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

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Answer 1

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

scanf(" %c", &ch); 

This will ignore any whitespaces, newlines or tabs.