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CHAPTER 5: Decision Making and Looping
CSEB113 PRINCIPLES of PROGRAMMING CSEB134 PROGRAMMING I
byBadariah Solemon
1BS (Sept 2012)
Topics1. Decision making using:
– Simple if selection– Simple if…else selection and using ?: operator– Handling multiple conditions– Nested if…else selection– if…else if selection– switch selection
2. Looping:– Design: Counter-controlled vs Sentinel-controlled– Using while loop– Using do…while loop– Using for loop
3. Data validation in selection and repetition structures4. Combinations of control structures5. Using continue and break statements
2BS (Sept 2012)
DECISION MAKING
Topic 1
BS (Sept 2012) 3
Intro to Decision Making • Re-call the Apples problem:
• In this problem, we have identified two conditions/ constraints:1. The quantity purchased must be more than zero2. The cost per Kg apples must be more than zero
• Because of those conditions, before the C program calculates the total cost of apples purchased, we must make it DECIDE that both conditions have been met.
• We can get our C programs to make decisions of this sort using Selection Structure
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Compute and display the total cost of apples given the number of kilogram (Kg) of apples purchased and the cost per Kg of apples
What is Selection Structure?• Take actions depending on the outcome of a condition. A condition is a
logical expression that is either True or False.• General forms of selection structure (SYNTAX):
BS (Sept 2012) 5
if (condition) then-statement;
else else-statement;
if(condition)then-statement;
(condition)? then-statement : else-statement;
if (condition) then-statement;
else if (condition) elseif-statement;
…else
else-statement;
switch (ControlVariable){ case constant 1:
statement; break;
case constant-n: statement;break;
default: statement;
}
1
2
3 4
SIMPLE if SELECTION
Topic 1-1
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if Selection• Is used when a statement or group of statements is to be executed when
the logical expression of the condition is TRUE.• Syntax:
– The expression of the condition must be specified using Relational and Equality operators as follows:
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if (condition)single then-
statement;
No semicolon at the end of the if statement.
Type Operator Meaning Example expression
Result if x=5 and y=4
Relational > x is greater than y x > y 5 > 4 is TRUE
< x is less than y x < y 5 < 4 is FALSE
>= x is greater than or equal to y x >= y 5 >= 4 is TRUE
<= x is less than or equal to y x <= y 5 <= 4 is FALSE
Equality == x is equal to y x == y 5 == 4 is FALSE
!= x is not equal to y x != y 5 != 4 is TRUE
an expression that can return true or false
Example – Relational Operator
• Case Apples: the algorithm is refined by making the program decide that the following condition is met:– The quantity purchased must be more than zero
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BeginPrompt and get QtyApplePrompt and get Costif QtyApple > 0
Compute: TotalCost = Cost * QtyApple End if Display TotalCostEnd
Prompt and get QtyApple
Compute: TotalCost = Cost * QtyApple
Print TotalCost
Begin
End
Prompt and get Cost
QtyApple > 0? TF
Example – Relational Operator
• Based on the refined algorithm, the C program is as follows:
BS (Sept 2012) 9
if with Compound Statements
• In the example that we have seen so far, there is only one statement to be executed after the if statement.
• To execute more than one statement after the condition is satisfied, we have to put curly braces { } around those statements.
• Syntax:
• Example:
BS (Sept 2012) 10
if (QtyApple > 0){
printf(“Calculating the total cost\n”);Total = Cost * QtyApple;
}
Compute: TotalCost = Cost * QtyApple
QtyApple > 0? TF
Print message
if (condition){
multiple then-statements;}
Example – Equality Operator• This example demonstrates the use of equality operators ==
and !=• Note that == is different that =. Why?
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Prompt and get status
Print “Kids”
status == 1? TF
Print “Adults”
status != 0? TF
ExerciseWhat is the output of this program? the If values entered are:a) 789 and 12b) 44 and 44c) 3 and 9901
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SIMPLE if…else SELECTION
Topic 1-2
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Simple if..else• Is used when a statement or group of statements is
to be executed when the logical expression of the condition is FALSE.
