The program doesn't stop on scanf("%c", &ch) line, why? [duplicate]

Let's say you input 2 when you're reading for num. The actual input stream will be 2\n (\n is the newline character). 2 goes into the num, and there remains \n, which goes into ch. To avoid this, add a whitespace in format specifier.

scanf(" %c", &ch); 

This will ignore any whitespaces, newlines or tabs.

The reason behind this is the newline \n character left behind by previous scanf, when pressing Enter key, for the next read of scanf. When the statement

scanf("%c", &ch);   

executed then it reads that \n left behind by the previous scanf.
To eat up this \n you can use a space before %c specifier. A space before the %c specifier is able to eat up any number of white-space characters.

scanf(" %c", &ch);   
       ^ a space