PROGRAM LOGIC CONTROL :
SEQUENCE, SELECTION AND REPETITION
Introduction
Computer
programs are made up of three basic constructs:
§ Sequence
§ Selection
§ Repetition (iteration)
All of the
constructs will be represented by three methods:
§ Flow Charts
§ Nassi Schneiderman Charts (N-S Charts)
§ Structured English
Structured
English and N-S Charts are examples of methods used to express program
design. Flow charts are used to express
the logic program of a constructs and should never be used to express program
design.
A flow chart
represents a mental picture that programmers should carry in their minds as an
aid to memorising the logic of the program constructs.
N-S
charts are graphical representation of program constructs that ensures program
designers use the constructs that are available to them in the chosen
high-level language.
Structured
English is non-graphical methods that also ensures that a program designer uses
the constructs that are available within the high-level language.
SEQUENCE
This
describes a sequence of actions that a program carries out one after
another, unconditionally.
SELECTION
Selection
is the program construct that allows a program to choose between different
actions. It allows for alternative paths
to be taken through a program.
The
selection constructs used in C are :
a)
if
b)
Nested if
c)
if…..else
d)
switch
a)
if
 |
Flow Charts :
Format :
if
(condition)
{
statement(s);
body executes only once if test is TRUE
}
Meaning
If the parentheses or condition after the word ‘if’ is TRUE then the
statement that follows after the condition will be executed. If the condition is FALSE then the statement
block will ignored.
Example :
|
//example
of if statements
#include
<stdio.h>
void
main()
{
int guess_input;
printf("Please enter a
whole number\n");
scanf("%d",
&guess_input);
if (guess_input==1)
{
printf("Right
number !\n");
printf("Well
Done.");;
}
}
|
Explanations:
In the above program, if the user types in the number is 1 then the statements printf( “Right number !\n”); and printf(
"Well Done.\n” ); will be executed. If guess_input is not equal to 1
then the statements will be ignored and the program will terminates.
MDo not put a semicolon after the PARENTHESES
of the relational test. Semicolons go
after each statement inside the block.
b)
Nested
if
If statement can
be nested. Nesting means that one if
statement is part of the body of another if statement.
 |
Flow Charts :
Format :
if
(condition1)
{
if
(condition2)
{
statement(s);
}
}
Meaning
If both if condition are TRUE,
then only the statements that follow after the second if will be executed.
Example :
|
// example of nested if statements
#include <stdio.h>
void main()
{
int
num1, num2;
printf("Please
enter two integer numbers:");
scanf("%d
%d",&num1,&num2);
if
(num1==1)
{
if
(num2==1)
{
printf("You
typed 1 and 1 !\n");;
}
}
}
|
Explanations:
In the example above the inner if statement will not be
reached unless both if statements are TRUE,
and the cout statement will not be executed unless both if statements
are TRUE.
c)
if…..else
 |
Flow Charts
Format:
if (condition)
{ statements
1; }
else
{ statements
2; }
Meaning:
If the condition expression is TRUE then the following statements
after the if will be executed. If it is
FALSE, then the statements after else will be executed.
Example :
|
/* Program
that demonstrate the use of if-else -if */
#include
<stdio.h>
void main()
{
int guess_input;
printf("Please enter an
integer number between 1 and 3 :");
scanf("%d",&guess_input);
if (guess_input==1)
{
printf("Number
One\n");
}
else if (guess_input==2)
{
printf("Number
Two\n");
}
else
if (guess_input==3)
{
printf("Number
Three\n");
}
else
{
printf("Your
number is not in the range\n");
}
printf("See you
again!\n");
}
|
If guess_input is equal to 1 then the statements printf(“Number
One\n”); is executed.
If guess_input is equal to 2 then the statements printf(“Number
Two\n”); is executed.
If guess_input is equal to 3 then the statements printf(“Number
Three\n”); is executed.
If guess_input os not 1, 2 or 3 the statements printf(“Your
number is not in the range”);
will be executed.
