Welcome to Programming and Data Structures

Now that we have declared some variables, we can use them to store values. We do that with an assignment statement. An assignment statement has the form variable = value;. This example shows a few assignments, and the comments illustrate different ways people sometimes talk about assignment statements:

int hour;
int minute;
int second;
hour = 11; // assign the value 11 to the variable hour
minute = 59; // 59 gets the value 59
second = 30; // set second to 30

The variable type that you declare must match the type of the value assigned to it. A type mismatch will generate a compile error. For example, you cannot store a string in an int variable:

The type of hour is int. It can only store whole number values, not strings like "six". It would also be an error to try to store "6" as that is still a string, not a number.

This rule is sometimes a source of confusion, because there are many ways that you can convert values from one type to another, and C++ sometimes converts things automatically. But for now you should remember that as a general rule variables and values have the same type, and we’ll talk about special cases later.

Variables must be initialized (assigned for the first time) before they can be used. You can declare a variable and then assign a value later, as in the previous example. You can also declare and initialize on the same line:

int main() {
    int hour = 11;      // Declare hour and assign a value
    int minute = 59;    // Declare minute and assign a value
}

When a variable is not given a value, it is said to be uninitialized. In C++, an uninitialized variable has an unknown value. (It will contain whatever value is represented by the bits that happened to be in the memory where it was placed!). Using an uninitialized variable can cause the program to behave unpredictably and is never desired. Fortunately, the compiler will generally provide you with a warning. The program below has this issue:

If we ignore the warning and run the code anyway, the output will be unpredictable. The value of hour will be whatever value is represented by the bits that were already in the memory location chosen for the variable! Try running the program below multiple times. You should see different values appear for the uninitialized variable.

Once a variable has been declared, it is a syntax error to redeclare it: