Question about format string in scanf function

user_15346569 0
Viewed 190

For each of the following pairs of scanf format strings, indicate whether or not the two strings are equivalent. If they're not, show how they can be distinguished:

(b) "%d-%d-%d" versus "%d -%d -%d"

So in this case, my answer was that they were not equivalent. Because non-white-space characters except conversion specifier which start with %, cannot be preceded by spaces, it will not match with the non-white-space character. So in the first case, no spaces will be allowed after the first and second integer, while in the second case, any number of spaces will be allowed after the first 2 integers.

But I saw that the book had a different answer. It said that they were both equivalent to each other. Is this the mistake of the book? Or am I just wrong with the concept of format string in the scanf function?

scanf c
2 Answers

The book is wrong. As per the specification of the 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).
  • Non-whitespace character, except format specifier (%): Any character that is not either a whitespace character (blank, newline or tab) or part of a format specifier (which begin with a % character) causes the function to read the next character from the stream, compare it to this non-whitespace character and if it matches, it is discarded and the function continues with the next character of format. If the character does not match, the function fails, returning and leaving subsequent characters of the stream unread.

So in first case when scanf arrives to the %d and gets the input, next is the - which means that scanf will expect next in the stream to see the non-whitespae character - and not any other whitespace character. So the legal input is 1- 2, but not 1 -2

In the second case, after first %d, scanf will allow the whitespace and than will arrive to non-whitespace, so it will allow the input 1 - 2 by the above definitions.

user_28167030

"%d-%d-%d" differs from "%d -%d -%d" and the difference has nothing to do with "%d".

Format "-" scans over input "-" and stops on the first space of input " -".

Format " -" scans over inputs "-" and " -" as the " " in the format matches 0 or more white-space characters in the input.

A directive composed of white-space character(s) is executed by reading input up to the first nonwhite-space character (which remains unread), or until no more characters can be read. The directive never fails. C17dr § 7.21.6.2 5

Had the question been: "%d-%d-%d" versus "%d- %d- %d",

These 2 are functionally identical.
We would need to dive input arcane stdin input errors to divine a potential difference.

user_24103590
Related