What is the effect of trailing white space in a scanf() format string?

Question

What is difference between scanf(\"%d\") and scanf(\"%d \") in this code, where the difference is the trailing blank in the format string?

Answer 1

A white-space character (space, newline, horizontal and vertical tab) in a format string matches any number of white-space characters in the input.
In your first case

  scanf("%d  ",&j);

when it encounters the white-space char (WSC) ' ' then it will eat all the white spaces input by user including \n on pressing Enter and it will expect to enter a non-WSC . In this case your program will terminate by pressing Ctrl + Z.