Control Structures

1
Chapter 10

• Control Structures

2
Flow of Control

• Sequence
• Selection
• Repetition

3
Selection if construct

• Syntax if (expression)
• statement
• expression is the condition for the if
  construct.
• If expression is evaluated to non-zero (true),
  statement is executed.
• If expression is evaluated to zero (false),
  statement is skipped.

4
if construct

• Syntax if (expression)
• statement

5
Compound-statement Action

• Syntax if (expression)
• compound-statement

6
Flags

• Flag an integer variable that simulates a
  Boolean variable. Contains 0 (false) or 1 (true).

7
Naming a Flag

• Appropriate naming is desirable.
• Example a flag attended implies
• attended if it contains 1 (true)
• did not attend if it contains 0 (false)

8
Nested if statement

• Example

• Above could be rewritten as

9
Short-circuit evaluation

• Evaluation stops as soon as value of expression
  is known. Evaluation is from left to right.
• For logical AND (), if the partial expression
  is false, the whole expression is false.
• For logical OR (), if the partial expression is
  true, the whole expression is true.

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Short-circuit evaluation

• Example

11
Complementing a Condition

• Comlementing or negating a logical expression
  means changing the polarity.
• Examples
• !(a 30) is equivalent to (a ! 30)
• !(a gt b) is equivalent to (a lt b)

12
DeMorgans Theorem

• NOT(a AND b) same as NOT(a) OR NOT (b)
• NOT(a OR b) same as NOT(a) AND NOT(b)
• In C
• !(expr1 expr2) same as !(expr1)
  !(expr2)
• !(expr1 expr2) same as !(expr1)
  !(expr2)

13
DeMorgans Theorem

• Example

14
Common Mistakes

• Do not use and ! on floating-point numbers.
• Mixing up with .
• Wrong placament of semi-colon, resulting in empty
  statement

printf() is outside if construct.
15
Common Mistakes

• Translating condition in English to C

• Let lower be 10, and upper be 30.If x is 20,
  (lower lt x) is true, so it is evaluated to 1.
  Since (1 lt upper) is also true, condition is
  true!

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Common Mistakes

• Wrong condition

• Correct method

which is equivalent to this (since lt has a
higher precedence than )
17
Common Mistakes

• Wrong condition

• Correct method

18
Common Mistakes

• Forgetting the braces for compound statements

• Correct method

19
if-else construct

• Syntax
• if (expression)
• statement1
• else
• statement2

if (expression) compound-statement1
else compound- statement2
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if-else construct

• May be used to avoid redundant code

• Use if-else construct

21
if-else construct

• Another example

22
Style

• Two common styles

if (expression) compound-statement1 else
compound- statement2
if (expression) compound-statement1
else compound- statement2
23
Removing common statements

• Common statements in the then and else parts
  should be moved out of the if construct, if
  appropriate

24
Removing common statements

• After moving common statements out of if
  construct

25
Logical assignment for Flags

• Example

if-else statement may be replaced by an
assignment statement.
26
Logical assignment for Flags

• Another example

27
Nested if-else statements

• Example

28
Nested if-else statements

• Which if is the else associated with?

• else is associated with nearest if.

29
Nested if-else statements

• To override default association, use braces to
  mark out block.

30
Nested if-else statements

• Example

31
Nested if-else statements

• Example

32
Common Mistakes

• Wrong matching of else with if.
• Wrong placement of semi-colon.

33
Conditional Operator (?)

• Ternary operator
• condition ? expr1 expr2
• First operand is condition.
• If condition is true, take value of expr1
  otherwise, take value of expr2.

34
Conditional Operator (?)

• Example

equivalent to
35
Conditional Operator (?)

• Example

equivalent to
36
switch construct

• Multi-way selection statement

37
switch construct

• May only test constant integral expressions,
  i.e., expressions that evaluate to integers or
  characters.
• The vis are integral values the sis are
  compound statements.
• After expression is evaluated, control jumps to
  appropriate case label.
• break statements are inserted to avoid falling
  through.

38
switch construct

• Example

39
Repetition Structure

• Counter-controlled repetiton number of
  iterations known.
• Sentinel-controlled repetiton iterate until a
  sentinel value is entered, or terminating
  condition is true.

40
while construct

• Loop structure with pre-test condition.
• while (expression)
• statement
• expression is loop condition.
• If expression is true, statement in loop body is
  executed, and expression tested again.
• If expression is false, loop terminates.

41
while construct

• Example Print n asterisks.

• count_star is the loop control variable.

42
while construct

• Example Compute sum of first 100 positive
  integers.

43
while construct

• Which of these is/are same as previous code?

44
while construct

• Loop control variable.
• Initialisation before the loop is entered, the
  variable must be initialised.
• Testing condition involving the loop control
  variable is tested before the start of each loop
  iteration if condition is true, loop body is
  executed.
• Updating loop control variable is updated during
  each iteration (usually at the beginning or the
  end of the loop body).

45
Counter-control repetition

• A counter is used to keep track of number of
  iterations.

46
Sentinel-control repetition

• A sentinel is used to denote end of data.

47
do-while construct

• Loop structure with post-test condition.
• do
• statement
• while (expression)
• Loop body is executed at least once.

48
do-while construct

• Examples

49
Flag-controlled loops

• When loop condition is complex, flags may be used.

50
for construct

• Another pre-test loop structure.
• Provides more compact form for counter-controlled
  loops.
• for ( initialisation-expression
• loop-condition
• update-expression )
• statement

51
for construct

• The for construct is similar to this while
  construct.
• initialisation-expression
• while ( loop-condition )
• statement
• update-expression

52
for construct

• Example

53
for construct

• The initialisation-expression and
  update-expression are often comma-separated lists
  of expressions.
• The comma operator evaluates the list from left
  to right.

54
for construct

• Any of the three expressions in the for header
  may be omitted, but the semi-colons must stay.
• If initialisation-expression is omitted, you must
  perform necessary initialisation before loop.

55
for construct

• If update-expression is omitted, you must ensure
  that necessary update operations are done in loop
  body.

56
for construct

• If loop-condition is omitted, then the test is
  always true.
• This loop is infinite

57
Common Mistakes

• Wrong placement of semi-colon.

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Common Mistakes

• Omitting semi-colons in for header.
• Mixing up semi-colons with commas in for
  header.
• Off-by-one error, where the loop executes one
  more or one fewer iteration than intended. How
  many iterations does this loop execute?

59
break statement

• Used in loops, break causes execution to break
  out of the loop that contains the statement.

60
continue statement

• The continue statement causes execution to skip
  remaining loop body and proceed to next iteration.

61
Nested loops combined structures

• An example of nested for loops.

62
Nested loops combined structures

• Example 2

63
Nested loops combined structures

• Example 3

64
Homework

• Try exercises behind chapter 10.