Control Structures

1
Control Structures

• Remember that in structured programming, we use
  three basic control structures
• Sequence
• Selection
• Repetition
• So far, we have worked with sequence and
  selection
• Were now going to look at repetition

2
Repetition

• The repetition control structure allows us to
  execute a set of instructions more than once
• The number of times the loop is executed is
  determined by a condition (just like the
  conditions in selection statements)
• As long as the condition is true, the set of
  instructions will be executed again

3
Repetition

• Repetition (looping, iteration) performs a
  segment of code as long as condition remains true
  or until a condition is false

?
False
True
exit loop
4
Repetition Terminology

• A repetition control structure is often called a
  loop
• Another term you will hear for repetition is
  iteration
• The set of instructions the loop controls are
  called the body of the loop
• A sentinel value is a special value that
  indicates the end of an input sequence. It does
  not count as one of the input values.

5
Loop Variations

• Three basic types
• Pre-testing condition is checked before body is
  executed (the type we had in Jessica)
• Post-testing body is executed once, then the
  condition is checked to see whether the body
  should be executed again
• Counting specialized loops for handling cases
  where the loop is controlled by the value of a
  counter

6
Pre-testing loop

• Also called a leading decision loop
• May not execute the body at all
• Condition is tested at the beginning of the loop
• Condition must be changed within the loop

total_score 0 Get exam_score DO WHILE
exam_score not 999 print student_name,
exam_score total_score total_score
exam_score get exam_score ENDDO
7
DOWHILE Using a COUNTER
Print_student_records Set temp_count to
0 (initialize) DOWHILE temp_count lt 31
Prompt operator for daily_temp
Get daily_temp Display daily_temp
Add 1 to temp_count (increment) ENDDO
Print All temperatures printed END (How
many temperatures will be printed?)
8
Print_student_records Read student_name,
test_score DOWHILE test_score not 999
Print student_name, test_score
Read student_name, test_score ENDDO
Print All students printedENDNote priming
read before DOWHILE trailer record - exam_score
999
9
Example 1

• Given a bag of plastic chips with numbers on
  them, count the number of chips in the bag
  labeled with the number 2. Assume you cannot
  tell whether there are chips in the bag until you
  try to retrieve one

10
Example 2

• Design an algorithm which will prompt for,
  receive, and total a collection of payroll
  amounts entered at the terminal until a sentinel
  amount of -99 is entered. After the sentinel has
  been entered, display the total payroll amount on
  the screen.

11
Pre-testing Loops in C

• while (condition) statement block
• while (num1 gt 5) total num1 num1--

12
Updating Assignment Operators

• Shortcut versions of assignment operators

a b same as a a b add rvalue to
lvalue a - b same as a a - b subtract
rvalue from lvalue a b same as a a
b multiply a / b same as a a / b divide a
b same as a a b modulus Note if
rvalue is an expression, will evaluate it
first a b c equivalent to a a (b c)
13
Increment and Decrement Operators

• Increment operator
• a way to add 1 to a variable
• these are all the same
• count count 1
• count 1
• count
• count

• Decrement operator
• --
• a way to subtract 1 from a variable
• these are all the same
• count count - 1
• count - 1
• count--
• --count

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Example 2 in C

• Design an algorithm which will prompt for,
  receive, and total a collection of payroll
  amounts entered at the terminal until a sentinel
  amount of -99 is entered. After the sentinel has
  been entered, display the total payroll amount on
  the screen.

15
Practice Problem

• Write a while loop to compute the sum of all
  integers between first and second (including
  first and second), where first and second are
  integers and first lt second.

16
Practice Problem 2

• Write a program that sums a sequence of integers.
  Assume that the first integer read specifies the
  number of values remaining to be entered. Your
  program should read only one value each time
  scanf is executed. A typical input sequence might
  be 5 100 200 300 400 500

17
Practice Problem 3

• Write a program that finds the smallest of
  several integers. Assume that input will end
  when a sentinel value of 999 is read. Do not
  count 999 as one of the integers to consider.

18
Post-testing Loops in Pseudocode

• REPEAT statement blockUNTIL condition p is true
• Always executed at least once
• Stop when condition is true
• Much less common

19
Post-testing Example in Pseudocode
set_total to zero REPEAT Prompt for product
type Read product type Prompt for
product qty Read product qty Compute
product total Add product total to total
Ask if more purchases Get response UNTIL
response is NO
20
Post-testing Loops in C

• do statement block while (condition)
• do printf(Enter a nonnegative integer
  ) scanf(d, num) while (num lt 0)

21
Counted Loops in Pseudocode

• Used when exact number of iterations is known
• DO index initial_value to final_value statement
  blockENDDO
• DO i 1 to 10 Write iENDDO

22
Counter-controlled loops can do all these things
DO temperature_count 1 to 30 Prompt
operator for max_temp Add max_temp to
max_total ENDDO

• Repeat the statement block
• initialize the loop_index to the initial value
• increment the loop_index each pass
• test the value of the loop_index each pass
• terminate the loop when loop_index exceeds final
  value

(How many records will be processed?)
23
Example 1

• Write an algorithm to sum the numbers 1 to 10 and
  display the resulting sum.

24
Counting Loops in C

• for (initialization condition modification)
  statement block
• sum 0for (i 1 i lt 11 i) sum i

• initialization - assignment statement to set loop
  control variable
• condition - determines when loop will exit
• modification - defines how loop control variable
  will change

25
for vs. while loop
These are the same
initialization
i 0 while (i lt 6) print (d, i)
i i 1
for (i 0 i lt 6 i i 1) printf
(d, i)
condition
increment
26
Example 1 in C

• Write a function to sum the numbers 1 to 10 and
  display the resulting sum.

27
Practice Problem 1

• Write a program that sums a sequence of integers.
  Assume that the first integer read specifies the
  number of values remaining to be entered. Your
  program should read only one value each time
  scanf is executed. Use a for loop.

28
Practice Problem 2

• Write nested for loops to print the pattern on
  the right. Each printf should print a single
  asterisk.

29
Practice Problem 3

• Write a for loop that computes the number of
  integer values that can be divided evenly by 11
  or by 13 in the range of start through end, where
  start and end are integer variables.

30
Give the Output
Example 3 int r 10 while (r gt 0)
printf (d , r) r - 2
Example 1 int i 2 while (i lt 10)
printf (d, i)
Example 2 int x 2 do printf (d ,
x) while (x lt 10)
Example 4 for (r 7 r gt 3 r--) printf(d
, r)