When scanf is confronted with a non-digit it will not consume any input and return that zero integers were read. The non-digit will stay in the input for the next call to scanf that will behave the same as the first call, etc.
In answer to your question below. You could use fgetc to parse at least one character, but this will give the error messages for every character already typed. Typically I think you want to skip until a newline. To this end you could use fgets as suggested by poolie. Or you could add the following after scanf.
int ch;
if (isint == 0)
while ((ch = fgetc(stdin)) != EOF && ch != '\n')
{
/* Skip characters */
}
P.S: In your case it is probably better to put it just before the first printf in the loop.
