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CS 201 Lecture No 5

  Lecture Handout 

  Introduction to programming

  Lecture No. 5

  Reading Material

  Deitel & Deitel – C++ How to Program    chapter 2 

  2.4, 2.5, 2.6, 2.19,  

  2.20 

   Summary

  o  Conditional Statements

  o  if/else structure

  o  Logical Operators

  Sample Program 2

  o  Tips

  Conditional Statements (Decision Making)

  In every day life, we are often making decisions. We perform different tasks while taking

  decisions. For example, the statement ‘if the milk shop is open, bring one liter of milk

  while returning home from college’, involves this phenomenon. 

  In this statement, there is an element of decision making. We bring one litre of milk if the

  shop is open. And if the shop is closed, we come back to home without milk. 

  Thus we are making a decision on the condition that the shop is open. The decision-

  making process is everywhere in our daily life. We see that the college gives admission to

  a student if he has the required percentage in his previous examination and/or in the entry

  test. Similarly administration of a basketball team of the college decides that the students

  having height more than six feet can be members of the team. 

  In the previous lectures, we have written simple elementary programs. For writing

  interesting and useful programs, we have to introduce the decision making power in

  them. Now we will see what kind of decisions are there in programming and how these

  can be used.

  Every programming language provides a structure for decision making. 

  'C' also provides this structure. The statement used for decisions in 'C' language is known

  as the 'if statement'. The if statement has a simple structure. That is 

  if ( condition )

        Statement (or group of statements)

  The above statements mean, If condition is true, then execute the statement or a group of

  statements. Here the condition is a statement which explains the condition on which a

  decision will be made. We can understand it from the example that Ali can become the

  member of the basket ball team if he has a height more than six feet .In this case, the

  condition will be 

  if (Ali’s height is greater than six feet) 

        Ali can be a member of team

  We have written the condition in English language. Now let's see how we can implement

  this in terms of variables, operators and C statements. In the program, we will write the

  condition in parentheses, followed by a statement or group of statements to be executed. 

  Now here is the concept of block of statements. We use braces { } to make a group

  (block) of a number of statements. We put ‘{’ before first statement and ‘}’ after the last

  statement. Thus if we have to do many things after the if statement. The structure of if

  statement becomes as under

  if (condition)

  {

      statement;

      statement;

      .

      .

     statement; 

  }

  Note the indentation of the lines and semi-colon after each statement. Semi-colons are

  necessary after every C statement. The indentation is only a matter of style. It makes the

  code easy to read and understand from where a block starts, ends and what kind of block

  it is. It does not affect the logic of the program. But the braces can affect the logic. We

  can also write a comment line to state the purpose of code block.

  Let's consider a simple example to explain the if statement. Suppose, we have ages of two

  students (say for the time being we have got these ages in variables). These variables are-

  age1 and age2. Now we say that if the age1 is greater than age2, then display the

  statement ‘Student 1 is older than student 2’. 

  The coding for this program will be as below

  #include <iostream.h> 

  main()

  {

  int age1, age2;

              age1 = 12;

  age2 = 10;

  if  (age1 > age2)

       cout << “Student 1 is older than student 2”;

  }

  Here, in our code we see a new operator i.e. ‘ > ‘ (greater than) in the if statement. We

  need such operators (like greater than, less than, equal to etc) while making decisions.

  These operators are called 'relational operators'. These are almost the same relational

  operators we use in algebra. Following table summarizes the relational operators.

  Algebraic   In C   Example   Meaning

  language

  Greater than   >   >   x > y   x is greater than y

  Equal to   =   ==   x == y   x is equal to y

  Less than   <   <   x < y   x is less than y

  Greater than or   >   >=   x >= y   x is greater than or

  equal to   equal to y

  Less than or   <   <=   x <= y   x is less than or equal

  equal to   to y

  Not equal to   ≠  !=   x != y   x is not equal to y

  Note that there is no space between ==, >=, <= and !=. 

  These are considered as single operators.

  The operator == (equal to) is different from the operator =. We know that operator = is

  the assignment operator which is used in assignment statement to assign a value to a

  variable.

  Don't confuse the assignment operator (=) with equal to operator (==). If we write single

  = in condition of if statement. For example, if we write if ( x = 2 ), the compiler will not

  give error. This means that it is not a syntax error. The conditional expression in   if

  statement returns a value. In this case, x = 2 will also have some value but it will not in

  the form of true or false. So it will create a logical error. So be careful while using equal

  to condition in if statement.

  Flow Charting

  There are different techniques that are used to analyze and design a program. We will use

  the flow chart technique. A flow chart is a pictorial representation of a program. There

  are labeled geometrical symbols, together with the arrows connecting one symbol with

  other. 

  A flow chart helps in correctly designing the program by visually showing the sequence

  of instructions to be executed. A programmer can trace and rectify the logical errors by

  first drawing a flow chart and then simulating it.

