Basic Control Flow

Input Validation

Input validation is an application of the if statement that verifies the user has given reasonable input.
Example
```
double area;
cin >> area;
```
user types "five" and hits return! (Causing cin to fail).
After every input a good program will test:
```
if (cin.fail())
```
The actual input should also be validated, even if the type is correct.
```
if (area < 0)
```

Other strategies:

if (cin)
   /* the stream did not fail */
else
   /* the stream failed */