No matter what the number is,
the statements printf(“See you again!”); will be executed.
|
/* Program
that demonstrate the use of if-else -if */
#include
<stdio.h>
void main()
{
float grade;
printf("Please enter a
mark:");
scanf("%f",&mark);
if (mark >= 90 &&
mark <=100)
{
printf("A\n");
}
else if (mark >= 80
&& mark<=89)
{
printf("B\n");
}
else if (mark >= 70
&& mark<=79)
{
printf("C\n");
}
else if (mark >= 60
&& mark<=69)
{
printf("D\n");
}
else
{
printf("F\n");
}
}
|
|
/* Program
that demonstrate the use of if-else -if */
/*Program that
demonstrate the use of logical operators */
#include
<stdio.h>
void main()
{
int age;
printf("Please enter your
age:");
scanf("%d",&age);
if (age>=6 &&
age<=9)
{
printf("
You are enittiled to play this game\n");
}
else if (age<6)
{
printf("You
are too young for this game\n");
}
else if (age > 9)
{
printf("You
are too old for this game\n");
}
}
|
d) switch
Although the if-else-if ladder can perform multiple tests, it is
hardly elegant. As the number of if-else-if increases, the nesting and the
resulting indentation gets quite deep, making the program difficult to
read. Actually it can be coded using a
switch statement. The switch statements
is sometime called the multiple choices statement lets your program choose from
several alternatives.
 |
Flow Charts
Format :
switch (expression)
{
case
(expression1) : { one or more C statements; }
case
(expression2) : { one or more C statements; }
case (expression3) : { one or
more C statements; }
.
.
.
default
: { one or more C statements; }
}
Meaning :
·
expression
Can be integer expression. a character, a constant or a variable.
·
Sub-expression (expression1, expression2, expression3.......)
Can be any other integer expression , character, constant or
variable. The number of sub-expression is determined by your application.
·
The one or more C statements
Can be any block of C code.
If the block is only one statements long, you don’t need the braces , bur they are recommended.
·
The default line
It is optional
and does not have to be the last line of the switch body.
Example :
|
/* Program that demonstrate use of switch statement
*/
#include <stdio.h>
void main()
{
int
user_entry;
printf("Please
enter an integer number between 1 and 3:");
scanf("%d",&user_entry);
switch
(user_entry)
{
case
1: printf("Number One\n");
break;
case
2: printf("Number Two\n");
break;
case
3: printf("Number Three\n");
break;
case
4: printf("Number Four\n");
break;
default:
printf("Number is out of range.\n");
break;
}
}
|
break This
is generally included inside switch .
The break statement is optional.
However, if the break statement
is omitted, execution will continue on into the next case’s statement until
either a break or the end of the switch is reached.
default This
will cause a statement or a series of statements to executed if no matches are
found.5
REPETITION (ITERATION)
Definition of Loops
A loop may be defined as a smaller program the can be
executed several times in a main program.
There are
several techniques in achieving this in C :
a)
while
b)
do…while
c)
for
a)
while Loop
Flow Charts
 |
Format :
while
(condition)
{
statement
(s);
}
Meaning :
With the while statement, the condition is first tested and if the
condition is TRUE then the statement or statements within the loop will be
executed and if the condition is first tested is FALSE the loop will not be
executed at all. The condition
will be tested repeatedly until the condition is false.
Example :
|
/*
Program that demonstarte the while loop */
#include
<stdio.h>
void
main()
{
int num;
printf("Please enter any
number:");
scanf("%d",&num);
while (num>10)
{
printf("Please
enter any number:");
scanf("%d",&num);
}
printf("You have
entered number that less than
10...\n");
}
|
num>10 (num
> 10) is the test. The statement
within the loop will continually be executed as long as the condition is true.
b) do…while
Flow Charts
 |
Format:
do
{
statement
(s);
} while
(condition);
Meaning:
The do while loop checks the condition at the end of the loop. This means that the statement or statements
inside the do while loop will be executed again if the condition is TRUE. If the condition is FALSE, the do while loop will not be executed.
Example :
|
/* Program that demonstrate do while loop */
#include < stdio.h>
void main()
{
int
num;
do
{
printf(
"Please enter any number:");
scanf("%d",
&num);
}while
(num > 10);
printf(
"You already entered number that less than 10.. \n");
}
|
num > 10 This
the condition. The program will read numbers that being keyed in
by the user until the number is less
than 10.
|
/*Program that
demonstrate do while loop */
#include
<stdio.h>
void main()
{
int count;
count=0;
printf("Initially count
is %d",count);
do
{
count++;
printf(
"\nNow count is %d\n",count);
} while (count < 4);
}
|
count++ This
statement increases the value of count by one.
while (count < 4) This
is the test. If count is less than 4,
i.e. count us true, the statements in the loop is executed. when count is equal to 4, the program finishes.
Difference between do while
and while
Difference
between do…while and while construct is that, the program to be repeated within
the do while loop will always be executed at least once. Whereas
the while allows for the condition where the program to be repeated is
never executed.
Others
statements used in loops
The break statement
We’ve already used break to exit a switch. But break actually exits the nearest
enclosing for, while and do while as well.
