I'm sure people do that (even though it's not necessary per some year C standard) but generally the pattern is actually for converting things which are not already 0 or 1 into 0 or 1. For example, you might want to use it here:

    int num_empty_strings = !!(strlen(s1)) + !!(strlen(s2)) + !!(strlen(s3))

which is equivalent to:

    int num_empty_strings = (strlen(s1) != 0) + (strlen(s2) != 0) + (strlen(s3) != 0)

Which you use is really a matter of coding style.

That is entirely unneccessary. An == expression will always evaluate to 1 or 0. The !!x trick can be useful in some other situations, though.

Here’s a thing you could do (but I don’t know why):

+= !(c-’s’) - !(c-’p’)