Savitch Progr

1
Problem Solving with C The Object of Programming
Walter Savitch Chapter 2 C Basics Slides by
David B Teague, Western Carolina University a
constituent institution of The University of
North Carolina
2
C Basics

• Variables and Assignments
• Input and Output
• Data Types and Expressions
• Simple Flow of Control
• Program Style

3
C Basics 2.1 Variables and Assignments

• A C variable can hold a number or other types.
• A C variable ALWAYS has a value stored in it.
• A C variable is implemented as a memory
  location. Information stored is binary -- 0s and
  1s.
• We do not need to know nor do we care what
  address a particular variable occupies in memory.

4
Names Identifiers

• The name of a variable (or anything else in C
  that needs naming) is called an identifier.
• An identifier must be spelled starting with a
  letter or an underscore symbol (_) followed by
  any number of letters, digits, or underscore
  symbols.
• Valid Examples x x_1 _abc A2b
  ThisIsAVeryLongIdentifier
• Invalid Examples 12 3X change myFirst.c
  data-1
• Why are these invalid?
• There are identifiers that are reserved to the
  use of the C language, called keywords, or
  reserved words. (A complete list is in Appendix 1
  of the text.)

5
Names Identifiers

• Invalid Examples 12 3X change myFirst.c
  data-1
• Why are these invalid?
• 12 , change, and 3x do not start with a letter
• myFirst.c has a . character that is not
  permitted
• data-1 contains a - character that is not
  permitted.
• Identifier Spelling Rule
• Identifiers must start with a letter or
  underscore and have only letters, digits or
  underscores after the first.

6
Display 2.1 A C Program (1 of 2)

• include ltiostreamgt
• using namespace std
• int main( )
• int number_of_bars
• double one_weight, total_weight
• cout ltlt "Enter the number of candy bars in a
  package\n"
• cout ltlt "and the weight in ounces of one
  candy bar.\n"
• cout ltlt "Then press return.\n"
• cin gtgt number_of_bars
• cin gtgt one_weight
• total_weight one_weight number_of_bars
• cout ltlt number_of_bars ltlt " candy bars\n"
• cout ltlt one_weight ltlt " ounces each\n"
• cout ltlt "Total weight is " ltlt total_weight ltlt
  " ounces.\n"

7
Display 2.1 A C Program (2 of 2)

• cout ltlt "Try another brand.\n"
• cout ltlt "Enter the number of candy bars in a
  package\n"
• cout ltlt "and the weight in ounces of one
  candy bar.\n"
• cout ltlt "Then press return.\n"
• cin gtgt number_of_bars
• cin gtgt one_weight
• total_weight one_weight number_of_bars
• cout ltlt number_of_bars ltlt " candy bars\n"
• cout ltlt one_weight ltlt " ounces each\n"
• cout ltlt "Total weight is " ltlt total_weight ltlt
  " ounces.\n"
• cout ltlt "Perhaps an apple would be
  healthier.\n"
• return 0

8
Variable Declarations

• Every variable in C must be declared.
• A declaration introduces a name to the compiler
  and specifies the type of data that is to be
  stored in the variable.
• A variable declaration has the form
• Type_name variable_name1, variable_name2,
• Example
• double distance
• The kind of data held in a variable is it type.
• You must declare all variables prior to use.

9
Assignment Statements

• total_weight one_weight number_of_bars
• In an assignment statement, the right hand side
  expression is evaluated, then the variable on the
  left hand side is set to this value.
• Syntax variable expression
• distance rate time
• count count 2

10
Pitfall Uninitialized Variables

• A variable that has not been set by your program
  will have the value left there by the last
  program to use the memory associated with that
  variable. This is an UNINITIALIZED variable. It
  contains garbage in the root sense of the word.
• This is illegal and incorrect but few compilers
  will catch this error.

11
Variable initialization

• How do we prevent use of uninitialized variables?
• Take great care with your program that all
  variables receive a value prior to fetching a
  value from them.
• Initialize your variables when they are declared.
• Syntax Two different appearing but equivalent
  initializations
• int x 3 double pi 3.14159
• int x(3) double pi(3.14159)

12
Programming Tip

• Use meaningful variable names.
• Names that tie your programs code to the problem
  being solved will make your code easier to read
  and easier to debug.
• Anything that you can do to make your code
  readable is worth days in debugging time.

13
2.2 Input and OutputOutput using cout

• cout ltlt number_bars ltlt candy bars \n
• ltlt is the insertion operator
• It is illegal to break a string constant across
  two lines. This is ILLEGAL, though some compilers
  happily accept it.
• cout ltlt Mary had a little lamb. Its fleece was
  white as snow.\n

13
14
Include Directories and Namespaces

• include ltiostreamgt
• using namespace std
• These lines provide declarations necessary to
  make iostream library available.
• C divides collections of names into namespaces.
  To access names in a namespace, the second line
  above, the using directive, is sufficient. It
  means that your program can use any name in the
  namespace std.
• A C namespace usually contains more than just
  names. They usually contain code as well.
• Older compilers will require the older style,
  ltiostream.hgt, and such compilers may not like the
  using directive. If your compiler doesnt like
  the using directive, just omit it.