   Sample Program 1

  Now let’s see the usage of relational operators by an example. There are two students

  Amer and Amara. We take their ages from the user, compare them and tell who is older? 

  As there are two students to be compared in terms of age, we need to declare two

  variables to store their ages. We declare two variables AmerAge and AmaraAge of type

  int. The variable names are one continuous word as we can’t use spaces in a variable

  name. 

  Here is an important point about variables declaration. We should assign an initial value

  (preferably 0 for integers) to variables when we declare them. This is called initialization

  of variables. 

  We can do this in one line while declaring a variable like int x = 0; This statement will

  declare a variable of name x with data type int and will assign a value 0 to this variable.

  Initializing a variable in this way is just a matter of style. You can initialize a variable on

  a separate line after declaring it. It is a good programming practice to initialize a variable. 

  Now we prompt the user to enter Amer’s age and store it into variable AmerAge. Then

  similarly we get Amara’s age from the user in the variable AmaraAge.

  While comparing the ages, we will use the   if   statement to see whether Amer’s age is

  greater than Amara’s. We will use > (greater than) operator to compare the ages. This can

  be written as if ( AmerAge > AmaraAge) .

  With this if statement, we write the statement cout << "Amer is greater than Amara" ; 

  It’s a simple one line test i.e. ‘if Amer’s age is greater than Amara's’, then display the

  message ‘Amer is older than Amara’.

  The flow chart for the above problem is as under.

   The complete code of the program is given below.

  /* This program test that if the age of Amer is greater

  than Amara’s age and displays the result. */

  # include <iostream.h>

  main ( )

  {

  int AmerAge, AmaraAge;

  //prompt the user to enter Amer’s age

  cout << “Please enter Amer’s age     “ ;

  cin >> AmerAge;

  //prompt the user to enter Amara’s age

  cout << “Please enter Amara’s age     “ ;

  cin >> AmaraAge;

  //perform the test

  if  (AmerAge > AmaraAge )

  cout << “ Amer is older than Amara”;

  }

  In our program, we write a single statement with the if condition. This statement executes

  if the condition is true. If we want to execute more than one statements, then we have to

  enclose all these statements in curly brackets { }. This comprises a block of statements

  which will execute depending upon the condition. This block may contain a single

  statement just like in our problem. So we can write the if statement as follow.

  if (AmerAge > AmaraAge ) 

  {

  cout << " Amer is older than Amara";

  } 

  A sample execution of the program provides the following output. 

  Please enter Amer’s age          16

  Please enter Amara’s age        14

   Amer is older than Amara

  Now think what happens if the condition in the if statement is not true i.e. Amer’s age is

  not greater than Amara’s. In this case, if the user enters Amer’s age less than Amara’s,

  then our program does nothing. So to check this condition, another if statement after the

  first if statement is required. Then our program will be as:

  /* This program checks the age of Amer and  Amara’s and

  displays the appropriate  the message. The program is using

  two if statements.*/

  # include <iostream.h>

  main ( )

  {

  int AmerAge, AmaraAge;

  //prompt the user to enter Amer’s age

  cout << “Please enter Amer’s age     “ ;

  cin >> AmerAge;

  //prompt the user to enter Amara’s age

  cout << “Please enter Amara’s age     “ ;

  cin >> AmaraAge;

  //perform the test

  if (AmerAge > AmaraAge )

      {  

  cout << “ Amer is older than Amara”;

       }

  if (AmerAge < AmaraAge )

        {

  cout << “ Amer is younger than Amara”;

         }

  }

  Now our program decides properly about the ages entered by the user.

  After getting ages from the user, the   if   statements are tested and if statement will be

  executed if the condition evaluates to true. 

  If/else Structure

  We have seen that the   if   structure executes its block of statement(s) only when the

  condition is true, otherwise the statements are skipped. The if/else structure allows the

  programmer to specify that a different block of statement(s) is to be executed when the

  condition is false. The structure of if/else selection is as follows.

  if ( condition)

  {

  statement(s);

  } 

  else 

  {

  statement(s);

  }

  Thus using this structure we can write the construct of our program as 

  if (AmerAge > AmaraAge )

  { 

  cout << " Amer is older than Amara";

  }

  else 

  {

  cout << " Amer is younger than Amara";

  }

  In this construct, the program checks the condition in if statement .If the condition is true,

  then the line "Amer is greater than Amara" is printed. Otherwise (if condition is not true),

  the statement related to else is executed and the message "Amer is younger than Amara"

  is printed. Here in if/else structure an important thing is that the else part is executed for

  all the cases (conditions) other than the case which is stated in the if condition.

  And in the comparison, we know that there are three conditions i.e. first value is greater

  than the second value, first value is less than the second value and first value is equal to

  the second value. Here in the above program construct the else part competes the greater

  than conditions and covers both less than and equal to conditions. 

