Chapter 4 Control Structure: Loop

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Chapter 4 Control Structure Loop
Prof. Kuanquan Wang The School of Computer
Science and Technology Harbin Institute of
technology

• Knowledge
• Understand the various concepts of loop control
  structure
• Skill
• Be able to develop a program involving loop
  control structure

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Outline

• 4.1 Review and introduction to loop flow
  control structure
• 4.2 Loop control with counter
• 4.3 Loop control with condition test
• 4.4 Loop control with sentinel

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4.1 Review and introduction to loop flow control
structure
How many basic flow control structure?
There are 3 kinds of basic flow control structure.
Sequence
Selection
Repetition or loop
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How Loops are Controlled?
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4.2 Counter Controlled Loop
counter ? initial Value
false
test counter value
true
Step x
Update counter
Step n
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Counter Controlled Loop
counter initialValue
counter ? initial Value
false
test counter value
true
Step x
Update counter
Step n
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Example

• Draw a flowchart for the
• following problem
• Read 5 integer and display the
• value of their summation.

Input 5 integer n1, n2, n3, n4, n5 Output
The summation of n1, n2, .., n5 Input
example 2 3 4 5 6 Output example 20
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Assume input example 2 3 4 5 6
start
Input n1
Input n2
This flowchart does not use loop, hence we need
to use 6 different variables
Input n3
input n4
input n5
sum ? n1n2n3n4n5
output sum
end
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Counter-Controlled Loop
Assume input example 2 3 4 5 6
1
2
3
4
5
6
counter ? 1, sum ? 0
counter
0
2
5
9
14
20
sum
0 2
2 3
5 4
9 5
14 6
counter lt 6
false
1 lt 6 true
2 lt 6 true
3 lt 6 true
4 lt 6 true
5 lt 6 true
6 lt 6 false
true
input n
2
3
4
5
6
n
sum ? sum n
This loop is counter-controlled
The counter Increases by 1
Uses only 3 variables
counter
output sum
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Decreasing Counter-Controlled Loop
counter ? 5, sum ? 0
false
counter gt 0
true
input x
sum?sum x
counter--
output sum
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Loop for

• Condition is tested first
• Loop is controlled by a counter
• Syntaxes
• for (initial value condition update counter)
• statement
• Or
• for (initial value condition update counter)
  
• statement
• statement

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i ? 0, sum ? 0
false
i lt 5
int x, sum, i sum 0 for (i 0 i lt 5 i)
scanf(d,x) sum sum x
true
input x
sum?sum x
i
printf(d,sum)
output sum
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for statement
???

• Example
• for ( num 1 num lt 3 num )
• printf(d\t, num)

num
1 _
printf(have come to exit\n)
_
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for statement
1

• Example
• for (num 1 num lt 3 num )
• printf(d\t, num)

num
printf(have come to exit\n)
_
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for statement
1
Example for (num 1 num lt 3 num )
printf(d\t, num)
num
printf(have come to exit\n)
_
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for statement
1

• Example
• for (num 1 num lt 3 num )
• printf(d\t, num)

num
printf(have come to exit\n)
1 _
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for statement
2

• Example
• for (num 1 num lt 3 num )
• printf(d\t, num)

num
printf(have come to exit\n)
1 _
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for statement
2

• Example
• for (num 1 num lt 3 num )
• printf(d\t, num)

num
printf(have come to exit\n)
1 _
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for statement
2

• Example
• for (num 1 num lt 3 num )
• printf(d\t, num)

num
printf(have come to exit\n)
1 2 _
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for statement
3

• Example
• for (num 1 num lt 3 num )
• printf(d\t, num)

num
printf(have come to exit\n)
1 2 _
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for statement
3

• Example
• for (num 1 num lt 3 num )
• printf(d\t, num)

num
printf(have come to exit\n)
1 2 _
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for statement
3

• Example
• for (num 1 num lt 3 num )
• printf(d\t, num)

num
printf(have come to exit\n)
1 2 3 _
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for statement
4

• Example
• for (num 1 num lt 3 num )
• printf(d\t, num)

num
printf(have come to exit\n)
1 2 3 _
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for statement
4

