Iterate on files of a folder, with specific condition

Question

Using this solution with dirent.h, I'm trying to iterate on specific files of the current folder (those which have .wav extension and begin with 3 digits) with the following code :

(Important note: as I use MSVC++ 2010, it seems that I cannot use #include <regex>, and that I cannot use this as well because no C++11 support)

DIR *dir;
struct dirent *ent;
if ((dir = opendir (".")) != NULL) {
  while ((ent = readdir (dir)) != NULL) {
    printf ("%s\n", ent->d_name);
    //if substr(ent->d_name, 0, 3) ... // what to do here to 
                                      //  check if those 3 first char are digits?
    // int i = susbtr(ent->d_name, 0, 3).strtoi();        //  error here! how to parse 
                                                         // the 3 first char (digits) as int? 

    // if susbtr(ent->d_name, strlen(ent->d_name)-3) != "wav" // ...

  }
  closedir (dir);
} else {
  perror ("");
  return EXIT_FAILURE;
}

How to perform these tests with MSVC++2010 in which C+11 support is not fully present?

Answer 1

You would not actually check for wav extension, merely that the filename would end with these 3 letters...

There is no such function as substr in the C library to extract a slice from a string.

You should check that the filename length is at least 7: strlen(ent->d_name) >= 7, then check that the first 3 characters are digits but not the fourth using the isdigit function from <ctype.h> and finally compare the last 4 characters of the filename to ".wav" using strcmp or better strcasecmp. The latter may be called _stricmp in the Microsoft world. If neither of these is available, use tolower to compare the last 3 characters to 'w', 'a' and 'v'.

Here is an implementation of the relaxed requirement (any number of digits):

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

...

DIR *dir;
struct dirent *ent;
if ((dir = opendir(".")) != NULL) {
    while ((ent = readdir(dir)) != NULL) {
        char *name = ent->d_name;
        size_t length = strlen(name);
        if (length >= 5 &&
            isdigit(name[0]) &&
            name[length-4] == '.' &&
            tolower(name[length-3]) == 'w' &&
            tolower(name[length-2]) == 'a' &&
            tolower(name[length-1]) == 'v') {
               int num = atoi(name);
               printf("%s -> %d\n", name, num);
               /* do your thing */
        }
    }
    closedir(dir);
} else {
    perror ("");
    return EXIT_FAILURE;
}

Answer 2

Ok, this is a solution

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

int
main(void)
{
    const char *strings[] = {"123.wav", "1234 not-good.wav", "456.wav", "789.wav", "12 fail.wav"};
    int         i;
    int         number;

    for (i = 0 ; i < sizeof(strings) / sizeof(*strings) ; ++i)
    {
        size_t      length;
        int         count;
        const char *pointer;

        if ((sscanf(strings[i], "%d%n", &number, &count) != 1) || (count != 3))
            continue;
        length = strlen(strings[i]);
        if (length < 4)
            continue;
        pointer = strings[i] + length - 4;
        if (strncasecmp(pointer, ".wav", 4) != 0)
            continue;
        printf("%s matches and number is %d\n", strings[i], number);
    }
    return 0;
}

as you can see scanf() checks if there is an integer at the beginning of the string, skipping any possible white space characters too, and then captures then number of characters that were scanned, if that equals 3 then it proceeds to check the extension.

If you are using Windows replace strncasecmp() with _strnicmp().