GAINING CONTROL

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GAINING CONTROL

• CHAPTER 2

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2.1 Expressions and Statements

• Y 4 x 5
• does_nothing ()

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EXPRESSIONS

• Expressions are sequences of tokens that can be
  evaluated to numerical quantity.
• A TOKEN is a group of characters that belong
  together.
• ( ) -

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TOKENS

• They range from a single number or identifier to
  a more complicated sequence of tokens.
• X
• x 5
• 2 Y - 7 (-3/X 7Z)

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EXPRESSIONS

• Expressions can contain any of the operators in
  C.
• We first discuss arithmetic and assignment
  operators.
• Others are Bit Operators and Cast Operators.

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Operators

• assignment
• addition
• - unary minus and subtraction
• multiplication
• / division
• remainder

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Arithmetic Operations

• addition and unary plus
• - unary minus and subtraction
• multiplication
• / division
• remainder
• a (a/b) b (ab)

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Compound Assignment

• The first syntax shortcut is
  compound assignment
• The assignment statement
• a a 4
• can be shortened to
• a 4

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STATEMENTS

• A statement is a sequence of tokens terminated
  with a semicolon that can be recognized by the
  compiler.
• A statement differs from an expression in that
  statements do not have values.

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STATEMENTS

• The purpose of some statements like assignment
  statements is to change the values of variables.

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CONTROL STATEMENTS

• The purpose of control statements is to select
  which set of statements to execute in a given
  circumstance or to cause a sequence of statements
  to be executed more than once.

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CONTROL STATEMENTS

• if ( expression )
• statement
• if (expression)
• statement 1
• else
• statement 2
• while
• statement
• for (initialization test processing)
• statement

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Lvalues (lvalues) -LEFT

• The term lvalues refers to an expression that has
  a Location in memory.
• The name of an variable is an lvalue.

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Modifiable lvalues

• Modifiable lvalues are expressions whose values
  can either be changed or evaluated.
• Modifiable lvalues can be used on the left-hand
  side of an assignment statement.
• Left

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rvalue -RIGHT

• A rvalue can be evaluated but cannot be changed.
• An rvalue cannot be used on the left-hand side of
  an assignment statement.
• 
  Right

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rvalue

• Example single character token 5 is an rvalue
• the expression 2x 5 is also rvalue

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C Expressions- Classifications

• Expression Lvalue Rvalue
• x yes yes
• x 3 no yes
• y yes yes
• 2y - 7 no yes
• (-2/y 7 y) no yes

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2.2 BLOCKS AND COMPOUND STATEMENTS

• Can contain both declarations and executable
  code.
• Treated as a single statement.
• Delimited by a pair of braces.
• Variables declared in a block are unknown outside
  the block

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Block Examples

• x 4
• y x 3

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Block Examples

• int I
• I 5
• x (I) 3
• y I-4

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Compound Statements

• A compound statement is a sequence of statements
  that can be used anyplace in the syntax that a
  simple statement can be used.
• It is a common construct in most programming
  languages.

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Blocks

• In C the construct that implements a compound
  statement is called a block.
• The syntax of the C language allows more than
  just a sequence of statements in a block
• Variable declarations can be included with the
  executable statements

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Block

• A block must begin with an opening brace and
  terminate with a closing brace.
• .
• The contents of the block may consist of
  declarations, statements, both or neither.

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Block

• The contents of the block may consist of
  declarations, statements, both or neither.
• int I
• x (I) 3
• y I-4

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Block - Declaration

• A declarations in a block must appear before the
  statements.
• int f ( int)

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Block - Placement

• A block can be placed anywhere in a program that
  a simple statement can be placed.

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Local Variable

• When a variable is declared inside of a block, it
  is called a local variable, since the compiler
  will only recognize its name from its point of
  declaration to the end of the block.

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Local Variable

• The statements in an inner block can access any
  variable that the surrounding function can access.

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Varaiables

• Surrounding Block
• int 5
• Inner Block
• int i 3

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Style

• A C program or function will be more readable
  when all variables used within a single function
  are given different names.
• The compiler can keep variables declared inside
  and outside of a block straight, but human beings
  may have problems.
• Declarations should appear in one place.
• Auxiliary declarations in a block are possible.

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2.3 IF and if-else Statements

• This conditional statement allows a program to
  test a condition and then choose which code to
  execute next.
• The choice depends on the outcome of that test.
• If (expression)
• statement

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If-else Statement

• If (expression)
• statement 1
• else
• statement 2
• If expression evaluates to true (nonzero),
  statement 1 is executed,
• If expression evaluates to false (zero),
  statement 2 is executed.

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Nested or Compound if-else

• If (expression)
• statement
• else if (expression)
• statement
• else if (expression)
• statement
• else
• statement

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