• Syntax:
• Example:
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if (condition)Single then-statement ;
elseSingle else-statement ;
if (QtyApple > 0)Total = Cost * QtyApple;
else printf(“Invalid quantity!\n”);
Compute: TotalCost = Cost
* QtyApple
Print message
QtyApple > 0? TF
No semicolon at the end of the if and else.
if…else with Compound Statements
• put curly braces { } around a group of statements• Syntax: Example:
BS (Sept 2012) 15
if (condition){
Multiple then-statements ;}else{
Multiple else-statements;}
if (QtyApple > 0){
printf(“Calculating the total cost\n”);Total = Cost * QtyApple;
}else{
printf(“Invalid quantity!\n”); Total = 0.0;
}
Compute: TotalCost = Cost * QtyApple
QtyApple > 0? TF
Print message
Total = 0.0
Print message – invalid quantity
Other Variations of Syntax of if…else
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1. With one then-statement and multiple else-statements. Syntax: if (condition)
Single then-statement;else{
Multiple else-statements;}
if (QtyApple > 0)Total = Cost * QtyApple;
else{
printf(“Invalid quantity!\n”); Total = 0.0;
flag = ‘I’;}
Compute: TotalCost = Cost * QtyApple
QtyApple > 0? TF
Total = 0.0
Print message – invalid quantity
flag = ‘I’
Example:
Other Variations of Syntax of if…else
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if (condition){
Multiple then-statements;}else
single else-statement ;
2. With multiple then-statements and one else-statement.
Syntax:
if (QtyApple > 0){
Total = Cost * QtyApple; flag = ‘V’;
}else
printf(“Invalid quantity!\n”);
Example:Compute:
TotalCost = Cost * QtyApple
QtyApple > 0? TF
Print message – invalid quantity
flag = ‘I’
Effect of Omitting { }• This what might happens if the { } are omitted
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int score;
printf(“Enter the score: ”);scanf(“%d”,&score);
if (score >= 60) { printf(“You have done very well\n”);
printf(“I’ll give you a present\n”);}else printf(“You have failed the course\n”); printf(“Sorry no present for you\n”); printf(“Go and study more”);
Enter the score: 75You have done very wellI’ll give you a presentSorry no present for youGo and study more
Test your skill• What happens if we omit the { }?
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int score;
printf(“Enter the score: ”);scanf(“%d”,&score);
if (score >= 60)
printf(“You have done very well\n”);printf(“I’ll give you a present\n”);
else{ printf(“You have failed the course\n”); printf(“Sorry no present for you\n”); printf(“Go and study more”);}
Conditional Operator ( ? : )• Used to simplify an if…else statement.• Syntax:
• The statement above is equivalent to:
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( condition) ? Single then-statement : single else-statement;
if (condition) single then-statement;else
single else-statement;
Example• if…else statement:
• Conditional statement:
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if (total > 60)printf("Passed!!\n");
elseprintf("Failed!!\n");
printf("%s!!\n", total > 60? "Passed" : "Failed");
total > 60 ? printf("Passed\n") : printf(“Failed\n");
Print “Failed”
total > 0? TF
Print “Passed”
OR
Example 2
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#include <stdio.h>void main(void){
char grade;int marks;printf("Enter marks");scanf("%d\n", &marks);
grade = (marks > 60)? 'P': 'F';
printf("\nGrade = %c\n", grade);
}
(marks > 60)? printf("%c",'P'): printf("%c",'F');
HANDLING MULTIPLE CONDITIONS
Topic 1-3
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Multiple Conditions – Apples Case
• Re-call that in the Apples Case problem, we have identified two conditions/ constraints:1. The quantity purchased must be more than zero2. The cost per Kg apples must be more than zero
• So far, we have handled only the first condition in these statements:
• We can get our program to check multiple conditions in one logical expression, using logical operator &&. Example:
BS (Sept 2012) 24
if (QtyApple > 0)Total = Cost * QtyApple;
if (QtyApple > 0 && Cost > 0)Total = Cost * QtyApple;
Logical Operators• To connect two conditions:
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Operator Read as Evaluation Example
&& AND All the conditions must be true for the whole expression to be true.
if (x == 10 && y == 9)
The if condition is true only when value x is 10, AND value of y is 9.
|| OR The truth of one condition is enough to make the whole expression true
if (x == 10 || y == 9)
The if condition is true when either one of x OR y has the right value.
! NOT Reverse the meaning of a condition
if (!(points > 90))
Means if points NOT bigger than 90
Results of a Logical Expression
• Each logical expression with multiple conditions has either True or False value, depending of the value of each condition in it.
• This table lists possible result of a logical expression. Symbols A and B indicate conditions.
