Variables And Data Types

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!! Identifiers

As in all programming languages, we use identifiers to name things such as variables, functions, data types, classes, etc. Identifiers in C++:

• can consist of characters, digits, and the underscore
• must begin with a character
• are case sensitive
• can be of any length, though the first 32 characters must be unique.

Variables

Variables in C++ must be explicitly declared as they are in Java.

int grade,
    sum,
    total;

double average,
       salary;

In C++, variables are not initialized when they are created. You can declare a variable and initialize it at the same time

int total = 0;
double taxRate = 0.05;

Data Types

C++ provides a number of primitive data types including

Literal values can be used in C++ programs just like in any other language. In C++, literals for the basic types are specified as follows

Assignments

Assignments in C++ use the the single equal sign (=) as in Java. Variables are declared to store a specific data type can only be assigned a value of that type.

grade = 89;
taxRate = 0.155;

Attempting to sign incompatable types will result in a compilation error.

Constant Declarations

Constant variables or named literals are defined using the keyword const placed in front of the variable declaration. A constant declaration requires an initial value assigned to the variable

const float PI = 3.1415926;
const int NUM_VALUES = 4;

Once a value has been assigned to a constant variable, it can not be changed. Constant variables are typically named in all uppercase to distinguish them from other variables.

Mathematical Operators

Mathematical expressions are constructed using the mathematical operators

avg = (grade1 + grade2 + grade3) / 3.0;

which are the same as those in Java

  +  -  *  /  %  ( )

If both operands are integers, then integer arithmetic is performed and the result will be an integer value. If one or both of the operands are real values, then real arithmetic is performed and the result is a real value. (For operator precedence, see the order of precedence table.)

Type Casting

There are two approaches for typecasting in C++: the C method or the new C++ method. In C, a value can be typecast or forced to a new type by specifying the value within parentheses

int count = 3;
double avg;
     :
avg = (grade1 + grade2 + grade3) / (float) count

while in C++ the type to which a value is being typecast is used as method

avg = (grade1 + grade2 + grade3) / float( count )

Type Definitions

In addition to the primitive data types, users can create their own types by simply renaming existing types or constructing new types from existing ones usin the typedef instruction. In the following example,

typedef unsigned char byte;

we create a new type called byte from the exisiting type unsigned char. You can now use this new type in your program just as you would any other type. Internally the program will actually use an unsigned char type and thus the use of the new type is still goverened by that type.

byte value;
byte small = 4;

The general form of the typedef statement is

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