Control Structures

1
Control Structures

• In structured programming, we use three basic
  control structures
• Sequence
• Selection
• Repetition
• So far, we have worked with sequential
  algorithms, where each instruction is executed
  once, immediately after the one above it

2
Selection

• We use selection to handle making choices
• A condition that is evaluated to be either true
  or false determines the next action

True
False
Is a gt b?
Print Yes
Print No
IF 1 is greater than 2 THEN Print Yes ELSE
Print No ENDIF
3
Selection

• Needed in most programs (if they do anything
  interesting)
• What are some examples of decisions a program
  might have to make?

4
Selection - Variations

• Simple selection - IF, THEN, ELSE
• Simple selection with null false branch - IF,
  THEN
• Combined selection - AND, OR
• Nested selection
• Case structure

5
Simple Selection

• IF account_balance lt 300 THEN service_charge
  5.00ELSE service_charge 2.00ENDIF

6
Simple Selection in C

• if (condition) statement block
• else statement block
• if (x lt y)
• small x
• else
• small y

7
Truth and Falsehood in C

• The value of false in C is 0.
• Everything else is true.
• if (x-x) printf(This will never be
  printed\n) else printf(This will always
  be printed\n)

8
Relational Operators

• Used to test the relationship between two
  expressions or two variables
• Condition evaluates as
• false (0)
• true (non-zero integer)
• Note that stringing relational operators together
  doesnt work
• 2 lt x lt 7 will be true when x is 8 (Why?)

9
Cs Relational Operators

• Equal uses two equal signs
• gt Greater than
• lt Less than
• gt Greater than or equal to
• lt Less than or equal to
• ! Not equal
• Note no space in two character operators

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Examples
x 500, y 300 if (xy) if (x gt 100) if (y
gt250)
if (x ! 500) if ((x y) gt 700) if ((x / 2) lt
99)
11
Logical Operators

• Used to combine or negate expressions containing
  logical operators.
• used with compound conditions

and or ! not
12
Logical Operators
and -- if ((x 5) (y ! 10)) both
conditions must be true for statement to
evaluate as true or -- if ((x 7) (y gt
10)) either condition can be true for
statement to evaluate is true.
Only evaluates as false if both false not
-- ! if (! (x gt 7)) reverses the value of
expression to its immediate right. same
as if (x lt 7)
13
Logical or Boolean Expression

• Condition that is true (any non 0) or false (0)
• if (n ! 0), same as if (n)
• testing for truth same as testing for non-zero
  value
• You can use a preprocessor directive to give
  meaningful names to 1 and 0
• define TRUE 1
• define FALSE 0
• can write if (n TRUE)

14
Order of Precedence
( ) Parentheses highest , -, !
(not) Unary , /, Multiplication, Division
,Modulus , - Addition, Subtraction lt lt
gt gt Greater than, less than, or equal
! Equal, not equal And Or Assignment
statement lowest Rule Use parentheses to
make evaluation order clear
15
Conditions Practice

• 1. Given a 12, b 6, c 8
• true or false? ((a lt b)(b ! c)
• (!(a lt b) !(b gt c))
• ((a gt b)(a gt c)(b gt
  c))
• 2. What would the values of a, b, and c need to
  be for this statement to be true? (use 1,2,3)
• ((a gtc b gt c) a lt b))

16
Selection - Variations

• Simple selection - IF, THEN, ELSE
• Simple selection with null false branch - IF,
  THEN
• Combined selection - AND, OR
• Nested selection
• Case structure

17
Simple Selection

• IF account_balance lt 300 THEN service_charge
  5.00ELSE service_charge 2.00ENDIF

18
Simple Selection in C

• if (condition) statement block
• else statement block
• if (x lt y)
• small x
• else
• small y

19
Simple Selection with Null False Branch (no ELSE)

• Used when nothing needs to be done if the
  condition is false
• IF student_attendance part_time THEN add 1 to
  part_time_countENDIF

20
In C

• if (condition) statement block
• if (num1 gt num2)
• printf(Its bigger\n)

21
Combined Selection

• IF student_attendance part_time AND
  student_gender female THEN add 1 to
  fem_part_time_countENDIF
• IF (record_code 23 OR update_code delete)
  AND account_balance zero THEN delete customer
  recordENDIF

22
In C

• if (num1 gt num3 num2 gt num3) printf(The
  third is the smallest\n)
• if ((record_code 23 update_code D)
  account_balance 0)
• delete_record(current_record)

23
Nested Selection (Linear)

• IF record_code A THEN increment
  counter_AELSE IF record_code B THEN
  increment counter_B ELSE IF record_code
  C increment counter_C ELSE increment
  error_counter ENDIF ENDIFENDIF

24
Nested Selection (Non-linear)

• IF student_attendance part_time THEN IF
  student_gender female THEN IF student_age
  gt 21 THEN add 1 to mature_fem_pt_students
  ELSE add 1 to young_fem_pt_students
  ENDIF ELSE add 1 to male_pt_students ENDIF
  ELSE add 1 to full_time_studentsENDIF

