C++ Programming

Chapter 3: Types and Variables

Data types

The most common data types in C++ are:

type	explanation	example
int	integer	12
double	floating point	4.5
char	character	'm'
string	string	"monkey"
bool	boolean	true

We will take a closer look at the features of these types in later chapters of the course.

Defining a variable

In C++, the variable must be defined before it can be used. Defining a variable gives its type and name. For example, the following code creates a variable of type int, called a.

int a;

In the above case, the variable is uninitialized, so it may start with any value that is already in memory. So you can't rely on a variable to have a value initially, for example, 0. However, we can provide in connection with the definition the desired initial value for the variable as follows:

int a = 5;

You can define multiple variables at once, as long as their types are the same. For example, the following code defines two int variables:

int a, b;

The variable type is fixed, and cannot be changed after definition.

Variable handling

After you define a variable, you can assign it a new value. For example, the following code defines two variables and then changes their values.

int a = 2, b = 5;
cout << a << " " << b << "\n"; // 2 5
a = 3;
b = a+b;
cout << a << " " << b << "\n"; // 3 8

If the new value of a variable is calculated based on its old value, can use operators +=, -=, *=, /=, etc. For example, the following code snippets have the same meaning:

a = a+5; b = b*2;

a += 5; b *= 2;

Also, if the value of a variable increases or decreases by one, we can use ++ and -- operators. For example, the following codes have the same meaning:

a++; b--;

a = a+1; b = b-1;

More about expressions

Setting the variable with = is actually an expression, whose value is the new value of the variable. For example, in the following code, an expression a = 3 has a value of 3, so the code sets a to 3 and b to 5.

b = 2 + (a = 3);

This allows us to set the value of many variables at the same time. For example, the following code sets the variables a and b to 1:

a = b = 1;

In this case, the interpretation is that the variables are set right to left.

For ++ and -- operators the value of the expression depends on which side the operator is. If the operator is after the variable, the value of the expression is the old value of the variable, and if the operator is ahead of the variable, the expression value is the new value for the variable. The following codes illustrate this:

int a = 3;
cout << a++ << "\n"; // 3

int a = 3;
cout << ++a << "\n"; // 4

You can use a comma to combine multiple expressions into one expression whose value is the value of the last expression. For example, the value of a = 2, b = 5 is 5.

Changing variables

A handy C++ function is swap, which interchanges the contents of two variables:

swap(a,b);

This will produce the same result as the following code, with auxiliary variable t:

t = a;
a = b;
b = t;

Constants

A constant is a variable whose value cannot change after defining it. A constant variable is defined by the const keyword.

For example, the following code creates a constant variable, with a value of 5.

const int n = 5;

The benefit of such a definition is that the variable is blocked by the compiler from changing. For example, the following code does not work, because it is trying to change the value of a constant variable.

const int n = 5
n = 8; // doesn't work