CPS120: Introduction to Computer Science

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CPS120 Introduction to Computer Science

• Variables and Constants

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Anatomy of a C Program

• //Simple C Program
• //
• // Purpose To demonstrate the
• // parts of a simple C program
• include ltiostream.hgt
• main ( )
• cout ltlt "This is a program "
• return 0

• Comments //
• Compiler Directive
• Main Function ( )
• Braces
• Statements

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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

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Languages and Data Types

• Strong typing the requirement that only a value
  of the proper type can be stored into a variable
• A data type is a description of the set of values
  and the basic set of operations that can be
  applied to values of the type

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Data Types

• Integer numbers
• Real numbers
• Characters
• Boolean values
• Strings

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Declarations

• A declaration is a statement that associates an
  identifier with a variable, an action, or some
  other entity within the language that can be
  given a name so that the programmer can refer to
  that item by name

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Declarations in Various Languages
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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

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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

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Integers

• The range varies depending upon how many bytes
  are assigned to represent an integer value
• Some high-level languages provide several integer
  types of different sizes
• Operations that can be applied to integers are
  the standard arithmetic and relational operators

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Data Types - Whole Numbers

• In C, 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.

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Other Data Types

• unsigned char, short, unsigned int, long, and
  unsigned long for whole numbers

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Real Numbers

• Like the integer data type, the range varies
  depending on the number of bytes assigned to
  represent a real number
• Many high-level languages have two sizes of real
  numbers
• The operations that can be applied to real
  numbers are the same as those that can be applied
  to integer numbers

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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 10308
• A double variable can store a number as high as
  1.7 x 10308
• A double variable can store a number with up to
  15 digits of precision (significant digits)

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Other Data Types

• float and long double for decimal values

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Characters

• It takes one byte to represent characters in the
  ASCII character set
• Two bytes to represent characters in the Unicode
  character set
• Our English alphabet is represented in ASCII,
  which is a subset of Unicode

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Characters

• Applying arithmetic operations to characters
  doesnt make much sense
• Comparing characters does make sense, so the
  relational operators can be applied to characters
• The meaning of less than and greater than
  when applied to characters is comes before and
  comes after in the character set

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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.

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Strings

• A string is a sequence of characters considered
  as one data value
• For example This is a string.
• Containing 17 characters one uppercase letter,
  12 lowercase letters, three blanks, and a period
• The operations defined on strings vary from
  language to language
• They include concatenation of strings and
  comparison of strings in terms of lexicographic
  order

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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

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Boolean

• The Boolean data type consists of two values
  true and false
• Not all high-level languages support the Boolean
  data type
• If a language does not, then you can simulate
  Boolean values by saying that the Boolean value
  true is represented by 1 and false is represented
  by 0

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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

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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

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Remember

• A reserved word is a word in a language that has
  special meaning
• Case-sensitive means that uppercase and lowercase
  letters are not considered the same
• Remember, C is completely case sensitive

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Variable Names in C

• 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

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Conventions for Naming Variables

• Use a conventional method of making your
  variables easy to read at a quick glance. For
  example
• 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)
• Separate successive words with underscore
  characters ( _ ) (eg. semester_grade, card_value)
  
• Hungarian notation
• Begin with type (eg. iTestScore)

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Common Reserved Words

• break
• case
• char
• const
• default
• do
• double
• else
• extern

• float
• for
• if
• int
• long
• return
• switch
• void
• while

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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 program
• 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 685char letterGrade
  'A'string firstName "Paul"

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Assignment statement

• Assignment statement an action statement (not a
  declaration) that says to evaluate the expression
  on the right-hand side of the symbol and store
  that value into the place named on the left-hand
  side

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Sample Assignment Statements
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Constants

• Named constant A location in memory, referenced
  by an identifier, that contains a data value that
  cannot be changed

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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

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Use of Constants (Literals)

• Numeric
• 5
• 3.14159
• -17.29
• Characters
• 'a'
• '7'
• ''
• Strings (a sequence of symbols)
• "I will be an better person "

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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
• do start with letter or underscore
• dont use reserved words

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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 3.14159
• const double PA_SALES_TAX 0.06
• const int SPEED_OF_LIGHT 299792458 // commas
  can't be used here

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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)