Question

In the following if I write scanf with %d /%d" (with a space) then I can input a fraction seperated by spaces.

For example: The input can be 5/7 or 5 /7 or 5 / 7 (with spaces anywhere).

How does scanf in the code below read spaces between numbers that way but yet it is has only one space between %d /%d"?

#include <stdio.h>

int main()
{
   int i, j;
   scanf("%d /%d", &i, &j);
   return 0;
}

Answer 1

Almost all input format specifiers in scanf will automatically skip all whitespace before reading the actual data. E.g. the %d format specifier in scanf actually means <skip spaces> <read int>.

That means that if we used %d/%d format for reading fractions, it would stand for

<skip spaces> <read int> <require /> <skip spaces> <read int>

In this sequence we have everything necessary, except we don’t allow/skip spaces before the /. To fix that we have to add an explicit request to skip spaces before /, which in scanf format string is expressed by a space character. Adding a space character before / produces %d /%d format string, which stands for

<skip spaces> <read int> <skip spaces> <require /> <skip spaces> <read int>

This is exactly what we need and that is exactly what you have in your code.