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A B A && B A || B !A !B
True True True True False False
True False False True False True
False True False True True False
False False False False True True
Example
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#include<stdio.h>void main ( ){
int x=5, y=0;;
printf(“x=%d y=%d\n\n”, x,y);
if (x>0 && y>=0)printf(“x greater than zero and ” “y greater than or equal to zero\n\n”);
if (x==0 || y==0)printf(“x and y equal to zero\n\n”);
if (!(x==y))printf(“x is not equal to y\n”);
}
x=5 y=0
x greater than zero and y greater than or equal to zero
x and y equal to zero
X is not equal to y
Exercise• What is the output of this program? If input data are:
– f 25 - m 25 - F 25 - M 25 – f 17 - m 17 - F 17 - M 17
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#include<stdio.h>void main ( ){
char gender;int age;float loan;
printf(“Enter your gender (f/F or m/M) and age:”);scanf(“%c %d”, &gender, &age);
if (gender == ‘f’ && age > 20){
car_loan = 200000.00;printf(“Your car loan is %.2f\n”, loan);printf(“Well done.\n”);
}}
Evaluating Multiple Operators in a Logical Expression
• The C rules for evaluating logical expression with multiple mix of operators are parentheses rule, precedence rule and associativity rule.
• For a complete list, refer to Appendix C (Hanly &Koffman)
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Precedence Operation AssociativityHighest (evaluated first)
Lowest (evaluated last)
[ ] ( )!
< > <= >=== !=
&&||? :
LLLLLLR
Example
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#include<stdio.h>void main ( ){
int x=8, b=-3, c=0, x;
if(a) printf(“a=%d, !a=%d\n”, a, !a);if(b) printf(“b=%d, !b=%d\n”, a, !a);
if(c) printf(“Never gets printed\n”);else printf(“c=%d, !c=%d\n”, c, !c);
if (a>b && B>c || a==b) printf(“Answer is TRUE\n”);else printf(“Answer is FALSE\n”);
x = a>b || b>c && a==b;
printf(“x=%d, !x=%d\n”, x, !x);}
Single variable – returns True or False based on its value- 0 : False- 1(non-zero): True
a=8 !a=0b=-3 !a=0c=0 !c=1Answer is FALSEx=1 !x=0
logical expression – returns True or False
and the result can be assigned to an integer variable
NESTED if…else SELECTION
Topic 1-4
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What is Nested if..else?• if and if…else contained in another if…else selection.• However, as if…else statements become nested, programs
become harder to understand.– To improve readability, indent each pair of if…else statement
• Example syntax (*numbers of ‘if’s and the numbers of ‘else’s are not necessarily equal):
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if (outer-condition){ … //if outer is True, execute this block
if (inner1-condition){ inner1 -then-statement; } //if inner1 is True, execute this block
else{ inner1-else-statement n; } //if inner1 is False, execute this block
}else{ … //if outer is False, execute this block
if (inner2-condition){ inner 2-then-statement; } //if inner2 is True, execute this block
else{ inner2-else-statement n; } //if inner2 is False, execute this block
}
Example
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#include<stdio.h>void main ( ){
int day, time;
printf(“Enter day and time: ”);scanf(“%d %d”, &day, &time);
if (day>0 && day<=6){ if (time<=900) printf(“\nSleep\n”); else { if (time <=1900) printf(“\nWork\n”); else printf(“\nRelax\n”);
}}else{ if (time<=1100) printf(“\nSleep\n”); else printf(“\nHave fun\n”); }
}
Enter day and time: 3 1000
Relax
Draw a flowchart for this program
if…else if SELECTION
Topic 1-5
BS (Sept 2012) 34
What is if…else if selection?
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if (score >= 90) printf(“A\n”);
else if (score >= 80) printf(“B\n”);
else if (score >= 70) printf(“C\n”);
else if (score >= 60)
printf(“D\n”);
else
printf(“F\n”);
• One type of nested selection structure• If any one of the condition is already
satisfied, the other conditions will be ignored completely.
score>=90? TF
Print “A”
score>=80?
score>=60?
score>=70?
TF
TF
Print “F”
Print “C”
Print “B”
Print “D”
F T
Re-writing if…else if • if..else if statements can be re-written to multiple single
if statements. But this applies to condition that uses equality operator only. Example:
BS (Sept 2012) 36
…if (number == 1) printf(“One\n”);else if (number == 2) printf(“Two\n”);else if (number == 3) printf(“Three\n”);else printf(“Others\n”);
Enter the score: 2Two
Enter the score: 2Two
…if (number == 1) printf(“One\n”);if (number == 2) printf(“Two\n”);if (number == 3) printf(“Three\n”);if (number < 1 && number > 3) printf(“Others\n”);
Test your skill• What are the outputs of these programs segments? If value of
score entered is 85
• What’s the effect of re-writing the above if..else if statements to multiple if statements?
