Chapter 4 Control Structure: Loop Knowledge: Understand the various concepts of loop control structure Skill: Be able to develop a program involving loop

Chapter 4 Control Structure: Loop

Knowledge:Understand the various concepts of loop control structure

Skill:Be able to develop a program involving loop control structure

1

Prof. Kuanquan Wang

The School of Computer Science and Technology

Harbin Institute of technology

Outline

4.1 Review and introduction to loop flow control structure

4.2 Loop control with counter4.3 Loop control with condition test4.4 Loop control with sentinel

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

C Programming LanguageC Programming Language 44

How Loops are Controlled?How Loops are Controlled?

Sentinel ControlledCounter Controlled

•1, 2, 3, 4, …•…, 4, 3, 2, 1

Condition Controlled


4.2 Counter Controlled Loop4.2 Counter Controlled Loop

counter ← initial Value

test counter value

Step n

Step x

false

true

Update counter


Counter Controlled LoopCounter Controlled Loop

counter = initialValue

test counter value

Step n

Step x

false

true

Update counter

counter ← initial Value


Example:Example:

Can you identify the input and output???

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


Input n1

Input n2

Input n3

input n4

input n5

output sum

sum ← n1+n2+n3+n4+n5

start

2n1

Assume input example: 2 3 4 5 6

3n2

4n3

5n4

6n5

20sum

end

This flowchart does not use loop, hence

we need to use 6 different variables


Counter-Counter-Controlled Controlled LoopLoop

counter ← 1, sum ← 0

counter < 6

sum ← sum + n

false

true

counter++

output sum

input n

1counter

sum 0

1 < 6 true

2n

0 + 22

2

2 < 6 true

3

2 + 35

3

3 < 6 true

4

5 + 49

4

4 < 6 true

5

9 + 514

5

5 < 6 true

6

14 + 620

6

6 < 6 false

Assume input example:

2 3 4 5 6

This loop iscounter-controlled

The counter Increases by 1

Uses only 3 variables


Decreasing Counter-Controlled LoopDecreasing Counter-Controlled Loop

counter ← 5, sum ← 0

counter > 0

sum←sum+ x

false

true

counter--

output sum

input x


Loop : Loop : forfor

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;

}


i ← 0, sum ← 0

i < 5

sum←sum+ x

false

true

i++

output sum

input x

int x, sum, i;sum = 0;for (i = 0; i < 5; i++) {

scanf(“%d”,&x);sum = sum + x;

}printf(“%d”,sum);


forfor statement statement

Example:

for ( num = 1; num <= 3; num++ ) printf(“%d\t”, num);

num

???

_

1 _printf(“have come to exit\n”);


Example:

for (num = 1; num <= 3; num++ ) printf(“%d\t”, num);

num

1

_


printf(“have come to exit\n”);


Example:


num

1

_




Example:


num

1

1 _




Example:


num

2

1 _




Example:


num

2

1 _





Example:


num

2

1 2 _



Example:


num

3

1 2 _




Example:


num

3

1 2 _




Example:


num

3

1 2 3 _




Example:


num

4

1 2 3 _




Example:



num

4

1 2 3 _



Example:



num

4

1 2 3 have come to exit_



Loop StructureLoop Structure

Condition is tested first Condition is tested later

4.3 Loop control with condition test


false

true

Testing Condition First

condition

Step x

Step y

Step a

Step n


Testing Condition First

condition

Step x

Step y

Step a

Step n


Loop: whileLoop: while

Condition is tested first Loop is controlled by condition or a counter Syntax

while (condition) statement;

Orwhile (condition) {

statement;statement;

}


Step a

condition

Step n

Step x

false

true

Step y

Testing condition later


Step a

condition

Step n

Step x

false

true

Step y

Testing condition later


Do-while LoopDo-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;


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


Condition-ControlledCondition-Controlled LoopLoop

false

true

input m

m<0 || m>100

m>49 “PASS”

“FAIL”

true

false

“WRONG INPUT”

Assume m=110

m 110

110 < 0 || 110 >100

WRONG INPUT

Assume m=5

5

5 < 0 || 5 >100

5 > 49

FAIL

Assume m=57

57

57 < 0 || 57 >100

57 > 49

PASS

Condition-controlled loop with its condition being tested at the end


false

true

input m

m<0 || m>100

m>49 “PASS”

“FAIL”

true

false

“WRONG INPUT”

input m

Condition-controlled loop with its condition being tested first


int marks;scanf(“%d”,&marks);while (marks<0) | | (marks>100) {

printf(“WRONG INPUT”);scanf(“%d”,&marks);

}if (marks>49) {

printf(“PASS”);else

printf(“FAIL”);}

DoubleSelection


do-while statementdo-while statement

Example :printf(“Input start and end value : “);scanf(“%d %d”, &start, &end);do {

printf(“%c (%d)\n“, start, start);start++;

} while (start <= end) ;

_

???

start

???

end

Input start and end value : _Input start and end value : 65 67_

65 67

Input start and end value : 65 67A (65)_

66

66 <= 67

Input start and end value : 65 67A (65)B (66)_

67 <= 67

67

Input start and end value : 65 67A (65)B (66)C (67)_

68

68 <= 67


4.4 Sentinel-Controlled Loop4.4 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

Can you identify the input and output???

Input: A set of integers ending with a

negative integer or a zero

Output: Summation and Average of these integers


Input Example:

30 16 42 -9

Output Example:

Sum = 88

Average = 29.33

Sentinel Value


Now…What have you

understand?


x>0

sum←sum+x

false

true

input x

sum←0

input x

display sum

Try to understand What will happen if this statement is deleted???

?

What happened if these 2

statements exchange

places

sum←sum+x

input x

?


ExerciseExercise 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


Sentinel-controlled loop

true

x!=999

sum←sum+x

false

input x

sum=0

input x

output sum

#include <stdio.h>

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);

}


int sum, x; sum = 0; scanf(“%d”, &x); while (x != 999) { sum = sum + x; scanf(“%d”, &x); } printf(“\nThe sum : %d\n”, sum);

?

?x

sum 0

1

_1

1 != 999

0+11

1 3

33 != 999

1+34

1 3 23

2323 != 999

4+2327

1 3 23 999

999999 != 999

The sum : 27


Now…Let us have a

summary!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!

Documents

Chapter 4 Control Structure: Loop Knowledge: Understand the various concepts of loop control structure Skill: Be able to develop a program involving loop