It's saying that:
c = (getchar() != EOF)
means read a character from stdin
, then compare it against EOF. The result of this is 1 if true, 0 if false. That result is then assigned to c
. The character that was read is lost, hence 'undesired effect'.
It wants you to run this yourself in this way to see how you can produce 0 and 1 by sending EOF vs. any other character.