1. REPETITION CONTROL STRUCTURE
CSC 128: FUNDAMENTALS OF COMPUTER PROBLEM SOLVING
REPETITION CONTROL STRUCTURE

2. Contents
Introduction 1
for loop 2
while loop 3
do … while loop 4

3. Introduction
It is used when a statement or a block of statements need to be executed several times.
Programmers use the repetition structures, referred to more simply as a loop, when they need the computer to repeatedly process one or more program instructions until some condition is met, at which time the repetition structures end.
Repetition is also known as iteration or loop.

4. Introduction Two types of loop: Pretest loop Posttest loop
Evaluation occurs before the instructions within the loop are processed
Instruction may never be processed
while statement, for statement
Posttest loop
Evaluation occurs after the instructions within the loop are processed
Instructions will be processed at least once
do..while statement

5. Introduction – example
Let say, you want to display “ I love C++” 5 times.
I love C++
void main() {
cout << “ I love c++!\n”; }

6. Introduction – example
The idea of using a loop…
Pseudocode
Flow Chart
Begin
Repeat
output “I love C++”
End
I love C++
QUESTION:
HOW DO WE STOP THE LOOP???

7. Introduction – example
Adding a loop control variable
Pseudocode
Flow Chart
Begin
counter = 1
Repeat (if counter <= 5)
output “I love C++”
counter ++
End
counter = 1
counter <= 5 T
Variable counter is LCV F
I love C++
counter ++

8. Introduction – loop control variable
A LCV controls the number of times the statement or the block of statements is being executed.
Flow Chart
Pseudocode
Counter = 1
Begin
counter = 1
Repeat (if counter <= 5)
output “I love C++”
counter ++
End
Counter <= 5 T F
I love C++
Counter ++

9. Introduction Requirement of a repetition structure Flow Chart
Loop body
Initialize
LCV
Counter = 1
Counter <= 5 T F
I love C++
Evaluate
LCV (loop condition)
Counter ++
Update
LCV

10. Introduction Task associated with loop: sum = sum + variable
Counter – to determine number of items.
Is done by adding a constant, such as 1 or 2, to the value of a variable.
Accumulator – to find totals.
Is done by adding a variable to another variable.
counter = counter + 1
sum = sum + variable

11. Introduction Indicator – value use to end the loop Indicators
Sentinel control
Counter control
While(counter<=5)
sentinel=999;
While(number!=sentinel)
Counter control
sentinel control

12. The for loop

13. for loop Also called as a counted or indexed for loop
The general form of the for statement is:
The initial statement, loop condition, and update statement are called for loop control statements
Items in square brackets ([ ]) are optional.
for ([initial statement]; loop condition; [update statement])
statement;

14. for loop The for loop executes as follows:
for ([initial statement]; loop condition; [update statement])
statement;
The for loop executes as follows:
1. The initial statement executes.
2. The loop condition is evaluated. If the loop condition evaluates to true
i. Execute the for loop statement.
ii. Execute the update statement (the third expression in the parentheses).
Repeat Step 2 until the loop condition evaluates to false.
The initial statement usually initializes a variable.
In C++, for is a reserved word.

15. for loop – Example 1 Example : Displaying the numbers 1 through 3
initialization
condition
update
for (int count = 1; count <= 3; count = count + 1)
cout << count << endl;
Result: 1 2 3

16. for loop – Example 1 Using for loop to display ‘Welcome to C++’.
Pseudocode:
Start
For( set i to 1; i less than or equal to 3; add 1 to i)
display “welcome to C++”
Endfor
End

17. Display “welcome to C++”
for loop – Example 1
Start
Flowchart
i = 0
i <= 10 F
End T
Display “welcome to C++”
i ++

18. for loop – Example 2
Example: to create a program to display backward the first 10 non negative number.

19. for loop – Exercises
Exercise 1: create a program that display the first 10 positive odd integers.

20. for loop – Exercises
Exercise 1 - answer

21. for loop – Exercises
Exercise 2: how many time the following loop processed?
Answer:
for (int count = 6; count < 6; count = count + 1)
cout << count << endl;

22. for loop – Exercises
Exercise 3: how many time the following loop processed?
Answer:
for (int count = 4; count <= 10; count = count + 2)
cout << count << endl;

23. for loop – Example 3
Example: to calculate and display total of 3 numbers

24. for loop – Example 3 Pseudocode: Start Initialize total = 0
For(set counter to 1; counter less than or equal to 3; add 1 to counter)
input number
total = total + number
Endfor
Display total
End

25. for loop – Example 3 Flowchart Start counter=1, total = 0, F
for counter <= 3 F
Output total T
End
Input number
total = total + number
counter = counter + 1

26. for loop – Example 3 C++ program segment total = 0;
for (int count = 1; count <= 3; count = count + 1) {
cin>>number;
total = total + number; }
cout << “total:” <<total<<endl;