BS (Sept 2012) 37
if (score >= 90) printf(“A\n”);else if (score >= 80) printf(“B\n”);else if (score >= 70) printf(“C\n”);else if (score >= 60) printf(“D\n”);else printf(“F\n”);
if (score >= 90) printf(“A\n”);if (score >= 80) printf(“B\n”);if (score >= 70) printf(“C\n”);if (score >= 60) printf(“D\n”);if (score < 60) printf(“F\n”);
Exercise• Write a program that prompts the users to enter
the value of Ritcher scale number (n), and print its equivalent effect as a message to the users based on the following table:
BS (Sept 2012) 38
Ritcher scale number (n) Effect n < 5.0 Little or no damage 5.0 <= n < 5.5 Some damage 5.5 <= n < 6.5 Serious damage: walls may crack or
fall 6.5 <= n < 7.5 Disaster: house or building may
collapse n >= 7.5 Catastrophe: most buildings
destroyed
switch SELECTION
Topic 1-6
BS (Sept 2012) 39
switch Statement• A switch statement is used to choose one choice
from multiple cases and one default case.• Syntax:
BS (Sept 2012) 40
switch (ControlVariable) { case constant 1: statement;
break; case constant-n: statement;
break; default: statement;}
The break statement is needed so that once a case has been executed, it will skip all the other cases and go outside the switch statement.
The default clause is executed if the cases are not met.
Rules for switch Statement1. The value for ‘case’ must be integer or character only.
– Examples:
2. The value checked in each ‘case’ must be constant only and not values in range.
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switch (color) {
case ‘R’ : colorstr = ‘r’; break;
}
switch (number) {
case 1 :num += 2;break;
}
The value for each case is followed by colon (:).
if (number==1) num += 2;
if (color==‘R’) colorstr = ‘r’;
case 1 : √ case >= 1 : X
Example
#include <stdio.h>void main() {
int num;printf(“Enter number:");scanf("%d", &num);
switch (num) {
case 1: printf("Bad (D)\n");break;case 2: printf("Good (C)\n");break;case 3: printf("Excellent (A)\
n");break;default: printf(“False grade\n");
}}
BS (Sept 2012) 42
• The logic of this switch selection is similar to if…else if
Enter number: 3Excellent (A)
num==1? TF
Print “Bad (D)”
num==3?
num==2? TF
Print “False grade”
Print “Good (C)”
Print “Excellent (A)”
F T
Omitting break Statement
BS (Sept 2012) 43
#include <stdio.h>void main(){
int majorCode;
printf("Enter your majoring code: ");scanf("%d", &majorCode);switch(majorCode){
case 1 : case 3 :case 5 : printf(“\nScience Student\n”);
break;case 2 :case 4 : printf(“\nArt Student\n”);
} /* end_switch */}
If the break statement is omitted, the execution will be carried out to the next alternatives until the next break statement is found.
Enter your majoring code: 3
Science Student
Example• Observe the equivalent
logical expression. What can you conclude from these two program segments?
BS (Sept 2012) 44
char grade;
printf(“Enter the grade you scored:”);scanf(“%c”,&grade);
switch (grade) { case ‘a’:
case ‘A’:printf(“Excellent!!\n”);printf(“You brilliant..\
n”); break; case ‘b’:
case ‘B’:printf(“Job well done!!\
n”);printf(“You deserve it..\
n”); break;
default: printf(“undefined grade\n”);}
if (grade == ‘a’ || grade == ‘A’){ printf(“Excellent!!\n”); printf(“You brilliant..\n”);}else if (grade == ‘b’ || grade == ‘B’){ printf(“Job well done!!\n”); printf(“You deserve it..\n”);}else printf(“undefined grade\n”);
Exercise1. Using switch statement, write a program
that reads a positive integer number between 1 to 5, and prints the word equivalent to it.
For example, if the user enters 5, the program should print the word “Five” to the screen.
BS (Sept 2012) 45
LOOPING
Topic 2
BS (Sept 2012) 46
Intro to Looping
BS (Sept 2012) 47
• Re-call the Apples problem:
• Before this, we’ve learned to get our program to make decisions using Selection Structure.
• With the program, when a cashier keys in number of kilogram of apples purchased and the cost per Kg of apples the computer will output the total cost of apples purchased for one customer. However, to calculate the total cost for 100 customers, the cashier would need to execute this program 100 times!
• So, the methods by which it is possible to repeat processing without executing the program or writing the same statements over and over is called Looping or Iteration using Repetition Structure.
Compute and display the total cost of apples given the number of kilogram (Kg) of apples purchased and the cost per Kg of apples
What is Repetition Structure?
