Flow of Control Control Structures in CC

1
Flow of Control Control Structures in C/C

• Flow of control
• Order in which statements are executed
• Control structures in C/C
• Sequence structure
• statements executed in order in which they are
  placed in program
• essentially built into C/C (comes for free)
• Selection structures
• provide ability to select which statement will be
  executed next
• provide decision-making capability, essentially
• Repetition (or iteration) structures
• provide ability to go back repeat execution of
  set of statements
• provide looping capability, essentially

2
Flow of Control Control Structures in C/C
(contd)

• Overview - 7 Control Structures in C/C
• Sequence
• Order in which you place you statements in
  program
• Three types of selection
• if structure - single selection
• if-else structure - double (multiple when
  nested) selection
• switch structure - multiple selection
  (restricted)
• Three types of repetition
• for structure
• while structure
• do-while structure

3
Flow of Control Control Structures in C/C
(contd)

• An Aside - The goto Statement
• The notorious goto statement (C/C actually
  supports it!)
• Allows unconditional transfer of control to some
  other statement
• Crimes of goto statements
• Bad program structure - spaghetti code
• Tend to complicate programs - make them more
  difficult to read understand at best
• Verdict for goto statements (at least for this
  class)
• Theoretically - goto statement is never
  required - other control structures in C/C
  provide sufficient flexibility to handle all
  possible flow control requirements
• Dont use at all cost, lest youd have lots of
  explanation to do!

4
Flow of Control Control Structures in C/C
(contd)

• The if Statement
• Single selection structure
• Performs an action if condition is true
• Skips the action if condition is false
• Note conditions are relational expressions
• Example in pseudocode
• If students grade is gt 60
• Print Passed
• Example in C
• if (grade gt 60)
• cout ltlt "Passed" ltlt endl

5
Flow of Control Control Structures in C/C
(contd)

• The if Statement (contd)
• Flowchart Representation

Action
Condition
true
false
6
Flow of Control Control Structures in C/C
(contd)
The if-else Statement

• Double selection structure
• Performs an action if condition is true
• Performs a different action if condition is false
• Can be nested (chained) to do multiple selection

• Example in pseudocode
• If students grade is gt 60
• Print Passed
• else
• Print Failed

Example in C if (grade gt 60) cout ltlt
"Passed" ltlt endl else cout ltlt "Failed" ltlt
endl
7
Flow of Control Control Structures in C/C
(contd)

• The if-else Statement (contd)
• Flowchart Representation

Action 1
Action 2
Condition
true
false
8
Flow of Control Control Structures in C/C
(contd)

• Compound Statements

• Seen so far - only single-statement action for
  if or if-else
• Multiple-statement actions - enclose statements
  with
• Compound statement - set of statements contained
  in
• Can be placed anywhere a single statement can be
  placed
• A compound statement containing declarations is
  called a block
• Example

if (grade gt 60) cout ltlt "Passed" ltlt
endl else cout ltlt "Failed" ltlt endl cout
ltlt "Must take this course again." ltlt endl
9
Flow of Control Control Structures in C/C
(contd)

• Nested (Chained) if-else Statements

if (grade gt 90) cout ltlt "A" else if
(grade gt 80) cout ltlt "B" else if
(grade gt 70) cout ltlt "C" else
if (grade gt 60) cout ltlt "D"
else cout ltlt "F" cout ltlt endl
(popular alternative form) if (grade gt 90)
cout ltlt "A" else if (grade gt 80) cout ltlt
"B" else if (grade gt 70) cout ltlt "C" else
if (grade gt 60) cout ltlt "D" else cout ltlt
"F" cout ltlt endl
10
Flow of Control Control Structures in C/C
(contd)

• The switch Statement
• Multiple selection structure
• Test expression can assume series of integral
  values
• Performs different actions for different test
  expression values
• Less general than nested if-else statement
• integer test expression only
• test for equality only
• General form shown on right

switch (expression ) case value1
statement11 ... break case
value2 statement21 ...
break ... case valueN
statementN1 ... break default
statementD ...
11
Flow of Control Control Structures in C/C
(contd)

• The switch Statement (contd)
• Uses 4 keywords
• switch
• identifies start of switch statement
• expression in parentheses (switch expression)
  evaluated its value compared to various
  alternative (case) values within compound
  statement that follows
• switch expression must evaluate to an integer
  result (compilation error otherwise)

switch (expression) case value1
statement11 ... break case
value2 statement21 ...
break ... case valueN
statementN1 ... break default
statementD ...
12
Flow of Control Control Structures in C/C
(contd)

• The switch Statement (contd)
• Uses 4 keywords (contd)
• case
• labels individual values to be compared to value
  of switch expression (identifies possible entry
  points of switch statement)
• comparison done in order in which case values are
  listed
• execution begins with statement immediately
  following case value that matched value of switch
  expression

switch (expression) case value1
statement11 ... break case
value2 statement21 ...
break ... case valueN
statementN1 ... break default
statementD ...
13
Flow of Control Control Structures in C/C
(contd)

• The switch Statement (contd)
• Uses 4 keywords (contd)
• break
• identifies end of each case forces immediate
  exit from switch statement
• once entry point established, no further case
  evaluations done all statements within compound
  statement executed unless break statement is
  encountered
• if omitted, all statements in compound statement
  following entry point (including default case)
  are executed
• (incidentally) also used to force similar
  immediate exit in while, for do-while statements

switch (expression) case value1
statement11 ... break case
value2 statement21 ...
break ... case valueN
statementN1 ... break default
statementD ...
14
Flow of Control Control Structures in C/C
(contd)

• The switch Statement (contd)
• Uses 4 keywords (contd)
• default
• identifies default action when no match between
  case value switch expression value exists
• optional (if switch expression value does not
  match any case value, no statement is executed
  unless default is encountered)
• invariably placed at bottom of switch statement
  (by virtue of its purpose) break statement not
  needed for last switch case

switch (expression) case value1
statement11 ... break case
value2 statement21 ...
break ... case valueN
statementN1 ... break default
statementD ...
15
Flow of Control Control Structures in C/C
(contd)

• // Switch statement example
• include ltiostream.hgt
• int main(void)
• char choice
• cout ltlt "Type a character then press enter "
  ltlt endl
• switch ( cin gtgt choice )
• case 'r' case 'R'
• cout ltlt "RED" ltlt endl break
• case 'g' case 'G'
• cout ltlt "GREEN" ltlt endl break
• case 'b' case 'B'
• cout ltlt "BLUE" ltlt endl break
• default
• cout ltlt "NON-PRIMARY" ltlt endl
• return(0)

16
Flow of Control Control Structures in C/C
(contd)

• / How does this program (using nested if-else)
• compare with the preceding one? /
• include ltiostream.hgt
• int main(void)
• int choice
• cout ltlt "Type a character then press enter "
  ltlt endl
• cin gtgt choice
• if (choice 'r' choice 'R')
• cout ltlt "RED" ltlt endl
• else if (choice 'g' choice 'G')
• cout ltlt "GREEN" ltlt endl
• else if (choice 'b' choice 'B')
• cout ltlt "BLUE" ltlt endl
• else
• cout ltlt "NON-PRIMARY" ltlt endl
• return(0)