Question

How do you use the return value from scanf to make sure it is a double I’ve got?

double input;

do {  /* do this as long as it not a double */
  printf("Input?");
  scanf("%lf", &input);
} while(scanf("%lf", &input) != 1); /* this will not work */

Answer 1

scanf will return the number of items assigned. In your case, since the format string contains only %lf, it will return 1 exactly in the case where you got your double. The problem with your code is that you first call scanf inside the loop, which will read the double off the stream. Then, in your while condition, you call scanf again, but there isn’t another double to be read, so scanf does not do anything.

The way I would write your code would be something like

int no_assigned;
do {
    printf("Input?");
    no_assigned = scanf("%lf", &input);
} while (no_assigned != 1);

The extra variable is there because it feels to me like the scanf is code that should be inside the loop, not in the while condition, but that is really a personal preference; you can eliminate the extra variable and move (note, move, not copy) the scanf call inside the condition instead.

EDIT: And here’s the version using fgets that’s probably better:

double input;
char buffer[80];

do {
    printf("Input? ");
    fflush(stdout); // Make sure prompt is shown
    if (fgets(buffer, sizeof buffer, stdin) == NULL)
        break; // Got EOF; TODO: need to recognize this after loop
    // TODO: If input did not fit into buffer, discard until newline
} while (sscanf(buffer, "%lf", &input) != 1);