CPS120: Introduction to Computer Science Variables and Constants Lecture 8 - B

Variables Used to store values in virtually every computer program Used for “remembering” things during program execution Variables have names, types and values Values can change during execution

Data Types You need to first choose an appropriate data type when you use a variable.Values can either be: whole numbers decimal numbers letters (i.e. characters) whole words (i.e. string values

Choosing a Type Most computer languages have a select number of different data types You must select the proper data type for each variable that you use in a program in order to program efficiently This decreases memory (RAM) usage This increases the speed of your program

Data Types - Whole Numbers To store whole numbers in a variable, we use a variable of the int data type. An int variable uses 4 bytes of memory. An int variable can store a number as low as -2,147,483,648. An int variable can store a number as high as 2,147,483,647.

Data Types - Decimal Numbers To store decimal numbers in a variable, we use a variable of the double data type A double variable uses 8 bytes of memory A double variable can store a number as low as -1.7 x A double variable can store a number as high as 1.7 x A double variable can store a number with up to 15 digits of precision (significant digits)

Data Types - Characters To store a letter or a single character (such as #, $, *, etc.), we use a variable of the char data type. A char variable only uses 1 byte of memory. A char variable can only hold one letter, digit, or character.

Data Types – Words / Phrases To store a word or phrase (string value), we use a variable that is a string Technically string is not a data type You can think of it as a data type for now

Data Types – True and False The data type bool is useful to store true and false values Alternatively, we can simply use an int variable with either a 1 value (to represent true) or a 0 value (to represent false) if necessary

Other Data Types unsigned char, short, unsigned int, long, and unsigned long for whole numbers float and long double for decimal values

Using Variables in C++ Variables must be declared before they are used in C++. Get into the habit of doing this at the top of your functions char grade; // a students semester grade int numStudents; // number of students in our class double price; // price of item string userName; // user's name

Variable Names Variable names are technically known as identifiers Choose your own variable names but you must be careful to use valid ones. Otherwise, the compiler will be confused and errors will result. When choosing your variable names: do not use keywords that are defined in the programming language (Reserved Words) do not include spaces or other disallowed characters do not use more than 31 characters do begin the identifier with a letter Remember, C++ is completely case sensitive

Conventions for Naming Variables Use a conventional method of making your variables easy to read at a quick glance. For example: 1.Begin variable identifiers with lowercase letters (eg. score) if you wish to use more than one word within the identifier, you must capitalize the following words or parts of words (eg. semesterGrade, testScore) 2.Separate successive words with underscore characters ( _ ) (eg. semester_grade, card_value) 3.Hungarian notation Begin with type (eg. iTestScore)

Common Reserved Words break case char const default do double else extern float for if int long return switch void while

Initializing Variables C++ does not automatically initialize all variables to the value 0 If you do not initialize a variable to a certain value, the variable will have an indeterminate value that can corrupt the logic of your programindeterminate value You should usually initialize your variables at the same time that you declare them. This is done with a declaration statement that is also an initialization statement int numberOfPizzas = 3; double monthlyCarPayment = 685; char letterGrade = 'A'; string firstName = "Paul";

Constants Sometimes you need to use the same value many times throughout a program. In this case, it is proper to use a constant rather than a variable Constants allow you to give a name to a value used several times in a program The value never changes

Use of Constants (Literals) Numeric Characters 'a' '7' '*' Strings (a sequence of symbols "I will be an better person "

Naming Constants Constants are defined in a way that is similar to variables Select a data type and give the constant a name Any valid identifier name can be used to name a constant Start with letter or underscore Can’t use reserved words

Conventions for Naming Constants Traditionally, all uppercase letters have been used when naming constants Use the underscore character ( _ ) between consecutive words. This allows other programmers to be able to "pick out" your constants at a quick glance Examples: const double PI = const double PA_SALES_TAX = 0.06 const int SPEED_OF_LIGHT = ; // commas can't be used here

Type Compatibilities You cannot store a value of one type in a variable of a different type – a type mismatch occurs Promotion occurs automatically You can typecast Supply the name of the data type you want to use to interpret the variable followed by the variable placed in parenthesis C = PI * float (diameter);