# Lesson 4

### Summary

o Sample Program
o Examples of Expressions
o Use of Operators
o Tips

### Solution:

Lets first sort out the problem. In the problem we will take the ages of ten students from
the user. To store these ages we will use ten variables, one variable for each student’s
age. We will take the ages of students in whole numbers (in years only, like 10, 12, 15
etc), so we will use the variables of data type int. The variables declaration statement in
our program will be as follow:
int age1, age2, age3, age4, age5, age6, age7, age8, age9, age10;

We have declared all the ten variables in a single line by using comma separator ( , ).
This is a short method to declare a number of variables of the same data type.
After this we will add all the ages to get the total age and store this total age in a variable.
Then we will get the average age of the ten students by dividing this total age by 10. For
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the storage of total and average ages we need variables. For this purpose we use variable
TotalAge
for the total of ages and AverageAge for average of ages respectively.
int TotalAge, AverageAge;

We have declared AverageAge as int data type so it can store only whole numbers. The
average age of the class can be in real numbers with decimal point (for example if total
age is 173 then average age will be 17.3). But the division of integers will produce
integer result only and the decimal portion is truncated. If we need the actual result then
we should use real numbers (float or double) in our program.
Now we have declared variables for storing different values. In the next step we prompt
the user to enter the age of first student. We simply show a text line on the screen by
using the statement:
cout << “Please enter the age of first student : ” ;

So on the screen the sentence “Please enter the age of first student:” will appear.
Whenever we are requesting user to enter some information we need to be very clear i.e.
write such sentences that are self explanatory and user understands them thoroughly and
correctly. Now with the above sentence everyone can understand that age would be
entered for the first student. As we are expecting only whole numbers i.e. age in years
only i.e. 10, 12 etc, our program is not to expect ages as 13.5 or 12.3 or 12 years and 3
months etc. We can refine our sentence such, that the user understands precisely that the
age would be entered in whole number only.
After this we allow the user to enter the age. To, get the age entered by the user into a
variable
, we use the statement:
cin >> age1;

Lets have a look on the statement cin >> age1; cin is the counter part of the cout. Here

### cin is the input stream that gets data from the user and assigns it to the variable on its right side. We know that the sign >> indicates the direction of the flow of data. In our statement it means that data comes from user and is assigned to the variable age1, where age1 is a variable used for storing the age entered for student1. Similarly we get the ages of all the ten students and store them into respective variables. That means the age of first student in age1, the age of second student in age2 and so on up to 10 students. When cin

statement is reached in a program, the program stops execution and expects some input
from the user. So when cin >> age1; is executed, the program expects from the user to
type the age of the student1. After entering the age, the user has to press the 'enter key'.
Pressing 'enter key' conveys to the program that user has finished entering the input and

### Examples of Expressions

We have already seen the precedence of arithmetic operators. We have expressions for
different calculations in algebraic form, and in our programs we write them in the form of
C statements. Let’s discuss some more examples to get a better understanding.
We know about the quadratic equation in algebra, that is y = ax2 + bx + c. The quadratic
equation in C will be written as y = a * x * x + b * x + c. In C, it is not an equation but an
assignment statement. We can use parentheses in this statement, this will make the
expression statement easy to read and understand. Thus we can rewrite it as y = a * (x *
x) + (b * y) + c.

Note that we have no power operator in C, just use * to multiply the same value.
Here is another expression in algebra: x = ax + by + cz2. In C the above expression will
be as:
x = a * x + b * y + c * z * z

The * operator will be evaluated before the + operator. We can rewrite the above
statement with the use of parentheses. The same expressions can be written as:
x = (a * x) + (b * y) + c * ( z * z)

Lets have an other expression in algebra as x = a(x + b(y + cz2)). The parentheses in this
equation force the order of evaluation. This expression will be written in C as:
x = a * (x + b * (y + c * z * z))

While writing expressions in C we should keep in mind the precedence of the operators
and the order of evaluation of the expressions (expressions are evaluated from left to
right). Parentheses are used in complicated expressions. In algebra, there may be curly
brackets { } and square brackets [ ] in an expression but in C we have only parentheses
( ). Using parentheses, we can make a complex expression easy to read and understand
and can force the order of evaluation. We have to be very careful while using
parentheses, as parentheses at wrong place can cause an incorrect result. For example, a
statement x = 2 + 4 * 3 results x = 14. As * operator is of higher precedence, 4 * 3 is
evaluated first and then result 12 is added to 4 which gives the result 14. We can rewrite
this statement, with the use of parentheses to show it clearly, that multiplication is
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performed first. Thus we can write it as x = 2 + (4 * 3). But the same statement with
different parentheses like x = (2 + 4) * 3 will give the result 18, so we have to be careful
while using parenthesis and the evaluation order of the expression.
Similarly the equation (b2 – 4ac)/2a can be written as ( b * b – 4 * a * c) / ( 2 * a ). The
same statement without using parentheses will be as b * b – 4 * a * c / 2 * a. This is
wrong as it evaluates to b2 – 4ac/2a (i.e. 4ac is divided by 2a instead of (b2-4ac)).

### Use of Operators

Here are sample programs which will further explain the use of operators in
programming.

