Display error message on second loop of do-while - c

Question

If I have a while or do while loop in C, is there some (native) way I can have something happen on the second loop?
I ask for getting inputs; I have this:
int size;
do {
printf("Size of tower (0 <= x <= 23): ");
scanf("%i", &size);
} while (size > 23 || size < 0);
If the user inputs some value which is not between 0 and 23, I want to display an error message and ask for another value. Obviously I could do it like this:
int size;
int error = 0;
do {
if (error) { printf("Invalid size\n"); }
printf("Size of tower (0 <= x <= 23): ");
scanf("%i", &size);
error = 1;
} while (size > 23 || size < 0);
However, this feels gross. I'm looking for an elegant solution, and I figure that having something run on the second loop would work.

Answer 1

I think you want something like this:
int size = -1;
int MAX_TRIES = 10;
while (MAX_TRIES--)
{
printf("Size of tower (0 <= x < 23): ");
if (scanf("%i", &size) != 1)
{
printf("Read error!!\n");
break;
}
if (size >= 0 && size < 23)
{
break;
}
printf("Error: You entered '%d' which is not in the range 0 <= x < 23\n", size);
}
By writing it this way, you won't have to compute the negation of your boolean condition logic mentally while writing the code.
Also, checking for the return value of scanf() is important. Thanks to Weather Vane's comment for reminding this.
Further, it is probably better to limit the number of executions of this loop rather than letting it run till infinity. ( Thanks to Jonathan Leffler's comment )

Answer 2

The conversion and error reporting could be moved to a function to simplify the calling code. Input is taken by fgets and the value is parsed in the function by strtol. This function returns success or failure. Other values get back to the caller through pointers.
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <errno.h>
#include <limits.h>
int get_int_range ( char *line, char **next, char *delim, int *value, int min, int max);//prototype
int main( int argc, char *argv[])
{
char line[100] = {'\0'};//input buffer
int valid = 0;
int size = 0;
do {
printf ( "Size of tower (0 <= x <= 23) or quit\n");
fgets ( line, sizeof ( line), stdin);//read a line
if ( strcmp ( line, "quit\n") == 0) {
valid = 0;
break;// if quit is entered, exit loop
}
valid = get_int_range ( line, NULL, "\n", &size, 0, 23);// call to parse a value
} while ( !valid);// on failure, keep looping the above
if ( valid) {
printf ( "Size of tower is %d\n", size);
}
return 0;
}
//inputs
// char *line : pointer to text to be parsed
// char **next : pointer to pointer to allow modification of caller's pointer
// char *term : pointer to characters to be considered terminators
// int *value : pointer to int to allow modification of caller's int
// int min : minimum value of range
// int max : maximum value of range
// returns : 0 failure or 1 success
int get_int_range ( char *line, char **next, char *delim, int *value, int min, int max)
{
long int input = 0;
char *end = NULL;//will point to end of parsed value
if ( line == NULL) {
printf ( "no text to parse\n");
return 0;
}
if ( value == NULL) {
printf ( "unable to save parsed value\n");
return 0;
}
errno = 0;
input = strtol ( line, &end, 10);//get the integer from the line. end will point to the end of the parsed value
if ( ( errno == ERANGE && ( input == LONG_MAX || input == LONG_MIN))
|| ( errno != 0 && input == 0)){// parsing error from strtol
perror ( "input");
return 0;
}
if ( end == line) {// nothing was parsed. no digits
line[strcspn ( line, "\n")] = '\0';//remove newline
printf ( "input [%s] MUST be a number\n", line);
return 0;// return failure
}
// *end is the character that end points to
if ( *end != '\0' && !( delim && strchr ( delim, *end))) {// is *end '\0'? is *end in the set of term characters?
line[strcspn ( line, "\n")] = '\0';//remove newline
printf ( "problem with input terminator: [%s] \n", line);
return 0;
}
if ( input < min || input > max) {// parsed value is outside of range
printf ( "input out of range %d to %d\n", min, max);
return 0;
}
if ( next != NULL) {// if next is NULL, caller did not want pointer to end of parsed value
*next = end;// *next allows modification to caller's pointer
}
*value = input;// *value allows modification to callers int
return 1;// success
}

Answer 3

My preferred technique would involve duplicating the input, but would simplify your loop:
printf("Size of tower (0 <= x <= 23): "); // Try to get good input once.
scanf("%i", &size);
while (size < 0 || 23 < size ) { // While the user is wrong:
printf("Invalid size\n");
printf("Size of tower (0 <= x <= 23): "); // Try, try again.
scanf("%i", &size);
}