## 4. Variables

### Modifying Variables

Unlike variables in mathematics, those in programming can change their value. In other words, a variable can be assigned a new value any number of times. As the first example of this, we will show how to increment the value of a variable by one.

First, let us introduce a new variable and name it `a`:

```double a=4;
```

The value of variable `a` is 4. Now, let us increment `a` by one:

```a++;
```

After executing this statement, the value of `a` will be 5.

Besides incrementing a variable, another way to change the value of a variable is simply by assigning it a new value:

```a = 9-2;
```

By executing this statement, the value of `a` will now be 7. Therefore, this statement assigns a new value to the variable `a`.

Let us unify all of this into a complete program:

```#include <iostream>
using namespace std;

int main()
{
double a=4;
cout << "a: " << a << endl;
a++;
cout << "a: " << a << endl;
a = 9-2;
cout << "a: " << a << endl;
}
```

Executing this program will result in the following output:

```a: 4
a: 5
a: 7
```

You can't introduce two variables with the same name.[*] Thus, you cannot add this statement at the end of the given program:

```    double a = 3;
```

because this statement attempts to introduce the variable `a` again.

For the same reason, you cannot modify the next-to-last statement of this program to:

```    double a = 9-2;
```