Validate input to integer within a given range c - c

Question

I am trying to get my input validated between a range and to type integer. Currently I have the following code.The range needed is 0-100 for X and 0-50 for Y.
for(loop=0;loop<3;loop++){
printf("\n\nEnter MAGENTA X coordinate from 0 to 100:\n");
scanf("%d",&mg[loop].x);
printf("\nEnter MAGENTA Y coordinate from %d to 50:\n");
scanf("%d",&mg[loop].y);
}
I need the option of the user ending the input prematurely with perhaps by entering "end" or something like that. This should then move onto the next color which is Yellow.
Does anyone know how I could implement this using scanf?
------EDIT-----
I've implemented the following and it defaults to "failure". I increased the handle to 2 and got the error end is being use without being initialized.
for(loop=0;loop<3;loop++){
printf("\n\nEnter MAGENTA X coordinate from 0 to 100:\n");
if(scanf("%d%c",&mg[loop].x) != 1 || end !='\n')
printf("failure");
else
printf("\nEnter MAGENTA Y coordinate from 0 to 50:\n");
scanf("%d",&mg[loop].y); /*Use the same validation here as above*/
}

Answer 1

You can use scanf, that's fine. Why not? scanf returns the number of items successfully read, so that 0 indicates a format error. After such you want to discard the remaining input before asking for another one.
Then you test the read variables against the boundaries.
You can specify a special value for the user's wish to exit, perhaps some string like "exit" which makes scanf fail whereupon you check whether it's "exit". The break command comes handy when you want to exit from a loop.

Answer 2

You can use scanf() but error reporting tends to be trickier. Minimally adapting your code:
for (loop = 0; loop < 3; loop++)
{
int rc;
printf("\n\nEnter YELLOW X coordinate from 0 to 100:\n");
while ((rc = scanf("%d", &ylw[loop].x)) == 1 &&
(ylw[loop].x < 0 || ylw[loop].y > 100)
printf("Enter a value between 0 and 100\n");
if (rc != 1)
{
fprintf(stderr, "Error reading x-value.\n");
break;
}
printf("\nEnter YELLOW Y coordinate from 0 to 50:\n");
while ((rc = scanf("%d", &ylw[loop].y)) == 1 &&
(ylw[loop].x < 0 || ylw[loop].y > 50)
printf("Enter a value between 0 and 50\n");
if (rc != 1)
{
fprintf(stderr, "Error reading y-value.\n");
break;
}
}
However, that repetition cries out for a function — albeit one with a modestly complex interface:
int prompt_for(const char *what, int min, int max, int err)
{
int value;
int rc;
printf("\nEnter %s from %d to %d:", what, min, max);
while ((rc = scanf("%d", &value)) == 1 && (value < min || value > max))
printf("Enter a value between %d and %d\n", min, max);
if (rc != 1)
{
fprintf(stderr, "Error reading %s.\n", what);
return err;
}
return value;
}
Sample use:
for (loop = 0; loop < 3; loop++)
{
ylw[loop].x = prompt_for("YELLOW X coordinate", 0, 100, -1);
if (ylw[loop].x == -1)
break;
ylw[loop].y = prompt_for("YELLOW Y coordinate", 0, 50, -1);
if (ylw[loop].y == -1)
break;
}
If you were using fgets() and sscanf(), you'd use:
int prompt_for(const char *what, int min, int max, int err)
{
int value;
int rc;
char line[4096];
printf("\nEnter %s from %d to %d:", what, min, max);
while (fgets(line, sizeof(line), stdin) != 0 &&
(rc = sscanf(line, "%d", &value)) == 1 &&
(value < min || value > max))
printf("Enter a value between %d and %d\n", min, max);
if (rc != 1)
{
fprintf(stderr, "Error reading %s.\n", what);
return err;
}
return value;
}
This isn't making much use of fgets(), though; it scores when you want multiple data items on a single line, because then you can report the whole line that was entered.
Warning: none of this code has been compiled

Answer 3

If you want to enforce that every input is on its own line of input (which is advisable when alternating prompt and reading from stdin), I recommend first reading a line into a temporary line buffer and then scanning that buffer with sscanf. Then check whether your input is valid in a last step.
Because this can clutter up your code, you might wright your own function for this:
/*
* Read an integer from stdin. Repeat until a valid number between
* lo and hi has been given. Returns 0 when a number was read and
* -1 when the input has run out.
*/
int read_int(int *x, int lo, int hi)
{
char buf[80];
char c;
for (;;) {
if (fgets(buf, sizeof(buf), stdin) == NULL) return -1;
if (sscanf(buf, "%d %c", x, &c) != 1 || *x < lo || *x > hi) {
printf("Please enter a single value between %d and %d.\n",
lo, hi);
} else {
break;
}
}
return 0;
}
You can then read your coordinates like so:
struct Coord ylw[3] = {{0, 0}};
int i;
for(i = 0; i < MAX; i++) {
printf("\n\nEnter YELLOW X coordinate #%d from 0 to 100:\n", i);
if (read_int(&ylw[i].x, 0, 100) < 0) break;
printf("\nEnter YELLOW Y coordinate #%d from 0 to 50:\n", i);
if (read_int(&ylw[i].y, 0, 50) < 0) break;
}
If the user types an eod-of-file character (Ctrl-D un Unix, Ctrl-Z on Windows), the number of coordinates can be cut short. If you want to end the input with a certain keyword, say "end", you can add this to your function by reading a string and checking first:
int read_int(int *x, int lo, int hi)
{
char buf[80];
char key[5];
char c;
for (;;) {
if (fgets(buf, sizeof(buf), stdin) == NULL) return -1;
if (sscanf(buf, "%4s ", key) == 1) {
if (strcmp(key, "end") == 0) return -1;
if (strcmp(key, "END") == 0) return -1;
}
if (sscanf(buf, "%d %c", x, &c) != 1 || *x < lo || *x > hi) {
printf("Please enter a single value between %d and %d.\n",
lo, hi);
} else {
break;
}
}
return 0;
}
(Scahhing a string from a string may seem redundant, but this way the user can type "end" after some spaces. You can also use a case-insensitive string comparison function.)