// pointer to a file
struct dirent *entry;
char txt[] = ".txt";
int txt_len = strlen(txt);
char * p;
char ch;
FILE * stream;
char file_path[512];
int i;
int name_len;

// pointer to the directory
DIR *dr = opendir(path);

// if the path is not a directory
if (dr == NULL) {
   printf("Could not open %s\n", path);
   return;
}

// get files from the directory
while ((entry = readdir(dr)) != NULL) {
   // 1) Find all the files ending with .txt and store in filelist.

   name_len = strlen(entry->d_name);
   // bad file name
   if (name_len <= txt_len) {
         // do nothing
   } else {
         p = entry->d_name + (name_len - txt_len);

         // file's name doesn't end with .txt
         if (strcmp(p, txt) != 0) {
            // do nothing
         } else {
            /*
            * 2) Read all files in the filelist one by one and count the
            * frequency of each alphabet letter only (a - z). The array
            * long alphabetfreq[] always has the up-to-date frequencies
            * of alphabet letters counted so far.
            * If the letter is upper case, convert
            * it to lower case first.
            */

            sprintf(file_path, "%s/%s", path, entry->d_name);

            stream = fopen(file_path, "r");

            // file cannot be opened for reading
            if (stream == NULL) {
               // do nothing
               // silently ignore it
            } else {
               // get each character from file
               while ((ch = fgetc(stream)) != EOF) {
                     // ch is a alphabet letter
                     if (isalpha(ch)) {
                        // convert it to its lower case
                        ch = tolower(ch);

                        // up-to-date frequencies of alphabet
                        // letters counted so far
                        alphabetfreq[(ch - 'a')]++;
                     }
               }

MAGIC SQUARE 1. Create a constant variable DIMENSION and a two-d...