Question

I have a c:\config folder which contains several configuration files (config_x).
I would like to be able to choose one of them at random (the program is written in C).
The number of files may change in the future – so needs to be able to query for number of files in the folder.

How can it be done?

Answer 1

Use FindFirstFile and FindNextFile in a loop. Keep a count of how many files you’ve seen so far (n). On each loop, randomly break out on a 1:n chance:

if(INVALID_HANDLE_VALUE!=(hFind = FindFirstFile(DirSpec, &FindFileData))) {
    for(i=0;;++i) {
        strcpy(fileChoice, FindFileData.cFileName);
        while(rand_s(&r));
        if( ((double)r) / ((double)i) < 1.0 ) break;
        if(!FindNextFile(hFind, &FindFileData)) break;
    }
}

You can find a proof of this method in The Art of Computer Programming, Volume 2, Section 3.4.2, by Donald E. Knuth.