count.c

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

void count(const char *str, int *counts)
{
    char alp[] = "aeiouy";
    int i, len = strlen(alp);
    for (i = 0; i < len; counts[i++] = 0);

    char *pos;
    while (*str != '\0')
    {
        if ((pos = strchr(alp, *str)) != 0)
            counts[pos - alp]++;
        str++;
    }
}

int main()
{
    unsigned const int N = 6;
    int counts[N] = { };
    char str[] = "Happy birthday, Denis Vasilyevich!";
    printf("str = %s\n", str);
    count(str, counts);
    char alp[] = "aeiouy", *p;
    for (p = alp; *p != '\0'; p++)
        printf("%c = %d ", *p, counts[p - alp]);
    printf("\n" );
}