• Used to repeat a block of statements a number of times (loop) until a certain condition is met without having to write the same statements multiple times.
• Two design of loops:
BS (Sept 2012) 48
• To execute a number of instructions from the program for a finite, pre-determined number of time
• Loop depends of arithmetic or conditional expression.
Counter-controlled
• To execute a number of instructions from the program indifinitely until the user tells it to stop or a special condition is met
• Loop depends on a sentinel value.
Sentinel-controlled
Types of Looping
BS (Sept 2012) 49
while (condition) LoopBody-statement;
do LoopBody-statement;
while (condition);
for (InitializationExp; Condition; UpdateExp)LoopBody-statement;
1
while
2
do…while
3
for
Exp = Expression
NO semicolon (;)
while LOOP
Topic 2-1
BS (Sept 2012) 50
while Statement• Syntax:
• Enclose a group of loop body statements within the braces { }
• As long as the condition is met (returns true), the statement inside the while loop will always get executed.
• When the condition is no longer met (returns false), the program will continue on with the next instruction (the one after the while loop).
BS (Sept 2012) 51
while (condition){
multiple loopBody-statements;}
while (condition)single LoopBody-statement;
an expression that can return true or false
1. Counter-controlled while Loop
• Used to execute a number of instructions from the program for a finite, pre-determined number of time
• Loop depends of arithmetic or conditional expression.
BS (Sept 2012) 52
...int total = 1;while (total <= 3) { printf(“Total = %d\n”, total); total++;}x++;...
total is the loop controlvariable
In this case, this loop will keep on looping until the counter total variable is 5. Once value of total is 6, the loop will terminate
total <= 3? TF
total++
total=1
Print value of totalx++
Total = 1Total = 2Total = 3
Example
BS (Sept 2012) 53
#include<stdio.h>main ( ){
printf(“Hello World\n”);printf(“Hello World\n”);printf(“Hello World\n”);printf(“Hello World\n”);printf(“Hello World\n”);
}
#include<stdio.h>main (){
int num = 1;
while(num < 6) {
printf("Hello World\n"); num++;}
}
Hello WorldHello WorldHello WorldHello WorldHello World
num < 6? TF
num++
num=1
Print “Hello World”End
Begin
Example 2• You may allow the user to set the number of
iteration as shown in example below :
BS (Sept 2012) 54
#include<stdio.h>void main (void){
int counter=1, n=0; printf(“Number of iteration?: “); scanf(“%d”, &n);
while(counter <= n) {
printf("Hello World\n"); counter++;}
}// in this example the output varies
depending on the value of n entered by the user
counter < n? TF
counter++
counter=1, n=0
Print “Hello World”End
Begin
Prompt and get n
Exercise1. For the following code fragment:
– How many times will the while loop body be executed?– What will be the end values computed for the variables sum and count?
2. Write a program that reads 3 integer numbers and prints the sum of the numbers. Repeat the reading and printing processes 10 times using while loop.
BS (Sept 2012) 55
sum =0;count = 2;while (count <= 5){
sum = sum + count;count = count + 3;
}
2. Sentinel-Controlled while Loop
• Counter control loop is used when we know beforehand how many iteration that the loop should execute.
• There will be cases where we (as the programmer) do not know how many times the loop should be executed, because the decision is up to the users.
• In this case, to terminate the loop, we need to use ‘sentinel controlled loop’ method
• In order to exit from the loop, the user must enter a unique data value, called a sentinel value.
• The sentinel value must be a value that could not normally occur as data.
BS (Sept 2012) 56
2. Sentinel-Controlled while Loop
• The algorithm for sentinel-controlled while loop:
• Consider this problem:– Write a program that reads several integer numbers from the user and
prints the sum of the numbers. The program stops reading numbers from the users when they enter ZERO.
BS (Sept 2012) 57
Read/assign a value to control variableWhile value of the control variable is not sentinel value
process the valueread the next valueend_while
Value != sentinel value?
