Checking if input is a number

**Median** · 09-11-2012

Hello everyone.

I am a complete begginer to C++. I started from scratch yesterday with the cprogramming tutorials, which are very helpful by the way.

I must ask you to bare with me with this question, since the only language i've learned so far is Visual Basic, so my thinking process isn't yet set to understand C++.

Imagine I'm writting a programme that allows me to convert temperature units. I'd use cin>>variable; to atributte a value to later be converted.

What happens if i enter 'a' instead of '100'? I'd like to reject any input that isn't numeric.

Thanks in advance.

**Median** · 09-11-2012

The variable would be of type double

**laserlight** · 09-11-2012

You could do something like:

Code:

if (cin >> variable)
{
    // Process variable.
}
else
{
    // Inform user of an input error.
}

More generally, you can read the input as a string then parse the string.

**Median** · 09-11-2012

Take this simple code

Code:

#include <iostream>
using namespace std;
int main()
{
    int var;
    while (1)
    {
          cin>>var;
          cout<<var;
    }
}

var is an integer, so when i input '1' it writes '1' afterwards, but if i write 'a', it goes on writting some number i dont understand. It does not break with an error or anything.

**laserlight** · 09-11-2012

So, how would you incorporate or otherwise make use of my suggestion in post #3?

**WaltP** · 09-11-2012

As laserlight suggested, use a string to input the values. They won't goof up if you enter the wrong thing.
Then you have to check each character in the string to make sure it's a valid number.
When you've concluded it's valid, convert the string to a number and continue, otherwise, scream at the user...

If you're really new, you might want to get a little farther along before tackling this type of verification. It can be complex. A couple more weeks might give you the background to do this easily.

**iMalc** · 09-11-2012

Originally Posted by WaltP

As laserlight suggested, use a string to input the values. They won't goof up if you enter the wrong thing.
Then you have to check each character in the string to make sure it's a valid number.
When you've concluded it's valid, convert the string to a number and continue, otherwise, scream at the user...

If you're really new, you might want to get a little farther along before tackling this type of verification. It can be complex. A couple more weeks might give you the background to do this easily.

Laserlight did not suggest such a thing.
That is one option, but the easy option is to just check the state of the stream after the read, which is what Laserlight's code does. The stream is implicitly convertible to bool. You still need to reset the stream back into a good state if the read fails though.

**WaltP** · 09-12-2012

He didn't?

Originally Posted by laserlight

More generally, you can read the input as a string then parse the string.

Yes he did. I just expanded on how...

**iMalc** · 09-12-2012

Whoops, you're right... brainfart.

**Median** · 09-12-2012

To be honest, I didn't understand any of the suggestions you guys made. I'll follow WaltP's advice and keep on learning for a couple of weeks before tackling this issue. I honestly thought it would be much more straightforward :P

In my (little) programming experience, with MatLab and Visual Basic, if I tried to input a string into a double type variable, the programme would stop with an error. In C++ it doesn't happen and I still don't understand why. I guess I don't yet understand how storing different type of data into variables really work.

**WaltP** · 09-12-2012

It's because when C/C++ attempts to read a number and gets a non-number, the input functions stop reading at that point.
The offending character stays in the buffer.
Next read the character is still there.
Next read the character is still there.
Next read the character is still there.
Next read -- ad nauseum.

You have to catch the error and clear out the buffer.

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