Thread: Restricting Character lengths

I am having trouble restricting the character lengths in this data entry form I am making using do/while with nested if statements. I have tried to alter these with strlen functions but i can't get it to constain the character length of the array. Basically, I want the person to be only able to input alpha numeric characters for the name of the dog's species with up to 22 characters only.

Code:

int main(void)
{
	/*variables*/
	int errors;
	int i;
	char dog_s[22] = {'\0'};

do 
{
errors = 0;
printf("Enter The breed, species of the dog");
printf(" , No more than twenty two characters\n");
scanf("%22s", dog_s);
fflush(stdin);
		
if (strlen(dog_s) < 0 || strlen(dog_s) > 22)
errors=1;
		
}
while (errors > 0);
for (i = 0 ; i <= (int)strlen(dog_s) ; ++i)
{
if (isdigit(dog_s[i]) == 0)
errors=0;
}
			
	
return 0;
}

Why don't you use fgets() instead of scanf()?

Jim

fflush(stdin) is wrong. Read why here and read about the solution here.

You can't prevent the user from typing certain characters, or have the program skip over them as they're typed. Keyboard input is line buffered meaning your program doesn't technically see the input until the user hits enter. You would need a special library like curses/ncurses for this, which is a lot of trouble. Try reading in a whole line with fgets, then check the length with strlen and make sure every character in the string is alphanumeric by using isalnum() from ctype.h.

Ok, I shall give it a try, could this be a source to why it isn't functioning to restrict character length?

Technically, yes, but probably not. fflush(stdin) results in undefined behavior. The thing about undefined behavior is, it's undefined. Anything or nothing at all, can happen. Usually fflush(stdin) just doesn't do anything, and the odds of it ruining your length check are really slim. Regardless, ditch that and try using fgets to read the input into a buffer, then check that buffer.

Originally Posted by anduril462

Technically, yes, but probably not. fflush(stdin) results in undefined behavior. The thing about undefined behavior is, it's undefined. Anything or nothing at all, can happen. Usually fflush(stdin) just doesn't do anything, and the odds of it ruining your length check are really slim. Regardless, ditch that and try using fgets to read the input into a buffer, then check that buffer.

code wise it would look like????

Read the two links in my first post (click the red text). They explain the problem and solution, including examples of using fgets.

The thing is if you use ... scanf("%22s",String);... the user can type as much as they want and the char array only sees one of...
a) the whole string if less than 22 characters
b) the first 22 characters if the user types more
c) the first word if there's more than one word (up to 22 characters)

You are constraining the array to bounds (although it should be ... dog_s[23]... to allow for the trailing null. But you can't constrain the typist