27. for loop – Exercises
Exercise 4: Suppose j, sum, and num are int variables, and the input values are 26, 34, 61, 4, and -1. What is the output of the code below?
cout << "Enter a number : ";
cin >> num;
for (j = 1; j <= 4; j++) {
sum = sum + num; }
cout << sum << endl;

28. for loop – Exercises
Exercise 4: answer

29. for loop
A semicolon at the end of the for statement (just before the body of the loop) is a semantic error. In this case, the action of the for loop is empty.
In the for statement, if the loop condition is omitted, it is assumed to be true.
In a for statement, you can omit all three statements—initial statement, loop condition, and update statement. The following is a legal for loop:

30. for loop
for (;;)
cout << "Hello" << endl;
This is an infinite loop, continuously printing the word Hello

31. The while loop

32. while loop Repeat or loop as long as the condition is true.
The general form of the while statement is:
while is a reserved word.
Statement can be simple or compound
while (expression) {
statement; }

33. while loop
while (expression) {
statement; }
Expression acts as a decision maker and is usually a logical expression
Statement is called the body of the loop
The parentheses are part of the syntax

34. while loop Expression provides an entry condition
while (expression) {
statement; }
Expression provides an entry condition
statement executes if the expression initially evaluates to true
loop condition is then reevaluated
statement continues to execute until the expression is no longer true

35. while loop Infinite loop continues to execute endlessly
can be avoided by including statements in the loop body that assure exit condition will eventually be false

36. while loop
The general form of while loop flowchart:

37. while loop
Example: to display the first five positive integers which increment by five.

38. while loop Pseudocode: Begin Initialize i to 0
While i is less than or equal to 20
Display i
add 5 to I (update)
End while
End

39. Go back and reevaluate the expression
while loop
Start
i = 0
Enter the
while statement
Expression evaluates
to zero
i <= 20
End
False condition
Expression evaluates to a nonzero number
True condition
Loop
Display i
i = i + 5
Go back and reevaluate the expression

40. while loop Example C++ program segment: i = 0; while ( i <= 20) {
2) A starting point / Initialization of the LCV
1) Loop Control
Variable (LCV)
i = 0;
while ( i <= 20) {
cout << i << “ “;
i = i + 5; }
3) Testing the loop repetition
condition
4) Updating the LCV

41. while loop Various form of while loops: Counter controlled
Sentinel controlled
Flag controlled

42. while loop – counter control
If you know exactly how many pieces of data need to be read, the while loop becomes a counter-controlled loop.
General syntax:

43. while loop – counter control
Counter controlled while loop includes the following:
Counter
A numeric variable used for counting something
Accumulator
Numeric variable used for accumulating something
Initializing
Assign a beginning value to the counter or accumulator; typically 0
Updating
Also called incrementing, means adding a number to the values stored in the counter or accumulator

44. while loop – counter control
Example
#include <iostream>
#include <conio>
int main() {
int count;
count = 1;
while (count <= 10)
cout << count << endl;
count++; }
getch();
return 0;
initializing
updating

45. while loop – counter control
Problem: Create a program that displays the word ‘Hello’ on the screen 10 times.
Solution:
Psedocode
Begin
Initialize lcv to 0
While lcv is less than 10
Display “Hello”
Update lcv
End while
End

46. while loop – counter control
Flowchart
Begin
Initialize
counter = 0
counter < 10
Hello
update
counter T F
End

47. while loop – counter control
Program and output
int main() {
int count;
count = 0;
while (count < 10)
cout << "Hello" << endl;
count++; }
getch();
return 0;

48. while loop – counter control
Exercise: Write a C++ statement associated to the following flowchart.
Begin
Initialize
counter = 10
counter < 100
Display counter
Multiplied by 2
Add 10 to
counter T F
End

49. while loop – counter control
Program and output
int main() {
int count;
count = 10;
while (count < 100)
cout << count * 2 << endl;
count += 10; }
getch();
return 0;

50. while loop – sentinel control
A sentinel-controlled while loop uses a special value called sentinel to control the loop.
Sentinel value is a special value that indicates the end of a set of data or of a process
Sentinel variable is tested in the condition and loop ends when sentinel is encountered

51. while loop – sentinel control
General syntax :

52. while loop – sentinel control
Example
#include <iostream>
#include <conio>
int main() {
char answer;
cout << "Do you want to quit (Y - yes, N - no) : ";
cin >> answer;
while (answer != 'Y')
cout << "Welcome to the program." << endl;
cout << "Do you want to quit (Y - Yes, N - No) : "; }
cout << "Bye.";
getch();
return 0;
Sentinel value

53. while loop – sentinel control
Output screen

54. while loop – sentinel control
Exercise: to create a program that process the loop as long as user enter an even number