The continue statement
A continue skips the rest of the loop body and causes
the loop test to be immediately evaluated (except in the for, which evaluates
action first).
|
// Programs
that demonstrate the use of break and continue statements
#include
<stdio.h>
#include<conio.h>
void main()
{
char test;
int number, quantity;
float unitPrice,
total;
number=1;
while (number==1)
{
printf("\nPlease
enter Quantity purchased :");
scanf("%d",&quantity);
printf("Please
enter Unit price :");
scanf("%f",&unitPrice);
total=quantity*unitPrice;
printf("total
: %.2f\n",total);
printf("Do
you want to continue [y/n] :");
test=getch();
if (test=='Y'
|| test=='y')
{
continue;
} // or number=0
else
{
break;
}
printf("End
process!!\n" ); }
}
|
c)
for Loop
The for statement encloses one or more statements that form the body
of the loop. These statements
in the loop are executed repeatedly a certain number of times . You as programmer, control the number of loop
repetitions.
Flow Charts
 |
Format:
for (start expression ; test expression; count expression)
{
statements;
}
Meaning:
C evaluates the start expression before the loop begins. Typically, the start expression is an
assignment statement ( such as ctr=1;)
but it can be legal expression you specify. C looks at and evaluates the start
expressions only once, at the top of the loop.
M
Do not put a semicolon after the right parentheses. If you do, the for loop “thinks” nothing each
time - until the test expression become False.
Every time the body of the loop repeats, the count expression executes, usually
incrementing or decrementing a variable.
The test expression evaluates to
True(non zero) or False(zero) and then
determines whether the body of the loop repeats again.
Example:
|
/* To introduce the for loop */
#include <stdio.h>
void main()
{
int
ctr;
for
(ctr=1; ctr<=10; ctr++)
{
printf("
for loop %d\n", ctr);
}
}
|
ctr=1 Start expression.
Start ctr at 1
ctr<=10 Test
expression. The looping will be executed
as long as ctr is less or equal to 10.
ctr++ Count
expression. Increment value of ctr by
one.
 |
? The for loop tests at the
top of the loop. If the test expression
is False when the for loop begins, the
body of the loop never executes.
Exercises - SELECTION ( if , if-else and switch)
Exercise 1
Read an integer value. Assume it is the
number of a month of the year; print out the name of that month.
Exercise 2
Given as input three integers representing
a date as day, month, year, print out the number day, month and year for the
following day's date.
Typical input: 28 2 1992
Typical output: Date following 28:02:1992
is 29:02:1992
Exercise 3
Write a program which reads two integer
values. If the first is less than the second, print the message up. If the
second is less than the first, print the message down If the numbers are equal,
print the message equal If there is an error reading the data, print a message
containing the word Error and perform exit( 0 );
Exercise 4
Write a C program to construct mini
calculator to do 5 arithmetic operations:
1. Add
2. Subtract
3. Multiply
4. Divide
5. Remainder
e.g output:
1. Add
2. Subtract
3. Multiply
4. Divide
5. Remainder
Please select the operation (enter the
number)as the above: 2
Please enter 2 numbers (subtract operation)
.
number 1: 2
number 2: 4
For the operation of 2-4, the result is -2
Exercise 5
Syabas Water Services wishes to compute the
water bill for its clients. It levies 9 cents per unit for the first 1000 units
of water consumed, 11 cents per unit for the next 1000 units and 15 cents per
units if the number of units consumed exceeds 2000. Compute and print the water
bill for the given meter readings..
Exercise 6
XYZ computes the weekly wages of its
employees as follows: the rate for the first 40 hours is 5 ringgits per hour,
the rate for the next 20 hours is 8 ringgits per hour and the rate is 10
ringgits per hour if the number of hours exceeds 60. The contribution of each
employee to Socso is 1 percent of the wage earned. Read the names and hours
worked for the company's employees and
print their pay slips.
Exercise 7
As a System Analysis at the University of
Virtual, You have to develope a program
to convert a mark to the grade and the
grade(gpa) value for a subject. The table of
conversion is shown below:
GRADE MARKS GPA
A 80-100 4.00
A- 75-79 3.67
B+ 70-74 3.33
B 65-69 3.00
B- 60-64 2.67
C+ 55-59 2.33
C 50-54 2.00
C- 45-49 1.67
D 40-44 1.00
F 0-39 0.00
Here
is an example of running program:
______________________________________
Enter
the mark to be converted : 55
The
Grade is :
C
The
gpa value is :
2.33
______________________________________
Name your program as :
group_your_name_mark_conversion.c (e.g: L01_yosof_kadase_mark_conversion.c) and
submit at yosof.net
p.s. Please write your name and student id
on the top of your program file( as comments )
Exercise 8
The Padaede Car Rental company charges
RM0.25/mile if the total mileage does not exceed 100.