25
Nested if statements
if (expression) statement if true else
if (expression) statement if true
else statement if false
if (expression) statement if true else if
(expression) statement if true else
statement if false
Rule else matches nearest unmatched if that
precedes it
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Consider this if (num lt 10) if (num gt 3)
printf (d is between 3 and 10, num)
else printf (d is less than 3,
num) if (num lt 10) / braces allow else to go
with first if / if (num gt 3) printf
(d is between 3 and 10, num) else
printf (d is greater than 10, num)
27
AND and Nested IFs

• IF student_attendance part_time THEN IF
  student_age gt 21 THEN increment
  mature_pt_student ENDIFENDIF
• IF student_attendance part_time AND
  student_age gt 21 THEN increment
  mature_pt_studentENDIF

28
Selection Example

• A program is required to read a customers name,
  a purchase amount and a tax code. The tax code
  has been validated and will be one of the
  following 0 tax exempt (0) 1 state sales tax
  only (3) 2 federal and state sales tax
  (5) 3 special sales tax (7)The program must
  then compute the sales tax and the total amount
  due and print the customers name, purchase
  amount, sales tax and total amount due.

29
Group Problem

• Design an algorithm which will prompt an operator
  for a students serial number and the students
  exam score out of 100. Your program is then to
  match the exam score to a letter grade and print
  the grade to the screen. The letter grade is to
  be calculated as follows 90 and
  above A 80-89 B 70-79 C 60-69 D belo
  w 60 F

30
Problem 1

• Write a nested if statement that displays an
  average daily temperature with a label for the
  type of day. If the average is 75 or above, it
  is a warm day. If the average is between 50 and
  74, it is a moderate day. If below 50, it is
  considered a cold day.

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Problem 2

• A company is raising its employees salaries
  based on the following tableemployee
  status years of service percent
  raiseFull-time Less than 5 years 4.0Full-time
  5 or more years 5.0Part-time Less than 5
  years 2.5Part-time 5 or more years 3.0If
  employee_status value is 1, the employee is
  full-time. If it is 2, the employee is
  part-time. Write a single nested if statement
  that computes the new salary of an employee given
  his or her employee_status, years_of_service, and
  salary.

32
Pseudocode Case Structure

• Alternative to some linear nested if statements
• CASE OF some variable value_1 statement
  block_1 value_2 statement block_2 value_n
  statement block_n other statement
  block_otherENDCASE

33
Case Example

• CASE OF record_code A increment
  counter_A B increment counter_B C
  increment counter_C other increment
  error_counterENDCASE

34
C Switch Statement
switch (expression) case
val1 statements break case val2
statements break case val3 statements
break default statements
Note expression must evaluate to an integral
type int, char, short, long val1, val2, and
val3 are integral constant expressions default
is the case executed if none of the other cases
are satisfied. It is not required but is usually
a good idea.
35
break

• The break statement causes an immediate exit from
  the switch
• Without the break, execution falls through from
  each case to the next
• This enables us to specify multiple values that
  should cause the same code to be executed without
  having to repeat the code
• But we must be careful to include the break where
  it belongs
• A break is recommended, though not required, at
  the end of the last case

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Example

• switch (tax_code) case 0 tax_rate
  0.0 break case 1 tax_rate
  0.03 break case 2 tax_rate
  0.05 break case 3 tax_rate
  0.07 break default
  printf(tax_code out of range\n)

37
Switch example
switch (num 10) case 0 printf
(Ones digit is zero\n) break case
1 printf (Ones digit is one\n)
break case 2 printf (Ones digit is
two\n) break default
printf (Ones digit is d\n,num 10)

38
Switch example
switch (CountFrom) case 5 printf
(5\n) case 4 printf (4\n) case 3
printf (3\n) case 2 printf (2\n)
case 1 printf (1\n)
printf (Ignition\n) printf (Blast
off!\n)
39
Selection Example with CASE

• A program is required to read a customers name,
  a purchase amount and a tax code. The tax has
  been validated and will be one of the
  following 0 tax exempt (0) 1 state sales tax
  only (3) 2 federal and state sales tax
  (5) 3 special sales tax (7)The program must
  then compute the sales tax and the total amount
  due and print the customers name, purchase
  amount, sales tax and total amount due.

40
Problem 3

• A video rental shop uses the following type
  classification scheme for movie video cassettes
• Code Movie Category
• A Action and Adventure
• C Comedy
• D Drama
• F Family
• H Horror
• M Musical
• Any other code value is an error and should print
  out a warning message. Write a switch statement
  that tests the value of type_code and increments
  the number of videos for the appropriate category

41
Group Problem 2

• Design a program which will read two numbers and
  an integer code from the screen. The value of
  the code should be 10, 20, 30, or 40. If the
  value of the code is 10 then compute the sum of
  the two numbers. If the code is 20 compute the
  difference (first minus second). If the code is
  30, compute the product of the two numbers. If
  the code is 40 and the second number is not zero,
  then compute the quotient (first number divided
  by second). The program is then to print the two
  numbers, the integer code and the computed result
  to the screen. If problems arise with the data
  (incorrect codes or zero divisors), print
  appropriate error messages.