A Sample Program:
Let's try to implement what we have learned so far:
Problem Statement:
Calculate the average age of a class of five students. Prompt the user
to enter the age of each student.
Solution:
Lets first sort out the problem. In the problem we will take the ages
of five students from the user. To store these ages we will use five variables,
one variable for each student’s age. As the age is stored in whole
numbers 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;
We have declared all the five 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 five students by dividing this total age by 5. For
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 88 then average age will be 17.6).
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.
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 students and
store them into respective variables. When cin statement is reached in
a program, the program stops execution and expects some input from the
user. 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 cin assigns the input value to the variable on the right hand
side which is age1 in this case.
Next, we add all these values and store the result to the variable TotalAge.
We use assignment operator for this purpose. On the right hand side of
the assignment operator, we write the expression to add the ages and store
the result in the variable, TotalAge on left hand side. For this purpose
we write the statement as follow:
TotalAge = age1 + age2 + age3
+ age4 + age5 ;
The expression on the right hand side uses many addition operators ( +
). As these operators have the same precedence, the expression is evaluated
from left to right. Thus first age1 is added to age2 and then the result
of this is added to age3 and then this result is added to age4 and so
on. Now we divide this TotalAge by 10 and get the average age. We store
this average age in the variable i.e. AverageAge by writing the statement:
AverageAge = TotalAge / 10;
And at the end we display this average age on the screen by using the
following statement:
cout<<“The average
age of the students is: “<<AverageAge;
Here the string enclosed in the quotation marks, will be printed on the
screen as it is and the value of AverageAge will be printed on the screen.
The complete coding of the program is given below:
/* This program calculates the
average age of a class of five students after prompting the user to enter
the age of each student. */
#include <iostream.h>
main ()
{
// declaration of variables, the age will be in whole numbers
int age1, age2, age3, age4, age5;
int TotalAge, AverageAge;
// take ages of the students from the user
cout << “Please enter the age of student 1: ”;
cin >> age1;
cout << “Please enter the age of student 2: ”;
cin >> age2;
cout << “Please enter the age of student 3: ”;
cin >> age3;
cout << “Please enter the age of student 4: ”;
cin >> age4;
cout << “Please enter the age of student 5: ”;
cin >> age5;
// calculate the total age and average age
TotalAge = age1 + age2 + age3 + age4 + age5;
AverageAge = TotalAge / 5;
// Display the result ( average age )
cout << “Average age of class is: “ << AverageAge;
}
A sample output of the above program is given below.
Please enter the age
of student 1: 12
Please enter the age of student 2: 13
Please enter the age of student 3: 11
Please enter the age of student 4: 14
Please enter the age of student 5: 13
Average age of class is: 12
In the above output the total age of the students is 63 and the actual
average should be 12.6 but as we are using integer data types so the decimal
part is truncated and the whole number 12 is assigned to the variable
AverageAge.
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 have no power operator in C, just use * to multiply the same value.
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. 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 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.
