ... )
{
    int ci;
    int i;
    int alpha[26] = {0};
    int max = 0;
    int max_i = 0;
    int total = c;

    printf("%d\n\n", c);

    va_list letters;

    va_start(letters, c);

    while((i = va_arg(letters, int))!= 0)
    {
        ci = i - 97;
        printf("%d\n", ci);
    }

    alpha[ci] = c;

    printf("%d", alpha[ci]);

    va_end(letters);

    return total;
}

Answer 1

The main text says 'stop as soon as it finds two the same'; the code says 'fancyMostFrequentChar' which means you have to count all characters and report the most frequent one. Different operations.

First duplicate

For the first, if alpha[ci] is not zero, you've found a duplicate; return it (remembering va_end). Otherwise, set alpha[c1] to 1. For the second, you have to process all arguments and then find the most frequent. You have to process the first argument specially. You don't need to check whether it's a duplicate; it isn't. But you do need to set the right entry in alpha to 1. Note that this code doesn't barf if you pass a non-letter, and it treats upper and lower case letters as the same.

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

char first_duplicate(char c, ...);

int main(void)
{
    char c = first_duplicate('a', 'b', 'c', 'd', 'd', 'b', 'a', '\0');

    if (c == 'd')
        printf("Hooray!\n");
    else
        printf("fail whale :(\n");

    return 0;
}

static inline int map_char(unsigned char u)
{
    if (isalpha(u))
        return tolower(u) - 'a';
    return 26;
}

char first_duplicate(char c, ...)
{
    int i;
    int alpha[26] = {0};
    va_list letters;
    va_start(letters, c);

    alpha[map_char(c)] = 1;

    while ((i = va_arg(letters, int)) != 0)
    {
        int n = map_char(i);
        if (n != 26 && alpha[n] != 0)
        {
            va_end(letters);
            return n + 'a';
        }
        alpha[n]++;
    }
    va_end(letters);
    return c;   // No duplicates spotted - return first letter
}

Most frequent letter

For the most frequent letter, you have to process the entire argument list, and then find which entry has the most occurrences.

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

char fancyMostFrequentChar(char c, ...);

int main(void)
{
    char c = fancyMostFrequentChar('a', 'b', 'c', 'd', 'd', 'b', 'a', '\0');

    if (c == 'a')
        printf("Hooray!\n");
    else
        printf("fail whale :(\n");

    return 0;
}

static inline int map_char(unsigned char u)
{
    if (isalpha(u))
        return tolower(u) - 'a';
    return 26;
}

char fancyMostFrequentChar(char c, ...)
{
    int i;
    int alpha[26] = {0};
    va_list letters;
    va_start(letters, c);

    alpha[map_char(c)] = 1;

    while ((i = va_arg(letters, int)) != 0)
    {
        int n = map_char(i);
        if (n != 26)
            alpha[n]++;
    }
    va_end(letters);

    int maxval = alpha[0];
    int maxpos = 0;
    for (i = 1; i < 26; i++)
    {
        if (alpha[i] > maxval)
        {
            maxval = alpha[i];
            maxpos = i;
        }
    }
    return maxpos + 'a';
}

Both compile cleanly and produce Hooray! when run.

Note that it would be more sensible, and plausible, to pass a single character string to the function, which would not need to use <stdarg.h> at all. It could also produce different answers based on user input, etc. The code using variable arguments is more or less stuck using a fixed length of argument list (though you could have different variables providing the values (first_duplicate(a, b, c, d, e, f, g, '\0');) if you wanted to).