14
15
Escape sequences

• The \ (backslash) preceeding a character tells
  the compiler that the next character does not
  have the same meaning as the character by itself.
• An escape sequence is types as two characters
  with no space between them.
• \\ is a real backslash character, not the escape
  character, a backslash that does not have the
  property of changing the meaning of the next
  character.
• \n newline
• \t tab character (same as control-h)
• \a alert, or bell
• \ double quote (that does not end a string
  literal).

16
Formatting for Numbers with a Decimal Point

• The following statements will cause your floating
  point output to be displayed with 2 places of
  decimals and will always show the decimal point
  even when the output has a zero fractional part.
• cout.setf(iosfixed)
• cout.setf(iosshowpoint)
• cout.precision(2)
• //Output format78.50

17
Input using cin

• When a program reaches a cin gtgt statement, it
  waits for input. You are responsible for
  prompting the user of your program for proper
  input.
• Syntax
• cin gtgt number gtgt size
• cin gtgt time_to_go
• gtgt points_needed

18
Designing input and output

• Echoing your input is frequently requested by
  problem statements. Even when not requested, it
  is usually better to echo your input.

19
2.3 Data Type and ExpressionsThe types int and
double

• From the C point of view, 2 and 2.0 are very
  different.
• 2 has type int has no fractional part
• 2.0 has type double has a fractional part,
  even
• if the
  fractional part is 0.
• An int and a float are stored differently.

20
Other Number types

• Memory
• Type used Range
  Precision
• short 2 bytes -32,767
  NA
• (a.k.a. short int) to 32,767
• int 4 bytes -2,147483,647
  NA
• to
  2,147483,647
• long 4 bytes same as int
  NA
• float 4 bytes approximately
  7 digits
• 10-38
  to 1038
• double 8 bytes approximately
  15 digits
• 
  10-308 to 10308
• long double 10 bytes approximately
  15 digits
• 
  10-4932 to 104932

21
The Types char and bool

• Memory
• char is a special type that is designed to hold
  single members of the ASCII character set.
• Some vendors have extended ASCII character
  encoding to include more characters than upper
  and lower case letters, digits and punctuation.
  (Notably IBM, on the PC, which has been adopted
  in nearly all Microsoft software.)
• cstring (from C) and string (from the Standard
  Library) are for more than one char value. More
  on these in a later chapter.
• bool is a type that was added to C in the1995
  Draft ANSI Standard. There are only two values
  true and false. Almost all compilers support the
  bool type.

22
Display 2.3 The Type char

• include ltiostreamgt // Blank lines removed to
  fit code to space available.
• using namespace std
• int main( )
• char symbol1, symbol2, symbol3
• cout ltlt "Enter two initials, without any
  periods\n"
• cin gtgt symbol1 gtgt symbol2
• cout ltlt "The two initials are\n"
• cout ltlt symbol1 ltlt symbol2 ltlt endl
• cout ltlt "Once more with a space\n"
• symbol3 ' '
• cout ltlt symbol1 ltlt symbol3 ltlt symbol2 ltlt
  endl
• cout ltlt "That's all."
• return 0

23
Arithmetic Operators and Expressions

• Common arithmetic operators are encoded
• Addition Multiplication
• Subtraction - Division /
• These operators are used to make arithmetic
  expressions
• one_weight number_of_weights
• This expression is said to return a value that
  automagically appears where the expression is,
  and is used in an outside expression. Here the
  sum is done, value returned, and this value is
  used as a factor in the multiplication.
• one_weight (number_of_weights off_set)
• (See precedence, later in the text.)

24
Automagically

• Automagically means automatically, as if by
  magic.

25
Arithmetic Operators and Expressions

• Arithmetic with and behaves as expected for
  both integer and floating point types.
• Arithmetic for / behaves as expected for floating
  point numbers.
• Arithmetic for / behaves in a somewhat surprising
  way for integer types (short, int, long)
• Division for integers is TRUNCATING - it discards
  the fractional part.

26
Arithmetic Operators and Expressionsdivision /,
and modulus , for integer values

• Division /, and modulus are complementary
  operations. Mod, or modulus, , works ONLY for
  integer types.
• 4 12 / 3 is the
  quotient
• 3 12
• 12
• 0 12 3 12 mod 3
  is the remainder
• 4 14 / 3 quotient 3
  THIS IS NOT 4.66
• 3 14
• 12
• 2 14 3 remainder
  after dividion
• See the PITFALL - Whole Number Division, page 70.
  

