Thread: validating inputs in code

i have a code which compares between these two variables. date1 and date2. The program runs when the user input two correct dates eg: 20/10/2003 20/14/2004 then it will read and display values in between the date. That means if the user enters other characters like alphabets or other weird characters the program will prompt an error and not run.

Only with the correct input(int) then it can run. how can i implement this in that case? probably in real expressions or switch case? thanks

this is my code:





/*
* date_compare compares two dates, returning <0, 0, >0 if
* date1<date2, date1==date2, date1>date2, respectively
*/
int date_compare(Date *date1, Date *date2)
{
if(date1->year < date2->year)
{
//printf("Year is smaller");
return -1;
}
else if (date1->year > date2->year)
{
//printf("Year is bigger");
return 1;
}
else
{
if (date1->month < date2->month)
{
//printf("Month is smaller");
return -1;
}

else if (date1->month > date2->month)
{
//printf("Month is bigger");
return 1;
}
else
{
if(date1->day < date2->day)
{
//printf("Day is smaller");
return -1;
}
else if(date1->day > date2->day)
{
//printf("Day is bigger");
return 1;
}
else
{
//printf("equal");
return 0;
}

}
}
}
/*
* date_destroy returns any storage associated with `d' to the system
*/
void date_destroy(Date *d){
if(d != NULL){
free(d);
}
}




[/CODE]

FYI, they're called regular expressions, not real expressions. I don't think they're available in standard C, but there is a POSIX extension supporting them and there are 3rd party libraries that provide regex functions.

There are several other options as well:
1. If available, use strptime.
2. Split the string using strtok. Use strtol or strtoul to convert to int/long, it has better error detection (atoi has none). You can use the endptr parameter to check if there is garbage at the end of datestr.
3. Use sscanf to parse datestr and extract the year, month and day. You will need to make sure there is no garbage at the end of your datestr.

If you go for option 2 or 3, you need to remember that not all months have 31 days, some have 30 and February has 28 (29 in a leap year). A switch statement here is good for checking max day based on the month, especially since you can take advantage of fall-through (see below). Google will turn up formulas for determining leap year, to help you figure out if February should have 28 or 29 days for the given year.

Code:

switch (month) {
    case 1:  // fall-through
    case 3:  // fall-through
    case 5:  // fall-through
    ...
    case 12:
        max_days = 31;
        break;

    case 4:  // fall-through
    case 6:  // fall-through
    ...
}
if (day > max_days)
    printf("Bad date, month %d has %d days", month, max_days);  // usually a good idea to tell the user why their input was invalid

It's a good idea to leave a comment stating that you explicitly want the cases to fall through, just to be clear you didn't forget a break;

20/14/2004 - does not looks like valid date

strncopy could leave your string without nul-terminator - so you need to put it manually before applying and string manipulating routines that rely on nul-terminator existense

And I personally prefer using strtoul for parsing such data over strtok - with it you do not need local copy

In case of failed parsing or wrong data - you have a memory leak - better move your memory allocation to the line 32

Code:

Date *date_duplicate(Date *d)
{
    Date *cloneDate = malloc(sizeof(*cloneDate));
    if(cloneDate )
        *cloneDate = *d;
     
    return cloneDate;
}

looks easier, checks return value of malloc and you do not need cast malloc in C - read FAQ

Before considering regular expressions dont underestimate the power of good old scanf, you can check make sure the number of input parameters and form "mm/dd/yyyy" etc is correct. If it's not, you can stop already because its not a correct date. Otherwise move on to check other things like numerical ranges, leap years etc. Then place the items like day, month, year into your date structure.

Code:

// read a date like "mm/dd/yyyy" where mm is a month 1 <= mm <= 12,
// dd is a day 1 <= dd <= 31 and yyyy is a year 1900 <= yyyy <= 9999
// return true if correct format and valid date; false otherwise
bool valid(const char *datestr)
{
    int month, day, year;
    char dummy;
    
    if (sscanf(datestr, " %d/%d/%d %c", &month, &day, &year, &dummy) != 3)
        return false;
    if (month < 1 || month > 12)
        return false;
    if (day < 1 || day > 31)
        return false;
    if (year < 1 || year > 9999)
        return false;
    return true;
}

int main()
{
    printf("%s is %s\n", "12/25/2013/", (valid("12/25/2013/") ? "valid" : "invalid!"));
    printf("%s is %s\n", " 12/25/2013", (valid(" 12/25/2013") ? "valid" : "invalid!"));
    printf("%s is %s\n", " 12/25/2013 !", (valid(" 12/25/2013 !") ? "valid" : "invalid!"));
    printf("%s is %s\n", "25/12/2013", (valid("25/12/2013") ? "valid" : "invalid!"));    
    return 0;
}

In the example the leading blank in the scanf format allows 0 or more leading blanks in the input. The dummy parameter prevents non-blank inputs which are invalid like "!"

Where is your definition of Date? Is it a struct. Also it seems you have an extra closing brace at the end of your input. Also if you have a problem compiling you wont get far if you dont say what the exact error was. I mean what is the message that is printed