• Example
• for (num 1 num lt 3 num )
• printf(d\t, num)
• printf(have come to exit\n)

num
1 2 3 _
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for statement
4

• Example
• for (num 1 num lt 3 num )
• printf(d\t, num)
• printf(have come to exit\n)

num
1 2 3 have come to exit_
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Loop Structure
4.3 Loop control with condition test
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Testing Condition First
false
true
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Testing Condition First
Step a
condition
Step x
Step y
Step n
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Loop while

• Condition is tested first
• Loop is controlled by condition or a counter
• Syntax
• while (condition)
• statement
• Or
• while (condition)
• statement
• statement

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Testing condition later
Step a
Step x
Step y
true
condition
false
Step n
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Testing condition later
Step a
Step x
Step y
true
condition
false
Step n
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Do-while Loop

• Statements in the loop are executed first (at
  least once), and condition is tested last
• Loop is controlled by a condition or counter
• Syntax
• do
• statement
• statement
• while (condition)
• statement

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• Example Draw a flowchart for this problem
• Given an exam marks as input, display the
  appropriate message based on the rules below
• If marks is greater than 49, display PASS,
  otherwise display FAIL
• However, for input outside the 0-100 range,
  display WRONG INPUT and prompt the user to
  input again until a valid input is entered

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Condition-Controlled Loop
Assume m110
Assume m5
Assume m57
110
5
57
m
input m
WRONG INPUT
true
mlt0 mgt100
110 lt 0 110 gt100
5 lt 0 5 gt100
57 lt 0 57 gt100
false
true
Condition-controlled loop with its condition
being tested at the end
mgt49
5 gt 49
57 gt 49
PASS
false
FAIL
WRONG INPUT
FAIL
PASS
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input m
false
mlt0 mgt100
true
WRONG INPUT
input m
Condition-controlled loop with its condition
being tested first
true
mgt49
PASS
false
FAIL
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int marks scanf(d,marks) while (markslt0)
(marksgt100) printf(WRONG INPUT) scanf(d
,marks) if (marksgt49) printf(PASS) else
printf(FAIL)
Double Selection
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do-while statement
???
???
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start
end

• Example
• printf(Input start and end value )
• scanf(d d, start, end)
• do
• printf(c (d)\n, start, start)
• start
• while (start lt end)

66 lt 67
67 lt 67
68 lt 67
_
Input start and end value _
Input start and end value 65 67_
Input start and end value 65 67 A (65) _
Input start and end value 65 67 A (65) B (66) _
Input start and end value 65 67 A (65) B (66) C
(67) _
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4.4 Sentinel-Controlled Loop

• Draw a flowchart for a problem which
• Receive a number of positive integers and display
  the summation and average of these integers.
• A negative or zero input indicate the end of
  input process

Input A set of integers ending with a negative
integer or a zero
Output Summation and Average of these integers
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• Input Example
• 30 16 42 -9
• Output Example
• Sum 88
• Average 29.33

Sentinel Value
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What will happen if this statement is deleted???
?
Try to understand
sum?0
input x
What happened if these 2 statements exchange
places
false
xgt0
true
sum?sumx
input x
?
input x
sum?sumx
display sum
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Exercise

• Given a set of integers with the last one being
  999
• Display the summation of all the integers.
• Input example
• 1 3 23 999
• Output example
• Sum 27

Draw the flowchart for this problem
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Sentinel-controlled loop
include ltstdio.hgt void main() int sum, x
sum 0 scanf(d, x) while (x !
999) sum sum x scanf(d,
x) printf(The sum d\n, sum)
true
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int sum, x sum 0 scanf(d,
x) while (x ! 999) sum sum x
scanf(d, x) printf(\nThe sum
d\n, sum)
1
3
23
999
1 ! 999
3 ! 999
23 ! 999
999 ! 999
0
01
1
13
4
423
27
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1
1 3
1 3 23
1 3 23 999
The sum 27
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• The concept of Loop Structure
• 3 methods to control loop structure
• Counter Controlled, Condition Controlled,
  Sentinel Controlled
• 3 statements in C language
• for, while, do while

Thank you for your attention! Now let us have a
break!
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