### Problem Statement:

Write a program that takes a four digits integer from user and shows the digits on the
screen separately i.e. if user enters 7531, it displays 7,5,3,1 separately.

### Solution:

Let’s first analyze the problem and find out the way how to program it.

### Analysis:

First of all, we will sort the problem and find out how we can find digits of an integer.
We know that when we divide a number by 10, we get the last digit of the number as
remainder. For example when we divide 2415 by 10 we get 5 as remainder. Similarly
3476 divided by 10 gives the remainder 6. We will use this logic in our problem to get
the digits of the number. First of all, we declare two variables for storing number and the
digit. Let’s say that we have a number 1234 to show its digits separately. In our program
we will use modulus operator ( % ) to get the remainder. So we get the first digit of the
number 1234 by taking its modulus with 10 (i.e. 1234 % 10). This will give us the digit 4.
We will show this digit on the screen by using cout statement. After this we have to find
the next digit. For this we will divide the number by 10 to remove its last digit. Here for
example the answer of 1234 divided by 10 is 123.4, we need only three digits and not the
decimal part. In C we know that the integer division truncates the decimal part to give the
result in whole number only. We will use integer division in our program and declare our
variable for storing the number as int data type. We will divide the number 1234 by 10
(i.e. 1234 / 10). Thus we will get the number with remaining three digits i.e. 123. Here is
a point to be noted that how can we deal with this new number (123)?
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There are two ways, one is that we declare a new variable of type int and assign the value
of this new number to it. In this way we have to declare more variables that mean more
memory will be used. The second way is to reuse the same variable (where number was
already stored). As we have seen earlier that we can reassign values to variables like in
the statement x = x + 1, which means, add 1 to the value of x and assign this resultant
value again to x. In this way we are reusing the variable x. We will do the same but use
the division operator instead of addition operator according to our need. For this purpose
we will write number = number / 10. After this statement we have value 123 in the
variable number.
Again we will get the remainder of this number with the use of modulus operator,
dividing the number by 10 (i.e. 123 % 10). Now we will get 3 and display it on the
screen. To get the new number with two digits, divide the number by 10. Once again, we
get the next digit of the number (i.e. 12) by using the modulus operator with 10, get the
digit 2 and display it on the screen. Again get the new number by dividing it by 10
(i.e. 1). We can show it directly, as it is the last digit, or take remainder by using modulus
operator with 10. In this way, we get all the digits of the number.
Now let’s write the program in C by following the analysis we have made. The complete
C program for the above problem is given below. It is easy to understand as we are
already familiar with the statements used in it.
/* A program that takes a four digits integer from user and shows the digits on the screen
separately i.e. if user enters 7531, it displays 7,5,3,1 separately. */
#include <iostream.h>
main()
{
// declare variables
int number, digit;
// prompt the user for input
cout << "Please enter 4-digit number:";
cin >> number;
// get the first digit and display it on screen
digit = number % 10;
cout << "The digits are: ";
cout << digit << ", ";
// get the remaining three digits number
number = number / 10;
// get the next digit and display it
digit = number % 10;
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cout << digit << ", ";
// get the remaining two digits number
number = number / 10;
// get the next digit and display it
digit = number % 10;
cout << digit << ", ";
// get the remaining one digit number
number = number / 10;
// get the next digit and display it
digit = number % 10;
cout << digit;
}
A sample output of the above program is given below.

### Problem Statement:

Write a program that takes radius of a circle from the user and calculates the diameter,
circumference and area of the circle and display the result.

### Solution:

In this problem we take the input (radius of a circle) from the user. For that we can use
cin
statement to prompt the user to enter the radius of a circle. We store this radius in a
variable. We also need other variables to store diameter, circumference and area of the
circle. To obtain the correct result, we declare these variables of type float, instead of int
data type, as we know that the int data type stores the whole numbers only. Here in our
problem the area or circumference of the circle can be in decimal values. After getting the
radius we use the formulae to find the diameter, circumference and area of the circle and
then display these results on the screen. The solution of this program in coding form is
given below.
The digits are: 8, 7, 6, 5
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/* Following program takes the radius of a circle from the user and calculates the
diameter, circumference and area of the circle and displays the result. */
#include <iostream.h>
main ()
{
// declare variables
// prompt the user for radius of a circle
// calculate the diameter, circumference and area of the circle
// implementing formula i.e. diameter = 2 r circumference = 2 ח r and area = ח r2
diameter = radius * 2 ;
circumference = 2 * 3.14 * radius ; // 3.14 is the value of ח (Pi)
// display the results
cout << "The diameter of the circle is : " << diameter ;
cout << "The circumference of the circle is : " << circumference ;
cout << "The area of the circle is : " << area ;
}
A sample output of the above program is given below.

### Tips

o Use descriptive names for variables
o Indent the code for better readability and understanding
o Use parenthesis for clarity and to force the order of evaluation in an expression
o Reuse the variables for better usage of memory
o Take care of division by zero
o Analyze the problem properly, and then start coding (i.e. first think and then write)
The diameter of the circle is : 10
The circumference of the circle is : 31.4
The area of the circle is : 78.5