Why am I getting a "statement with no effect" warning?

The following is my code

/* Initialise default without options input. */
options -> processHiddens = false;
options -> timeResolution = DEFAULT_MOD_TIMES;
options -> performSync = true;
options -> recursive = false;
options -> print = false;
options -> updateStatus = true;
options -> verbose = false;
options -> programname = malloc(BUFSIZ);
options -> programname = argv[0];

while ((opt = getopt(argc, argv, OPTLIST)) != -1)
{
    switch (opt)
    {
        case 'a':
            !(options -> processHiddens);
        case 'm':
            options -> timeResolution = atoi(optarg);
        case 'n':
            !(options -> performSync);
        case 'p':
            !(options -> print);
        case 'r':
            !(options -> recursive);
        case 'u':
            !(options -> updateStatus);
        case 'v':
            !(options -> verbose);
        default:
            argc = -1;
    }
}

I'm trying to flip the Boolean statement around each time an option is entered, hence doing something like

!(options -> processHiddens);

instead of just

options -> processHiddens = true;

However, I'm getting the following warning when compiling:

mysync.c: In function ‘main’:
mysync.c:32: warning: statement with no effect
mysync.c:36: warning: statement with no effect
mysync.c:38: warning: statement with no effect
mysync.c:40: warning: statement with no effect
mysync.c:42: warning: statement with no effect
mysync.c:44: warning: statement with no effect

Because !(options -> processHiddens) is an expression, and you're not assigning the result to anything. You need something like:

options->processHiddens = !options->processHiddens;