TF
Get a value
Process value
Get next value
• The sentinel value in this case is ZERO
#include <stdio.h>void main(void){
int num, sum = 0;
printf(“Enter a number [zero to end]: ”);scanf(“%d”,&num);
while (num != 0){
sum += num;printf(“Enter a number [zero to end]: ”);scanf(“%d”,&num);
}printf(“Sum = %d\n”, sum);
}
Example
BS (Sept 2012) 58
Sentinel value ZERO will terminate the loop
Enter a number [zero to end]: 3Enter a number [zero to end]: -6Enter a number [zero to end]: 10Enter a number [zero to end]: 0Sum = 7
num != 0? TF
Prompt and get num
sum += num
Prompt and get num
Print value of sum
Example
BS (Sept 2012) 59
#include <stdio.h>main (){ int sum=0, score=0, count=0; printf("Enter first score or (-99 to quit):"); scanf("%d", &score);
while (score != -99) { count++;
sum += score; printf("Enter next score or (-99 to quit):"); scanf("%d", &score); } printf("Sum of %d scores: %d", count, sum);}
Sentinel value -99 will terminate the loop
score!= -99? TF
Prompt and get score
sum += score
Prompt and get score
Print value of count and sum
count++
Enter first score (or -99 to quit): 80Enter next score (or -99 to quit): 77Enter next score (or -99 to quit): -99Sum of 2 scores: 157
Exercise1. Write a program that calculates and prints the
average of several real numbers. Assume the last value read is the sentinel 9.9. Use a while loop to accomplish the task. Sample input/ouput:
2. Write a program that computes and displays the sum of a collection of Celsius temperatures entered until a sentinel value of -275 is entered.
BS (Sept 2012) 60
10.0 12.3 5.6 21.3 9.9
Average: 8.6
do…while LOOP
Topic 2-2
BS (Sept 2012) 61
1. Counter-controlled do/while Loop
• Syntax:
• the LoopBody-statement inside it will be executed once no matter what.
• Then only the condition will be checked to decide whether the loop should be executed again or just continue with the rest of the program.
BS (Sept 2012) 62
do {
multiple LoopBody-statements;} while (condition); Semicolon here is a MUST
do single LoopBody-statement;
while (condition);
NO semicolon
Example
BS (Sept 2012) 63
#include<stdio.h>main ( ){
printf(“Hello World\n”);printf(“Hello World\n”);printf(“Hello World\n”);printf(“Hello World\n”);printf(“Hello World\n”);
}
#include<stdio.h>main (){
int num = 1;
do {
printf("Hello World\n");num++;
} while(num <=5); }
End
Begin
num <= 5? TF
num++
num=1
Print “Hello World”
Hello WorldHello WorldHello WorldHello WorldHello World
Example 2• You may allow the user to set the number of
iteration as shown in example below :
BS (Sept 2012) 64
#include<stdio.h>void main (void){
int counter=1, n=0; printf(“Number of iteration?: “); scanf(“%d”, &n);
do { printf("Hello World\n"); counter++;} while(counter <= n);
}// in this example the output varies
depending on the value of n entered by the user
Prompt and get n
End
Begin
counter <= n? TF
counter++
num=1
Print “Hello World”
while Loop vs do..while Loop
BS (Sept 2012) 65
int total = 10;
while (total < 10) { printf(“Total = %d\n”, total); total++;}
printf(“Bye”);
int total = 10;
do { printf(“Total = %d\n”, total); total++;} while (total < 10);
printf(“Bye”);
total < 10? TF
total++
total=10
Print value of totalPrint “Bye”
total < 10? TF
total++
total=10
Print value of total
Print “Bye”
while loop
do…while loop
Exercise1. For the following code fragment:
– How many times will the do…while loop body be executed?– What will be the end values computed for the variables sum and count?
2. Re-write the program that reads 3 integer numbers and prints the sum of the numbers, which keeps on repeating the reading and printing processes 10 times using do…while loop.
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sum =0;count = 2;do {
sum = sum + count;count = count + 3;
} while (count <= 5);
2. Sentinel-controlled do/while Loop
• The algorithm for sentinel-controlled do…while loop:
• Example:
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Start doprocess the valueread a value to the control variable While value of the control variable is not sentinel value
int sum=0, score=0, count=0; do {
count++;sum += score;printf("Enter score or (-99 to quit):");scanf("%d", &score); } while (score != -99);
printf("\nSum of %d scores: %d", count, sum);
Enter score (or -99 to quit): -99
Sum of 1 scores: 0
Exercise1. Re-write the program that calculates and prints
the average of several real numbers after the last value read is the sentinel 9.9 by using a do…while loop to accomplish the task. Sample input/output:
2. Re-write the program that computes and displays the sum of a collection of Celsius temperatures entered until a sentinel value of -275 is entered using a do…while loop.
