A whitespace character in a scanf format causes it to explicitly read and ignore as many whitespace characters as it can. So with
scanf("%d ", ..., after reading a number, it will continue to read characters, discarding all whitespace until it sees a non-whitespace character on the input. That non-whitespace character will be left as the next character to be read by an input function.
With your code:
printf("enter a value for j "); scanf("%d ",&j); printf("j is %d \n", j);
it will print the first line and then wait for you to enter a number, and then continue to wait for something after the number. So if you just type 5Enter, it will appear to hang — you need to type in another line with some non-whitespace character on it to continue. If you then type 6Enter, that will become the value for
i, so your screen will look something like:
enter a value for j 5 6 j is 5 enter a value for i i is 6
Also, since most scanf %-conversions also skip leading whitespace (all except for
%n), spaces before %-conversions are irrelevant (
" %d" will act identically). So for the most part, you should avoid spaces in scanf conversions unless you know you specifically need them for their peculiar effect.