2.3. Variables¶
One of the most powerful features of a programming language is the ability to manipulate variables. A variable is a named location that stores a value.
Just as there are different types of values (integer, character, etc.), there are different types of variables. When you create a new variable, you have to declare what type it is.
Note
In C++, integer variables are declared as type int and character variables
are declared as type char.
The following statement creates a new variable named fred that has type char.
char fred;
This kind of statement is called a declaration.
The type of a variable determines what kind of values it can store. A
char variable can store a character, and it should come as no surprise
that int variables can store integers.
There are several types in C++ that can store string values, but we are
going to skip that for now (see Chapter 7).
To create an integer variable, the syntax is
int bob;
where bob is the arbitrary name you made up for the variable. In general, you will want to make up variable names that indicate what you plan to do with the variable. For example, if you saw these variable declarations:
char first_letter;
char last_letter;
int hour, minute;
you could probably make a good guess at what values would be stored in
them. This example also demonstrates the syntax for declaring multiple
variables with the same type: hour and minute are both integers (int
type).
Although the language allows this, it should be avoided. It is easy to accidentally create uninitialized variables, one of the most common sources of error in C++ programs. Consider the following:
What do you think this program will output?
What should it output?
Uninitialized variables are an example of undefined behavior.
The C++ language does specify what should happen here. Most compilers will warn that you did this, but none will stop you from compiling and running this program unless you ask it to.
The value of hour could be 0, but might also be
any value constructed from the bits that just happened
to be stored in memory when hour was created.
Many compilers will allow something along the lines of:
-Werror=uninitialized
To force an error if an uninitialized variable is detected.
Naming conventions
C++ is a multi-paradigm language with a long history. Over the years many different groups have developed different programming styles for C++. None of them are wrong as long as they apply their rules consistently.
Since this book intends to teach C++, it follows the conventions used in the C++ Standard Library. For more information about naming conventions, see the Naming conventions section of the C++ Core Guidelines, edited by Bjarne Stroustrup and Herb Sutter.
If you continue programming in C++, you will undoubtedly encounter variables that use other styles.
That’s ok. You can’t follow everyone’s conventions.
Q-2: A(n) is a name given to a location in memory used to keep track of a value.
Q-3: Take a look at the code block below:
char tom;
It’s an example of a(n) statement.
int main() { int x = 7; int y = 10; char c = '8'; while (x < y) { cout << c << '\n';; x++; } double d = 9; int z = x + y; cout << "It's the year " << 2000 + z << "!"; }
int main() {
char init1 = 'K';
string init2 = "T";
cout << init1 << "+" << init2 << '\n';
string init3 = "C";
char init4 = 'J';
cout << init3 << '+' << init4 << '\n';
string c = "Carved their initials in a tree!";
cout << c;
}
-
Q-6: Match the variable to the kind of value it can store.
Ideally, you want your variables to be named according to what they represent. Not the case here! Try again!
- char joe;
- 'x'
- string ten;
- "Joe"
- int x;
- 10
Write code that creates the variables name, first_initial, and number_of_siblings IN THAT ORDER. It is up to you to choose the correct types for these variables.
More to Explore
From cppreference.com
C++ identifiers
C++ Core Guidelines naming conventions.
Naming conventions from Google C++ style guide.