55. while loop – sentinel control
Solution
Flowchart
Begin
End
number % 2 == 0
Get another
number F
Prompt for
a number
Get T

56. while loop – sentinel control
Program
int main() {
int number;
cout << "Enter a number : ";
cin >> number;
while (number % 2 == 0)
cout << "Enter the next number : "; }
cout <<"You have entered an odd number to terminate”
<<“the program.";
getch();
return 0;

57. while loop – sentinel control
Output

58. while loop – flag control
A flag-controlled while loop uses a bool variable to control the loop
The flag-controlled while loop takes the form:

59. while loop – flag control
Example
void main() {
bool found = false;
char continue;
while (!found)
cout << " Program continued..still want to continue"
<< " the loop? Press Y for yes, N for No"<< endl;
cin>>continue;
if(continue == ‘Y’)
found = false;
else
found = true; }
cout << "Program terminated";
getch();

60. The do … while loop

61. do … while loop The general form of a do...while statement is:
The statement executes first, and then the expression is evaluated.
If the expression evaluates to true, the statement executes again
As long as the expression in a do...while statement is true, the statement executes do
statement
while (expression);

62. do … while loop
General form of flowchart:

63. do … while loop
To avoid an infinite loop, the loop body must contain a statement that makes the expression false
The statement can be simple or compound. If compound, it must be in braces
do...while loop has an exit condition and always iterates at least once (unlike for and while)

64. do … while loop
Example: to display the first five positive integers which increment by five.

65. do … while loop Pseudocode: Begin Initialize i to 0 do Display i
add 5 to I (update)
While i is less than or equal to 20
End

66. Go back and reevaluate the expression
do … while loop
Start
Flowchart
i = 0
Enter
do statement
Expression evaluates
to zero
Display i
End
False condition
True condition
Expression evaluates to a nonzero number
Loop
i = i + 5
i <= 20
Go back and reevaluate the expression

67. do … while loop C++ program segment i = 0; do {
cout << i << “ “;
i = i + 5; }
while ( i <= 20)

68. do … while loop Exercise 1 a. The while loop produces nothing.
b. The do..while loop outputs the number 11 and also
changes the value of i to 16.

69. do … while loop
Exercise 2 – determine the output of the following program
int x = 20; do {
cout << x << endl;
x = x – 4; }
while (x > 10)

70. do … while loop
Exercise 3 – determine the output of the following program
int x = 1; do {
cout << x << endl;
x = x + 1; }
while (x < 5)

71. do … while loop
Exercise 4 – determine the output of the following program
int total = 1; do {
cout << total << endl;
total = total + 2; }
while (total >=3)

72. do … while loop
Answer: INFINITE LOOP!

73. Nested Control Structures

74. Nested loop
In many situations, it is very convenient to have a loop contained within another loop.
Such loops are called nested loops.
For each single trip, through the outer loop, the inner loop runs through its entire sequence.
Each time counter i increases by 1, the inner loop executes completely.

75. Nested loop Example of C++ program segment for (i = 0; i <= 5; i++){
cout << "\n i is now " << i << endl;
for (j = 1; j <= 4; j++)
cout << " j = " << j ; }

76. Nested loop How it works… i is now 0 Outer loop
j = 1 j = 2 j = 3 j = 4
i is now 1
i is now 2
i is now 3
i is now 4
i is now 5
Inner loop

77. Nested loop ** *** **** *****
Suppose we want to create the following pattern.
In the first line, we want to print one star, in the second line two stars and so on. * **
***
****
*****

78. Nested loop
Since five lines are to be printed, we start with the following for statement.
for (i = 1; i <= 5 ; i++)
The value of i in the first iteration is 1, in the second iteration it is 2, and so on
Can use the value of i as limit condition in another for loop nested within this loop to control the number of starts in a line.

79. Nested loop The syntax for (i = 1; i <= 5 ; i++) {
for (j = 1; j <= i; j++)
cout << "*";
cout << endl; }

80. Nested loop
What pattern does the code produce if we replace the first for statement with the following?
for (i = 5; i >= 1; i--)
Answer:
*****
****
*** ** *

81. The jump statements

82. break Causes an exit from loop or switch statement void main() {
int x;
for (x = 1; x<=10; x++)
if (x == 5)
break;
cout<< x <<“”; }
cout<< “ loop terminated at x:”<<x<<endl;
loop terminated at x: 5
Press any key to continue

83. continue
Skips the remaining statements in the loop and proceed with the next loop
void main() {
int x;
for (x = 1; x<=10; x++)
if (x == 5)
a=x;
continue; }
cout<< x <<“”;
cout<< “ the number”<<a<<endl;
cout<< “ is not printed”<<endl;
the number 5 is not printed
Press any key to continue

84. Thank You !