Verifying integer input in a union in C - c

Question

I'm trying to verify that what a user inputs in to one section of a union is actually an integer. I have been trying to use the isdigit function (see below) but I am having no success. All I need to do is make sure the user enters ONLY numbers for the date, but I am having a lot of trouble with it.
My code:
#define STRSIZE 30
#define PROFSIZE 30
#define NBRASSI 2
#define TRUE 1
#define FALSE 0
struct assignment
{
char name[STRSIZE];
char prof[PROFSIZE];
int duedate;
float value;
};
.
.
.
struct assignment populate_structure(struct assignment assi[], int assi_nbr)
{
int count;
int date_check = FALSE;
for (count = 0; count < assi_nbr; count++)
{
flushall();
printf("Enter Assignment #%d name (max %d chars):\n", count + 1,
STRSIZE);
gets(assi[count].name);
while (date_check == FALSE)
{
printf("Enter due date for Assignment #%d (YYYYMMDD):\n",
count + 1);
scanf("%d", &assi[count].duedate);
if (isdigit(assi[count].duedate))
{
date_check = TRUE;
}
else
{
printf("Invalid");
date_check = FALSE;
}
}
printf("Enter %% of final grade for Assignment #%d:\n", count + 1);
scanf("%f", &assi[count].value);
flushall();
printf("Enter Professor's name for Assignment #%d (max %d chars):\n",
count + 1, PROFSIZE);
gets(assi[count].prof);
printf("\n\n");
}
return assi[count];
}
It gives me no errors in Visual Studio, but when I run the program I get an Abort error whenever ANY value is input for assi[].duedate
If I remove if (isdigit(assi[count].duedate)), the program runs fine (so long as they only input an integer for duedate). Any help is greatly appreciated.

Answer 1

As the commenters already noted: isdigit() is for single characters only. Also: the way you use scanf() already assures the result to be a number.
But you should read a string instead, it is easier to test. For example
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
int main(void)
{
int c;
int position = 0;
int res;
char date[9] = {'\0'};
char input[20] = {'\0'};
puts("try if date is in the format \"YYYYMMDD\"");
res = scanf("%19s",input);
if(res != 1){
fprintf(stderr,"input was a mess, aborting\n");
goto __FAILURE;
}
c = input[position];
while(c != '\0'){
switch(position){
case 0:
if( c == '2' || c == '1' ){
date[position] = (char) c;
} else {
goto __FAILURE;
}
break;
case 1:
if( c == '9' || c == '0' ){
// check valid digit by looking back here
// (not implemented)
date[position] = (char) c;
} else {
goto __FAILURE;
}
break;
case 2:
if( isdigit(c) ){
date[position] = (char) c;
} else {
goto __FAILURE;
}
break;
case 3:
if( isdigit(c) ){
date[position] = (char) c;
} else {
goto __FAILURE;
}
break;
case 4: // month
if( c == '0' || c == '1' ){
date[position] = (char) c;
} else {
goto __FAILURE;
}
break;
case 5: // month
if( isdigit(c) ){
// check valid digit by looking back here
// (not implemented)
date[position] = (char) c;
} else {
goto __FAILURE;
}
break;
case 6: // day
if( c == '0' || c == '1' || c == '2' || c == '3'){
date[position] = (char) c;
} else {
goto __FAILURE;
}
break;
case 7: // day
if( isdigit(c)){
// check valid digit by looking back here
// (not implemented)
date[position] = (char) c;
} else {
goto __FAILURE;
}
break;
default:
break;
}
c = input[++position];
}
printf("Date was correct and is %s\n",date);
exit(EXIT_SUCCESS);
__FAILURE:
printf("Date was wrong at position %d with character %c or just too short/long\n",position,c);
exit(EXIT_FAILURE);
}
Fill out the rest (check if date is valid and in the correct range). You could also part the date into individual year/month/day sections instead of putting the string into another string and convert them into numbers via atoi ( you can use atoi `because you already made sure to have a valid number) or by converting manually inside the switch without extra variables and functions. Checking ranges arithmetically is simpler than comparing strings/characters.

Answer 2

Here is my offering - any invalid entry calls exit(1) though a better error handler would be preferred. The data is entered as a string, and then extracted and validated.
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
typedef struct {
int year;
int month;
int day;
} mydate_t;
int main(void){
int i;
mydate_t dat = {0};
int dayspermon[12] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
char str[12];
printf("Enter a date (YYYYMMDD): ");
fflush(stdout);
if(fgets(str, sizeof str, stdin) == NULL) {
exit(1);
}
for(i=0; i<8; i++) {
if(!isdigit(str[i])) {
exit(1); // also fails when the string is too short
}
}
// extract
for(i=0; i<4; i++) {
dat.year = dat.year * 10 + str[i] - '0';
}
for(i=4; i<6; i++) {
dat.month = dat.month * 10 + str[i] - '0';
}
for(i=6; i<8; i++) {
dat.day = dat.day * 10 + str[i] - '0';
}
// validate
if(dat.year < 2000 || dat.year > 3000) // arbitrary range
exit(1);
if(dat.year % 4 == 0 && (dat.year % 100 != 0 || dat.year % 400 == 0)) {
dayspermon[1]++; // leap year adjustment
}
if(dat.month < 1 || dat.month > 12)
exit(1);
if(dat.day < 1 || dat.day > dayspermon[dat.month - 1])
exit(1);
printf("Date: %04d %02d %02d\n", dat.year, dat.month, dat.day);
}