Validating input values with an if/else statement in C++

Question

So I've looked around but I'm apparently either not looking in the right place or I'm misunderstanding something.

The directions given are that the program can only accept values between 2.0 and 5.0 for vault heights. I get the feeling it has to do with my if condition but I'm not sure how to correct it.

When I input the height during debugging, it jumps to my else statement, and doesn't give me a chance to re-enter the new input.

//Retrieving name and first vault and date
cout << "Please enter the name of the pole vaulter.\n";
getline(cin, name);
cout << "Please enter the height of the first vault attempt.\n";
cin >> vault1;
if(5.0 >= vault1 >= 2.0) {
    cout << "What date was this vault accomplished?\n";
    getline(cin, date1);
} else {
    cout << "Only jumps between 2.0 meters and 5.0 meters are valid inputs.       Please enter the height of the first vault attempt.\n";
    cin >> vault1;
    cout << "What date was this vault accomplished?\n";
    getline(cin, date1);
}

Cheers and hth. - Alf · Accepted Answer · 2013-02-09 02:06:33Z

4

The expression

5.0>=vault1>=2.0

should work fine in Python, but in C++ it's parsed as

(5.0>=vault1) >= 2.0

where the result of the first comparison is compared to 2.0, yielding a nonsense result.

In C++ write that instead as

2.0 <= vault1 && vault1 <= 5.0

If you perform this check in more than one place, then it can help on clarity and maintainability define a suitably named boolean function.

answered Feb 9, 2013 at 2:06

Cheers and hth. - Alf

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5 Comments

Heather T Over a year ago

Yeah, the check is being performed on all three vault height inputs.\

johnsyweb Over a year ago

I wish I could +2 for that last sentence.

Rapptz Over a year ago

@HeatherT you have a stray '\n' in the buffer.

Cheers and hth. - Alf Over a year ago

as rapptz says. one way to get rid of it is to use the ignore method.

Heather T Over a year ago

Not sure where you mean I have a stray new line. Prof. also checks for readability so I try to start a new line of text after every question.

billz · Accepted Answer · 2013-02-09 02:11:53Z

1

C++ doesn't support mathematical expression.

Change:

if (5.0>=vault1>=2.0)

To:

if (vault1 > 5.0 || valut1 < 2.0)
{
   // invalid vault1
}
else
{
   // valid vault1
}

OR if you want to use &&

if (vault1 <= 5.0 && valut1 > 2.0)
{
   // valid vault1
}

edited Feb 9, 2013 at 2:11

answered Feb 9, 2013 at 2:04

billz

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7 Comments

Heather T Over a year ago

Thanks Billz! I thought it was something like that but I wasn't sure how to fix it.

Rapptz Over a year ago

Shouldn't it be if(vault1 <= 5.0 && vault1 >= 2.0)? Since it's a mathematical expression?

Cheers and hth. - Alf Over a year ago

@Rapptz: no, he's checking for invalid. but, he got the inclusive thing wrong...

Rapptz Over a year ago

If he's checking for invalid then you can negate the expression, i.e. if(!(vault1 <= 5.0 && vault1 >= 2.0))?

Cheers and hth. - Alf Over a year ago

yepps, that's de morgan's theorem

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