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10.0 12.3 5.6 21.3 9.9
Average: 8.6
for LOOP
Topic 2-3
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• Syntax:
InitializationExp : initialize the loop control variableCondition : determine whether the condition or test expression
returns True or falseUpdateExp : change value of the loop control variable at the end
of each loop
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for Loop
for (InitializationExp; Condition; UpdateExp)single LoopBody-statement;
MUST semicolons
NO semicolon
for (InitializationExp; Condition; UpdateExp){
multiple LoopBody-statements;}
1. Counter-controlled for Loop
• Example:
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int total;for (total = 1; total <= 3; total++) printf(“Total = %d\n”, total);
Control variable
total <= 3? TF
total++
total=1
Print value of total
Total = 1Total = 2Total = 3
Example 2• You may allow the user to set the number of
iteration as shown in example below :
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#include<stdio.h>void main (void){
int counter=1, n=0; printf("Number of iteration?: "); scanf("%d", &n);
for(counter=1; counter<=n; counter+++) printf("Hello World\n");}
// in this example the output varies depending on the value of n entered by the user
counter <= n? TF
counter++
counter=1, n=0
Print “Hello World”End
Begin
Prompt and get n
while Loop vs for Loop
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int total = 1;
while (total <= 3) { printf(“Total = %d\n”, total); total++;}printf(“Bye”);
total <= 3? TF
total++
total=1
Print value of totalPrint “Bye”
int total;
for(total=1; total<=3; total++) { printf(“Total = %d\n”, total);}printf(“Bye”);
total <= 3? TF
total++
total=1
Print value of totalPrint “Bye”
Both produce same output and have similar order of execution!Because using a for loop is just another way of writing a while loop.
Differences between while Loops and for Loops
• Although the two are similar in their order of execution, they are different as follows:
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Item for loop while loop
Initialization expression
Is one of the loop expressions Must be given prior to the loop
Condition (or test expression )
Is one of the loop expressions Is one of the loop expressions
Update expression Is one of the loop expressions Must be in the loop body
When number of iteration is known
Is very convenient Is less convenient
When number of iteration is unknown
Is less convenient Is ver y convenient
Omitting for Loop Expressions
• It is also possible to omit one or more of the for loop expressions.
• HOW? • Example:
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int num=1;
for (;num<=5;num++) { printf("Hello World\n");}
1. Assign initial value to control variable
2. Place semicolon without the expression
2. Sentinel-controlled for Loop
• Example:
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#include <stdio.h>main (){ int sum=0, score; printf("Enter first score (or -99 to quit):");
for ( scanf("%d", &score); score != -99; scanf("%d", &score)) { sum += score; printf("Enter next score (or -99 to quit):"); } printf("Sum of all scores: %d", sum);}
DATA VALIDATION
Topic 3
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Intro to Data Validation• Good programmers would ensure that only valid data are
entered and processed by their programs.• Say for example we want to write a program that reads
the score marks from the user, and print its equivalent grade.
• Say that the valid score marks range is between 0 to100. So, if user keys in value other than 0 to100, the program should do something such as the following:– Option 1: Tell the users that they have entered a wrong
input and terminate the program.– Option 2: Tell the users that they have entered a wrong
input and ask them to reenter the input.
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Data Validation: if…else ifOption 1: Tell the users that they have entered a wrong input
and terminate the program.
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printf(“Enter the score: ”);scanf(“%d”,&score);
if (score >= 90 && score <= 100) printf(“A\n”);
else if (score >= 80 && score < 90) printf(“B\n”);
else if (score >= 70 && score < 80) printf(“C\n”);
else if (score >= 60 && score < 70) printf(“D\n”);
else if (score >= 0 && score < 60 printf(“F\n”);
else printf(“Error, input should only be between 0 –
100 \n”);
Data Validation: while LoopOption 2(1): Tell the users that they have entered a wrong
input and ask them to reenter the input using while loop.
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printf(“Enter score: ”); scanf(“%d”, &score);
while (score < 0 || score > 100) {
printf(“Sorry, input must be between 0 – 100\n”); printf(“Re-enter the score: ”); scanf(“%d”,&score);
}if (score >= 90 && score <= 100) printf(“A\n”);else if (score >= 80 && score < 90) printf(“B\n”);else if (score >= 70 && score < 80) printf(“C\n”);else if (score >= 60 && score < 70) printf(“D\n”);else printf(“F\n”);
Sentinel-controlled while loop with multiple sentinel values in the range of < zero or > 100
Data Validation: do..while Loop
Option 2(2): Tell the users that they have entered a wrong input and ask them to reenter the input using do…while loop.
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do{printf(“Enter score: ”);scanf(“%d”,&score);if (score < 0 || score > 100) printf(“Sorry, input must be between 0 – 100\n”);
}while (score < 0 || score > 100);if (score >= 90 && score <= 100) printf(“A\n”);else if (score >= 80 && score < 90) printf(“B\n”);else if (score >= 70 && score < 80) printf(“C\n”);else if (score >= 60 && score < 70) printf(“D\n”);else printf(“F\n”);
Sentinel-controlled do…while loop with multiple sentinel values in the range of < zero or > 100
Data Validation: for LoopOption 2(3): Tell the users that they have entered a wrong
input and ask them to reenter the input using for loop.
