Control Structures

1
Selection
2
Control Structures

• Structures that allow the programmer to control
  the flow of execution of program statements
• They fall into 2 categoriesselection and
  repetition
• Some Examples
• if statements
• switch statements
• for loops
• while loops

3
Boolean Expressions

• The Boolean data type
• a representation for true and false values
• true has the value of not zero
• false has the value of zero
• not a built-in data type
• User-defined type
• typedef int Boolean
• define TRUE 1
• define FALSE 0

4
boolean.h

• a header file that you create so you can use the
  Boolean type in all your applications
• define TRUE 1
• define FALSE 0
• typedef int Boolean
• Must include boolean.h to use the boolean data
  type
• Why string quotes rather than brackets for
  boolean.h?

5
Typedefs

• typedef ltdata typegt ltnew type namegt
• typedef is a reserved word
• creates a synonym for some data type
• Why use typedefs?
• Gives more meaning to the types of values
• typedef float salaryType
• typedef float percentType

6
Relational Operators

• Operators used for relating numbers to each other
• equal to
• not equal to !
• less than lt
• greater than gt
• less than or equal to lt
• greater than or equal to gt
• Used in boolean expressions
• Arithmetic expressions can be used in boolean
  expressions
• (10 4 3 - 8) lt (18 30 / 4 -20)

7
Logical Operators

• Operators used to connect boolean expressions
• - logical and, binary
• - logical or, binary
• ! - logical not, unary
• Used to perform boolean logic
• To avoid confusion and to make code more
  readable, it is best to parenthesize comparisons
  using logical operators
• !((3 gt 4) (6 gt 5 6 lt 12))
• (( -5.0 -6.2) ((7 lt 3) (6 (3 3))))

8
Short Circuit Evaluation

• An optimization
• Only evaluates as many expressions as are
  necessary
• Using
• returns TRUE as soon as any expression evaluates
  to TRUE
• Using
• returns FALSE as soon as any expression evaluates
  to FALSE
• !((3 gt 4) (6 gt 5 6 lt 12))

9
if Statements

• if ( ltboolean expressiongt )
• ltstatementgt
• A decision making statement used to make a
  program execute some statements only when a
  certain condition is true
• If the boolean expression evaluates to TRUE, then
  the statement is executed
• If the boolean expression evaluates to FALSE,
  then the statement after the if statement is
  executed
• There must be parenthesis around the expression

10
Example

• float sum, num
• sum 0
• scanf(d, num)
• if (num gt 0.0)
• sum sum num
• printf(The sum is d\n, num)

11
Compound Statements

• Several simple statements associated with each
  other in a block
• The block is denoted by
• doSomething()
• doSomethingElse()
• doYetAnotherThing()
• Each statement in a block ends with a semi-colon
• The closing bracket of a block does NOT end with
  a semi-colon
• That would cause a syntax error

12

• Compound statements are commonly used with if
  statements
• They are needed for the case where there are
  multiple actions that need to take place if the
  boolean expression evaluates to TRUE
• Simple statements can also be expressed using the
  block notation
• if (xgt0)
• x 5
• printf(d is a positive integer., x)

13
Whats the Output?

• int a 5
• int b 10
• if (a lt b)
• b a
• printf(d d\n,
• a,b)
• printf(d d\n,a,b)

• int a 10
• int b 5
• if ((a lt b)(b-a gt 0))
• a b
• b--
• prinf(d d\n,a,b)
• prinf(d d\n,a,b)

14
Lets Write Some if Statements

• Write a program that
• Reads 3 real numbers
• Counts the number of positive reals
• Accumulates the sum of the positive reals

15
Two-Way Selection

• if statements only provide one way branching
• if else statements provide two way branching
• Syntax
• if ( ltboolean expressiongt )
• lttrue statementgt
• else
• lt false statementgt

16
Control Flow of an if else Statement

• If the boolean expression evaluates to TRUE, the
  statement following the expression is executed
• Program control returns to the line of code
  following the complete if else statement
• If the boolean expression evaluates to FALSE, the
  statement following the else is executed
• Program control returns to the line of code
  following the complete if else statement

17
An Example

• int temp 0
• int a 10
• int b 5
• if (a gt b)
• a
• else
• temp a
• a b
• b temp
• printf(d d\n,a,b)

18
Nested Selection Statements

• Since each selection statement is a simple
  statement, they can be used as statements that
  are part of other selection statements
• Why would you need this?
• Sometimes you have certain requirements for
  certain ranges of data
• Sometimes you are looking for certain
  combinations of values for variables

19
For Instance

• / a, b, and c represent the coefficients in the
  quadratic equation ax2 bx c 0 /
• float a,b,c
• if (a 0)
• if (b ! 0)
• / what goes here? /
• else if (c 0)
• / fill in the blank /
• else
• / last case /

20
Mutually Exclusive Conditions

• Only one can be true at a time
• Nesting selection statements prevents expressions
  from being evaluated where the result is going to
  always be false
• if (score gt 69)
• printf(Satisfactory)
• if ((score lt 69) (score gt 50))
• printf(Unsatisfactory)
• if (score lt 50)
• printf(Failing)

21
Lets Do It

• Problem Calculating traffic fines
• Fine scale
• 5 - 10 mph over limit, 25 per mph over the limit
• 11 - 15 mph over the limit, 35 per mph over the
  limit
• 16 - 20 mph over the limit, 40 per mph over the
  limit
• gt 20 over the limit, 1000
• Notice how important indentation is when nesting
  selection!

22
Switch Statements

• Multiway selection
• You are not restricted to 1 or 2 courses of
  action
• Syntax
• switch ( ltselectorgt )
• case ltlabel 1gt ltstatementgt
• break
• case ltlabel2gt ltstatementgt
• break
• default ltstatementsgt

23

• The ltselectorgt is a variable of any ordinal type
• e.g. int, char, typedef int myType
• The labels are what the selector is compared
  toeffectively
• if (selector label) / do whatever is
  following that
• 
  label until we hit a break /
• The ltstatementgt can be any statement, simple or
  complex
• e.g. assignment, selection, function calls
• The break says Im done with this branch and
  returns program control flow to the line
  following the switch statement
• if you leave this out, a very common mistake,
  sequential evaluation of statements takes place

24
An Example
25
When can I use a switch statement?

• Whenever your selection is based on an ordinal
  type
• Soit is not a replacement for if and if else
  selection statements