Thread: Check input data

Originally Posted by Cevris

In my programm I want to give as input a number from 0 to 100 and I need to deal with the situation in which a bad user of the program might give character(s) as input (while they shouldn't!).

I've done this:

Code:

int num;
 printf("Give number of names:\n");
int flag=scanf("%d",&num);

while(flag!=1 || num<0 || num>100)
{
    printf("Give number of names:\n");
   flag=scanf("%d",&num);
}

When I run this piece of ..... code and give characters for input, I get some great output:
Give number of names:
Give number of names:
Give number of names:
Give number of names:
....and keeps going forever....

Anyone could share some ideas?

Get rid of the first scanf() call entirely. Switch your loop to a do .. while construct with the test at the bottom... initialize num to 0 and flag to 0... and try it again.

This works great but I know it's not desiredable:

Code:

#include <stdio.h>

int main (void)
{

int num = 0;
int flag = 0;

do
{
	printf("Give number of names: ");
	flag = scanf ("%d", &num);
	getchar();
	fflush (stdin);
	}while (flag != 1 || num < 1 || num > 100);

return 0;
/

Sorry, i'm just trying to get my head around using other methods instead of FFLUSH (STDIN); I decided after class I would try to figure this out which is what i'm doing, final exam tonight. In all my homework I used this, despite it being incorrect my professor never took issue to it.

Seriously fflush() is for output streams only... in your code it probably just causes a return from the call, and does nothing.
Frankly, it should cause a run time exception since it is an obvious misuse of the function.

Originally Posted by dolfaniss

Sorry, i'm just trying to get my head around using other methods instead of FFLUSH (STDIN); I decided after class I would try to figure this out which is what i'm doing, final exam tonight. In all my homework I used this, despite it being incorrect my professor never took issue to it.

Please, don't go teaching people how to do things the wrong way. If you can't give proper advice, best not to give any. If you can't figure out how to properly flush the input buffer, you're probably a little too novice to be running around handing out any advice. And if your professor knew you were using fflush(stdin) and didn't bust you for it, he's not doing his job well, but I'm guessing he didn't read every line of every program of each of his students. It's wrong, stop using it in your code and stop teaching it. Read the tutorial Salem linked to in post #2, and ask us for help if you need to (new thread please).

Originally Posted by xiaohuai

i guess the character is never read, so it's still there. and the scanf is trying to read it but fails, so it keeps looping without getting an input from you.
try to use a do while loop as commomTater suggested and add an if statement to check the flag:

Code:

 int num;
    int flag;
    do{
        printf("Give number of names:\n");
        flag=scanf("%d",&num);
        if(flag != 1)
        {
            char trash[10];
            scanf("%s",trash);
            continue;
        }
    }while(num<0 || num>100);
    return 0;

I run this, but if I give characters for the first time it terminates. If I give a number greater than 100 or less than 0 at first place it runs correctly. Why is that?

Code:

 int num;
    do{
         num = 0;
         printf("Give number of names (1 - 100) :  ");
         scanf(" %d",&num);                            
       }while(num<1 || num>100);

Originally Posted by CommonTater

Code:

 int num;
    do{
         num = 0;
         printf("Give number of names (1 - 100) :  ");
         scanf(" %d",&num);                            
       }while(num<1 || num>100);

Nope....If you check it you will see that it runs forever.

Ok here you go...

This is my exact test code, scoop and poop from my IDE (which is Pelles C, ver 6.50 rc 4)...

Code:

#include <stdio.h>


int main (void)
{ int num = 0;

  do
   { num = 0;
     printf("Enter a number (1 - 100) : ");
     scanf("%d",&num); }
  while ((num < 1) || (num > 100));

  printf ("\nSuccessful entry = %d\n",num);

  return 0; }

And here is a screen shot with invalid entries, blank lines, and finally a good entry...

Attachment 10593

Originally Posted by Cevris

I run this, but if I give characters for the first time it terminates. If I give a number greater than 100 or less than 0 at first place it runs correctly. Why is that?

Because who knows what value num has when you start? If you get to the end of the do-while with num=47, then the while condition is false and the loop terminates.

Originally Posted by tabstop

Because who knows what value num has when you start? If you get to the end of the do-while with num=47, then the while condition is false and the loop terminates.

Ok... now I see the problem... he's getting a prompt that says "Enter a number" but he's typing FRED or something... yes that will make it run on.

Interesting problem...

try this: int num = -1;