[Deque]

Thanks, that solved the problem. but I want to know the reason aswell .

There is still a newline in the buffer which is read into op as character. Flushing the buffer will help. Using a space in scanf triggers it to scan for whitespaces in order to ignore them. You can see this in the reference and the usage of the format parameter: http://www.cplusplus.com/reference/cstdio/scanf/

Whitespace character: the function will read and ignore any whitespace characters encountered before the next non-whitespace character (whitespace characters include spaces, newline and tab characters -- see isspace). A single whitespace in the format string validates any quantity of whitespace characters extracted from the stream (including none).

That means putting a space into the format paramter will make it ignore the newline in the buffer.

i have read somewhere that getchar() are vulnerable and is recommended not to use it.

scanf is vulnerable too (format string attacks).

whenever you declare a char data type, use %c or %s as the format specifier.
int - %d
long int - %ld
float - %f which I believe you already know
char - %s / %c

Your answer has nothing to do with the problem or the question.