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printf("Enter the score:");
for ( scanf("%d", &score); score < 0 || score > 100; scanf("%d", &score))
{ printf(“Sorry, input must be between 0 – 100\n”); printf("Enter the score:"); }
if (score >= 90 && score <= 100) printf(“A\n”);else if (score >= 80 && score < 90) printf(“B\n”);else if (score >= 70 && score < 80) printf(“C\n”);else if (score >= 60 && score < 70) printf(“D\n”);else printf(“F\n”);
Sentinel-controlled for loop with multiple sentinel values in the range of < zero or > 100
COMBINATIONS OF CONTROL STRUCTURES
Topic 4
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Possible Combinations• Possible combinations are limitless. Four basic forms of
combinations are as follows:1. One or more selection structures inside a repetition structure.2. One or more repetition structures inside a selection structure.3. Nested selection structures – one or more selection structures
inside a selection structure4. Nested repetition structures – one or more repetition
structures inside a repetition structure• This course covers the first three forms only and we have seen
examples of nested selection structures before.
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1. Selection Structures inside a Repetition Structure
• Example: a simple if..else inside a while loop
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x <= 3? TF
x++
x=1
Print “Kids”
Print “Bye”
Prompt and get status
status == 1?
Print “Adults”
TF
2. Repetition Structures inside a Selection Structure
• Example: a while loop inside a simple if..else
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status == 1?
TF
x++
x=1
Print “Kids”
Print “Bye”
Prompt and get status
x <= 3?
Print “Adults”
TF
x=1
Exercise1. Write C programs that calculate and
display the average of 10 floating point numbers read from user by implementing while, do…while and for loops to accomplish the task.
2. Write programs that keep printing the multiples of the integers 2, namely 2, 4, 8, 16, 32, 64 and 128. Your loop should terminate at 128.
Use while, do…while and for loops to implement this.
• Implement appropriate data validation in the programs
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continue AND break STATEMENTS
Topic 5
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The break statement• The continue and break statements are used to
modify the program flow when a selection structure or a repetition structure is used.
• The break statement can be used to forced exit of selection or terminate repetition structure.
• Example:
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for (num=1;num<=5;num++) { if (num==2) break; printf("Hello World\n"); }
When the value of num is equals to 2, the program will terminate from the for loop.
OUTPUT?
The break statement• You can use the break statement at any time. • This can be very useful if you want to stop running a
loop because a condition has been met other than the loop end condition.
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int i; i = 0; while ( i < 20 ) {
i++; if ( i == 10)
break; }
The while loop will run, as long i is smaller then twenty. But, the while loop must stop (break) if i equals to ten
The continue Statement• Can be used to skip the rest of the loop body
statements and continue with the next repetition of the loop (start from the top again - the loop variable must still be incremented).
• Example:
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for (num=1;num<=5;num++) { if (num==2) continue; /* end_if */ printf(“Number %d\n”,num); } /*end_for */
When the value of num is equal to 2, the program will skip the printf statement and continue with the for loop.
OUTPUT?
The continue Statement
• In a for loop, any modification to the control variable will be done before the condition is checked.
• In a while and do…while structures, the loop condition will be checked as soon as the continue statement is encountered to determine whether the loop will be continued .
• Example:
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int i=0;
while ( i < 20 ) { i++; continue; printf("Nothing to see\n"); }
In the example above, the printf function is never called because of the continue statement.
Summary1. We can get our C programs to make decisions using Selection
Structure including if, if…else, ?: operator, if…else if and switch.
2. Also we can get our programs to repeat (loop) processing without writing the same statements over and over using Repetition Structure including while loop, do…while loop, for loop
3. All the while, do…while , and for loops can be implemented as counter-controlled loop or sentinel-controlled loop.– When number of iteration is known, use counter-controlled
loop– When decision to proceed with iteration depends on a value
or a range of values, use sentinel-controlled loop
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Summary4. The expression of the condition in the selection and
repetition structures must be specified using Relational operators (such as >, >=, <, <=) and Equality operators (==, !=) and several conditions may be combined using Logical operators (&&, ||, and !).
5. You may combine sequence, selection and repetition structures in a C program
6. You may use the continue and break statements to modify the program flow when a selection structure or a repetition structure is used.
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