27
Arithmetic Operators and ExpressionsPrecedence

• When two operators appear in an arithmetic
  expression, there are PRECEDENCE rules that tell
  us what happens.
• Evaluate the expression,
• X Y Z
• by multiplying Y and Z first then the result is
  added to X.
• Rule Do inside most parentheses first, then
• multiplication and division next,
• additions and subtractions next, and
• break all ties left to right.

28
Arithmetic Operators and ExpressionsStyle

• Always follow your instructors style
  requirements.
• Our suggestion is that in writing an expression,
  you should write the expression with spaces
  either side of the operator
• We suggest this
• X Y Z
• but not this
• XYZ
• Always follow your instructors style
  requirements.

29
2.4 Simple Flow of ControlA simple branching
mechanism

• Making decision in computer programs requires
  changing the execution from next instruction next
  to some other instruction next. This is called
  flow of control.
• There are two types of flow of control selection
  and looping.
• Looping repeats an action, and will be discussed
  a later.
• Selection chooses between alternative actions.
• Selection
• if (expression) Control Expression
  returns a bool value
• action1 Affirmative clause.
  Executed if Expression is true
• else
• action2 Negative clause.
  Executed if Expression is false

30
Comparison Operators

• C provides comparison operators for making
  decisions in computer programs. These operators
  return a value of type bool true or false.
• Math C C
  Math
• Symbol English Notation Sample
  Equivalent
  
• equal to x 7
  2 y x 7 2y
• ? not equal to ! ans
  ! n ans ? n
• lt less than lt count lt m 3
  cout lt m 3
• ? less than lt time
  lt limit time ? limit
• or equal to
• gt greater than gt time gt
  limit time gt limit
• ? greater than gt age gt
  21 age ? 21
• or equal to

31
Logical Operators

• The and operator
• Syntax
• (Comparison_1) (Comparison_2)
• Example, in an assignment to a bool variable
• bool in_range
• in_range (0 lt score) (10 lt score)
• The or operator
• Example -- in an if-else statement
• if ( (x 1) (x y) )
• cout ltlt x is 1 or x equals y. \n
• else
• cout lt x is neither 1 nor equal to y.\n

32
PITFALL strings of inequalities

• Suppose x, y and z are integer values.
• if (x lt y lt x) // Unfortunately, this is
  WRONG BUT IT COMPILES.
• cout ltlt z is between x and y
• Here is why the expression is WRONG.
• In mathematics x lt y lt z is short hand for x lt y
  y lt z.
• In C, this is not true. It is still valid C,
  but isnt what you expect from the mathematics.
  In C the precedence rules require x lt y lt z be
  evaluated like this
• (x lt y) lt z
• The parenthesized expression returns a bool
  value. The lt requires the same type on both
  sides. The bool value gets converted to the int
  value 0 (for false) or 1 (for true). Then 0 ltz
  or 1 lt z compiles. And gives (most of the time) a
  wrong answer!

33
PITFALL using instead of

• if (x 12) // The
  should have been
• cout ltlt x is equal to 12
• else
• cout ltlt x is not equal to 12
• The second expression is NEVER executed,
  regardless of the value of x before this
  statement is encountered.
• WORSE, after this if statement executes, the
  expression x 12 HAS ASSIGNED the value 12 to
  x.
• Why? The expression x 12 returns the value 12,
  which is converted to the bool value true, which
  is used by the if.
• Always write
• if (12 x) // Compiler will say 12 x
  non-lvalue on left
• cout ltlt x is equal to 12
• else
• cout ltlt x is not equal to 12

34
Simple Loop Mechanisms

• Most programs include a mechanism to repeat a
  block of code multiple times. 30 Students, 30
  grades on each test, 100 workers, pay check
  generator block runs 100 times.
• C provides loops named
• while
• for
• do while
• The piece of code the loop executes is called the
  body.
• Each loop execution of the body is called an
  iteration.

35
Display 2.10 A while loop

• include ltiostreamgt
• using namespace std
• int main( )
• int count_down
• cout ltlt "How many greetings do you want? "
• cin gtgt count_down
• while (count_down gt 0)
• cout ltlt "Hello "
• count_down count_down - 1
• cout ltlt endl
• cout ltlt "That's all!\n"
• return 0

36
Display 2.13 A do-while loop

• include ltiostreamgt
• using namespace std
• int main( )
• char ans
• do
• cout ltlt "Hello\n"
• cout ltlt "Do you want another greeting?\n"
• ltlt "Press y for yes, n for no,\n"
• ltlt "and then press return "
• cin gtgt ans
• while (ans 'y' ans 'Y')
• cout ltlt "Good-Bye\n"
• return 0

37
Pitfall Stray semicolons

• Loop body with several statements
• while(bool expression) Do not put a
  semicolon here
• 
  This usually causes an
• several statements infinite
  loop. (Next section)
• Loop body with one statement
• while(bool expression) Do not put a
  semicolon here
• statement This
  usually causes an
• 
  infinite loop.

