runner$ cat freq_struct2.c
/* Calculate fequencies of letters in an input file */
#include <stdio.h>
#include <ctype.h>
struct table_tag {
        char letter;
        int count;
};
void bubble(struct table_tag table[]);
void swap(struct table_tag *table1, struct table_tag *table2);
void printfreq(struct table_tag table[], int tot);

void main(void)
{
        struct table_tag table[26];
        int i;       /* index for lc array */
        int tot = 0; /* total number of alpha characters */
        int ch;      /* int so that EOF will work */
        for (i = 0; i < 26; i++) {
                table[i].letter = (char) (i + 'a');
                table[i].count = 0;
        }
        while ((ch = getchar()) != EOF)
                if (isalpha(ch = tolower(ch))) {
                        tot++;
                        table[ch - 'a'].count++;
                }
        bubble(table);
        printfreq(table, tot);
}

/* bubble: sort lc array into decreasing order. Carry alf along */
void bubble(struct table_tag table[])
{
        int i, dum;
        for (dum = 0; dum < 25; dum++)
                for (i = 0; i < 25; i++)
                        if (table[i].count < table[i+1].count) 
                                swap(&table[i], &table[i+1]);
}
void swap(struct table_tag *table1, struct table_tag *table2)
{
        struct table_tag temp;
        temp = *table1;
        *table1 = *table2;
        *table2 = temp;
}

/* printfreq: print out the frequency table */
void printfreq(struct table_tag table[], int tot)
{
        int i;
        printf("Frequency count of letters, out of total: %d\n\n", tot);
        printf("  Letter   Frequency (%%)\n");
        for (i = 0; i < 26; i++)
                printf("%6c %13.3f%%\n", table[i].letter,
                   (double)table[i].count/tot*100.0);
}

runner$ freq_struct2
AaAaA
bbbBBbbbbb
ZzZzZZZzzZzzzzz  (ctrl-D)
Frequency count of letters, out of total: 30

  Letter   Frequency (%)
     z        50.000%
     b        33.333%
     a        16.667%