int frequency[26];
int ch;
FILE* txt_file = fopen (, );


for (ch = 0; ch < 26; ch++)
    frequency[ch] = 0;

while (1) {
    ch = fgetc(txt_file);
    if (ch == EOF) break; 

    
    if ('a' <= ch && ch <= 'z')      
        frequency[ch-'a']++;
    else if ('A' <= ch && ch <= 'Z') 
        frequency[ch-'A']++;
}