Hw do I check for valid input?

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I want to check if the input is in the time format HH:MM. Valid time is from 00:00 to 23:59. I did:

C#

printf("Please enter you age: ");
scanf("%f", &age);
if (age < 18) 
{
    printf("Too young, we can't serve you.\n");
    return 1;
}

C++

printf("Please enter the time in the format HH:MM: ");
scanf("%c%c%c%c%c", &h1, &h2, &colon, &m1, &m2);
if ((h1 < '0' || h1 > '2') || (h2 < '0' || h2 > '9') || (m1 < '0' || m1 > '5') || (m2 < '0' || m2 > '9')) 
{
    if (h1 == '2' && h2 > '3')
    {
            printf("The time is not in the format HH:MM.");
            return 1;
    }
}

but it doesn't work.

Posted 12-Apr-15 7:23am

yngwie123

Updated 12-Apr-15 11:38am

v7

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Richard MacCutchan · Accepted Answer · 2015-04-12T07:42:00

Solution 1

C++

if (h1 == '2' && h2 greater than '3')

If any character is invalid, this test is redundant at this point. It should be part of the initial if statement. Something like:

C++

if ((h1 < '0' || h1 > '2') || (h2 < '0' || h2 > '9') || (m1 < '0' || m1 > '5') || (m2 < '0' || m2 > '9') || (h1 == '2' && h2 > '3'))
{
        printf("The time is not in the format HH:MM.");
        return 1;
}

Posted 12-Apr-15 7:42am

Richard MacCutchan

Comments

Maciej Los 12-Apr-15 13:48pm

5ed!

yngwie123 12-Apr-15 14:38pm

It's confusing. Why do I get "The time is not in the format HH:MM." when I hit "10:00", for example? I tried various inputs but they all don't seem to work.

Frankie-C 12-Apr-15 15:07pm

In your original code you messed up the conditions for 'correct' value and 'wrong' one.
in the line:
if ((h1 < '0' || h1 > '2') || (h2 < '0' || h2 > '9') || (m1 < '0' || m1 > '5') || (m2 < '0' || m2 > '9'))
you checked all wrong conditions, you must have returned immediatly as 'wrong' format. But you added another check:
if (h1 == '2' && h2 greater than '3')
So all wrong inputs, but not having hour bigger that 23 become correct ;-)
Maybe in your checks reverting the conditions you got an error again making wrong all 'correct' values less than 23.
The correct coding of your test should have been:
if ((h1 < '0' || h1 > '2') || (h2 < '0' || h2 > '9') || (m1 < '0' || m1 > '5') || (m2 < '0' || m2 > '9'))
{
printf("The time is not in the format HH:MM.");
return 1;
}
else if (h1 == '2' && h2 > '3')
{
printf("The time is not in the format HH:MM.");
return 1;
}

yngwie123 12-Apr-15 15:40pm

It still doesn't work. Maybe it's logically wrong?

Frankie-C 12-Apr-15 16:31pm

What don't work? It works pretty good.
On what values it fails for you?

yngwie123 12-Apr-15 17:36pm

OK, it works but with the other piece of code I added it doesn't. Why?

Richard MacCutchan 13-Apr-15 3:32am

You are using %f for the age variable, you should use %d.

yngwie123 13-Apr-15 8:18am

"age" type is float but after giving a valid age it asks for the time in the format HH:MM and at that point, for some reason, whatever the input is, it says that the time isn't in the format HH:MM. I can't undrestand why this happens.

Richard MacCutchan 13-Apr-15 8:25am

Why is age a float, when you are only comparing it against the integer 18? Use an integer type to make it work properly. And your time check code is still wrong, try the chage that I suggested.

Richard MacCutchan 13-Apr-15 8:33am

Add the following line after each call to scanf():


fflush(stdin);

yngwie123 13-Apr-15 8:52am

I made it float because it can get values like "18.5". Well, I added this line after the "age" scanf and it works. Why did it happen at all and what does this line do? The HH:MM format isn't complete because it can get more than 5 chars like "12:24)" and it's valid. How can I check for input of 5 chars only?

Richard MacCutchan 13-Apr-15 9:10am

Unfortunately scanf is not the best method of getting input into a program, as it leaves CR/LF characters in the input buffer unless you call fflush. You can add a dummy format selector after the last %c to capture any characters past the HH:MM.

yngwie123 13-Apr-15 9:20am

OK, I'll remember this. I like the dummy format selector idea :)