joy joy - 1 year ago 129
C Question

scanf asking twice for input while I expect it to ask only one


char *method1(void)
static char a[4];
scanf ("%s\n", a);
return a;

int main(void)
char *h = method1();
printf ("%s\n", h);
return 0;

When I run the code above, the prompt is asking me twice for input (I only use scanf for once in the code). Why is that?

Answer Source

From my scanf manual page

 White space (such as
 blanks, tabs, or newlines) in the format string match any amount of white
 space, including none, in the input.  Everything else matches only

Thus with scanf ("%s\n", a) it will scan for a string followed by optional white space. Since after the first newline more whitespace may follow, scanf is not done after the first newline and looks what's next. You will notice that you can enter any number of newlines (or tabs or spaces) and scanf will still wait for more.

However, when you enter the second string, the sequence of whitespace is delimited and scanning stops.

Use scanf ("%s", a) to not scan trailing whitespace.