If the total mileage is over 100, the
company charges RM0.25/mile for the first 100 miles,
then it charges RM0.15/mile for any
additional mileage over 100. Write a program so that
if the clerk enters the number of miles,
the program would display the total price owed.
Here
is an example of running the program:
______________________________
Enter
the number of miles: 75
Total
Price = RM18.75
Press
any key to continue
______________________________
Name your program as:
group_your_name_mileage.c
(e.g: L01_yosof_kadase_mileage.c) and
submit at yosof.net
p.s. Please write your name and student id
on the top of your program file (as comments )
Exercise 9
The Padaede Car Rental company charges
RM1.00/mile if the total mileage does not exceed 100. If the total mileage is over 100, the company
charges RM1.00/mile for the first 100 miles, then it charges M0.50/mile for any additional mileage over
100. Write a program so that
if the clerk enters the number of miles,
the program would display the total price owed
Note:
You are required to use both SWITCH AND
IF-ELSE statements when writing the program.
Write a menu driven program which has the
following options:
A. Computes
the total price owed
B. Exit
The interface of the system should be as
follows:
Computes the total price owed for the given
mileage
-----------------------------------------------------------
A.
Computes the total price owed
B.
Exit
-----------------------------------------------------------
Please enter your choice :
If ‘A’ is entered:
Computes the total price owed for the given
mileage
-----------------------------------------------------------
Please enter the mileage (mile): 200
For the 200 miles , the total price
owed is RM 150.00.
The program MUST check whether the value of
unit input by the user is positive,
zero or negative. Error message MUST be
displayed if zero or negative values are typed in.
If ‘B’ is entered, the program shall end.
Otherwise, the program shall mention wrong
choice is entered and the program shall end.
Name your program as:
group_your_name_padaede_car_rental.c (e.g:
L01_yosof_kadase_padaede_car_rental.c) and submit at yosof.net
p.s. Please write your name and student id
on the top of your program file (as comments )
Exercises - LOOP ( while , do while and for)
Exercise 1
Enter (get input) 30 integer values within the range of 1 to 200.
Find how many of these values fall in the
range 1-50,51-100,101-150 and 151-200
Exercise 2
Construct a c program of a clock system.
e.g output:
current time:
2:54:58
Exercise 3
Construct a c program of a countdown system.
Exercise 4
Develop a program to produce a Time Scale Table. The program must use
at least 2 different loop statements.
Use the following formula to display your answer.
HOURS = 24 * DAYS.
MINITS = 60 * HOURS.
SECONDS = 60 * MINITS
Sample Output:
Time Scale Table
@@@@@@@@@@@@@@@@@
DAY/S HOUR/S MINIT/S SECOND/S
1 24 1440 86400
8 192 11520 691200
15 360 21600 1296000
22 528 31680 1900800
29 696 41760 2505600
36 864 51840 3110400
43 1032 61920 3715200
50 1200 72000 4320000
57 1368 82080 4924800
64 1536 92160 5529600
Do you want to continue [y/n]: n
Exercise 5
Write a program that can read numbers from 0 to 12 and print the following multiplication table of output:
1 X 7 = 7
2 X 7 = 14
..
..
..
12 X 7 = 48
Exercise 6
Write a program that can do the following task :
a) Read 10 integer numbers and print the largest value
b) Read 10 integer numbers and print the smallest value
c) Read 10 integer numbers and print the total
d) Read 10 integer numbers and print the average
Exercise 7
Write a program that can do the following task, with no specified input:
a) Read integer numbers and print the largest value
b) Read integer numbers and print the smallest value
c) Read integer numbers and print the total
d) Read integer numbers and print the average
Exercise 8
Write a program that calculates and prints the sum of the even integers from 2 to 30.
Exercise 9
Write a program that calculates and prints the product (multiplication) of the odd integers from 1 to 15.
Exercise 10
Write a program to get character from user within a certain range. Your program should be able to check for validity of the character to be within a certain range from .A. to .Z..
Sample program output as the following:
Enter a character between .A. to .Z.
a
Character is outside legal range! Try again!
Enter a character between .A. to .Z.
0
Character is outside legal range! Try again!
Enter a character between .A. to .Z.
R
Character is : R
Exercise 11
Write a program to get number from user within a certain range. Your program should be able to check for validity of the number to be within a certain range from 1 to 500 and .1 to terminate.
Sample program output as the following:
Enter a number between 1 to 500 and .1 to TERMINATE
0
Number is outside legal range! Try again!
Enter a number between 1 to 500 and .1 to TERMINATE
200
Number is : 200
Enter a number between 1 to 500 and .1 to TERMINATE
-1
End
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