38
Pitfall Stray semicolons

• Loop body with several statements
• do
• several statements
• while(bool expression) Dont
  forget the semicolon
• 
  here.
• Loop body with one statement
• do
• statement
• while(bool expression) Dont forget
  the semicolon
• 
  here.

39
Increment and decrement operators

• C provides the and -- operators, each in
  each of two forms, prefix and postfix.
• The text, for good teaching reasons, leaves the
  use of expressions using and -- to provide a
  value until later.
• For now, we use n as a synonym for n n 1
• and n-- for a synonym n n - 1

40
PITFALL Infinite Loops

• Any loop in which the Boolean control expression
  remains true will never terminate. A loop that
  does not terminate is called an infinite loop.
• Infinite loops are wrong, and worse, can be hard
  to find.
• Examples
• This loop terminates This is an
  infinite loop
• x 2 x 1
  
• while (x ! 12) while (x ! 12)
• cout ltlt x ltlt endl cout ltlt x ltlt endl
• x x 2 x x 2
• WHY?

41
Display 2.14 Charge Card Program (1 of 2)

• include ltiostreamgt
• using namespace std
• int main( )
• double balance 50.00
• int count 0
• cout ltlt "This program tells you how long it
  takes\n"
• ltlt "to accumulate a debt of 100,
  starting with\n"
• ltlt "an initial balance of 50 owed.\n"
• ltlt "The interest rate is 2 per
  month.\n"

42
Display 2.14 Charge Card Program (2 of 2)

• while (balance lt 100.00)
• balance balance 0.02 balance
• count
• cout ltlt "After " ltlt count ltlt " months,\n"
• cout.setf(iosfixed)
• cout.setf(iosshowpoint)
• cout.precision(2)
• cout ltlt "your balance due will be " ltlt balance
  ltlt endl
• return 0

43
2.5 Programming Style Comments

• The most difficult part of any programming
  language to learn to use properly is the comment.
  
• A comment should always tie the code to the
  problem being solved. In some circumstances, a
  comment could explain tricky code. (It is
  better to write clear code and omit the comment.)
• / comment in this style / may span more than
  one line.
• // these comments run from the // to the end of
  the line.
• The text uses the // style comments exclusively.
  If you do this too, then you can use
  /comments/ to comment out code while developing
  programs.

44
Indenting

• Indenting elements considered a group should be
  indented to look like a group.
• if-else, while, and do-while should be indented
  as in the sample code.
• The affirmative clause and negative clause of
  if-else statements should be indented more than
  surrounding code.
• The body of loops should be indented more than
  surrounding code.
• CONSISTENCY of style is more important than any
  particular style standard.

45
Program header comments

• Comments should be placed at the start of the
  program that describes the essential information
  about the program
• The file name
• The author
• The address or other means to contact the author
• The purpose of the program
• What the program does
• The date written or version number

46
Naming Constants

• Avoid magic numbers! Use declared constants.
• Example
• If the tax rate is 4 (0.04) today, it will go
  up. Make such numbers declared constants unless
  you want to hunt through your code to find and
  replace all instances of 0.04 and try to decide
  if it is a tax rate, then replace it by the new
  tax rate, 0.05.
• Name declared constants with the const keyword.
• Spell declared constant names with upper case
  letters,
• with successive words separated by underscores.
• Example
• const int BRANCH_COUNT 10
• const double TAX_RATE 0.04

47
Summary (1 of 2)

• Use meaningful name for variables.
• Check that variables have been declared before
  use, and have the correct data type.
• Be sure variables have been assigned a value
  before use. This can be done by initialization at
  definition, or by assigning a value before first
  use.
• Use enough parentheses to make the order of
  operations clear. Remember, code is meant to be
  read, which implies writing for an audience.
• Always have your program prompt the user for
  expected input. Always echo users input.
• An if-else statement chooses between two blocks
  of code to execute. An if statement chooses
  whether to execute a block of code.

48
Summary (2 of 2)

• A do-while always executes its body at least
  once. A while loop may not execute its body at
  all.
• Almost all numeric constants should be given
  meaningful names to be used instead of the
  numbers. Use the const modifier to do this.
• Use indenting, spacing, and line break patterns
  similar to the sample code to group sections of
  code such as the body of a while statement, or
  the affirmative and negative clauses of an
  if-else statement.
• Insert commentary to explain major subsections of
  your code, or to explain any unclear part of your
  program.
• Make your code clear. Remember, a program is
  meant to be read by programmers, not just
  compilers.