  Thus in the above program construct, the message "Amer is younger than Amara" is

  displayed even if Amer’s age is the same as Amara’s age. This is logically incorrect and

  so to make this correct, we should display the message "Amer is younger than or is of the

  same age as Amara". Now this statement describes both the cases other than the one

  ‘Amer is greater than Amara'.

  The use of else saves us from writing different   if   statements to compare different

  conditions, in this way it cover the range of checks to complete the comparison. 

  If we want to state the condition "Amer is greater than or is of the same age as Amara’s"

  then we use the greater than or equal to operator (i.e. >=) in the if statement and less than

  operator ( < ) in the else statement to complete the comparison. 

  It is very important to check all the conditions while making decisions for good, complete

  and logical results. Make sure that all cases are covered and there is no such case in

  which the program does not respond.

  Logical Operators

  There are many occasions when we face complex conditions to make a decision. This

  means that a decision depends upon more than one condition in different ways. Here we

  combine the conditions with AND or OR. For example, a boy can be selected in basket

  ball team only if he is more than 18 years old and has a height of 6 feet. In this statement

  a boy who wants to be selected in the basket ball team must have both the conditions

  fulfilled. This means that AND forces both the conditions to be true. Similarly we say

  that a person can be admitted to the university if he has a BCS degree OR BSC degree. In

  this statement, it is clear that a person will be admitted to the university if he has any one

  of the two degrees. 

  In programming we use logical operators ( && and || ) for AND and OR respectively

  with relational operators. These are binary operators and take two operands. These

  operators use logical expressions as operands, which return TRUE or FALSE.

  The following table (called truth table) can be used to get the result of the && operator

  and || operator with possible values of their operands. It is used to explain the result

  obtained by the && and || operators.

  Expression 1   Expression 2   Expression 1 &&   Expression 1  ||

  Expression 2   Expression 2

  True   False   false   True

  True   True   true   True

  False   False   false   False

  False   True   false   True

  The && operator has a higher precedence than the || operator. Both operators associate

  from left to right. An expressions containing && or || is evaluated only until truth or

  falsehood is known. Thus evaluation of the expression (age > 18) && (height > 6)  will

  stop immediately if age > 18 is false (i.e. the entire expression is false) and continue if

  age > 18 is true (i.e. the entire expression could still be true if the condition height > 6 is

  true ).

  There is another logical operator that is called logical negation. The sign ! is used for this

  operator. This operand enables a programmer to ‘reverse’ the meaning of a condition.

  This is a unary operator that has only a single condition as an operand. The operator ! is

  placed before a condition. If the original condition (without the ! operator) is false then

  the ! operator before it converts it to true and the statements attached to this are executed.

  Look at the following expression 

  if ( ! (age > 18 ))

        cout << “ The age is less than 18”;

  Here the cout statement will be executed if the original condition (age > 18) is false

  because the ! operator before it reverses this false to true.  

  The truth table for the logical negation operator ( ! ) is given below.

  Expression   ! Expression

  True   False

  False   True

  Sample Program 2

  Problem statement

  A shopkeeper announces a package for customers that he will give 10 % discount on all

  bills and if a bill amount is greater than 5000 then a discount of 15 %. Write a C program

  which takes amount of the bill from user and calculates the payable amount by applying

  the above discount criteria and display it on the screen.

  Solution   

  In this problem we are going to make decision on the basis of the bill amount, so we will

  be using   if statement. We declare three variables amount, discount and netPayable and

  initialize them. Next we prompt the user to enter the amount of the bill. After this we

  implement the   if statement to test the amount entered by the user. As we see in the

  problem statement that if the amount is greater than 5000 then the discount rate is 15 %

  otherwise (i.e. the amount is less than or equal to 5000) the discount rate is 10 %. So we

  check the amount in if statement. If it is greater than 5000 then the condition is true then

  the if block is executed otherwise if amount is not greater than 5000 then the else block is

  executed.

  The analysis and the flow of the program is shown by the following flow chart.  

  The complete program code is given below :

  /* This program calculates the discount amount for a customer. As different discount

  percentage applies on different amount so program is using if statement for deciding

  which discount is applicable and display the result. */

  # include <iostream.h>

  main ( )

  {

  double amount, discount, netPayable ;

  amount = 0 ;

  netPayable = 0 ; 

  discount = 0 ;

  // prompt the user to enter the bill amount

  cout << "Please enter the amount of the bill     " ;

  cin >> amount ;

  //test the conditions and calculate net payable

  if ( amount > 5000 )

     {

         //calculate amount at 15 % discount

          discount = amount * (15.0 / 100);   

          netPayable = amount - discount;

          cout << "The discount at the rate 15 % is Rupees     " << discount << endl; 

          cout << "The payable amount is Rupees    " << netPayable ;  

                 }

             else

                {

         // calculate amount at 10 % discount

                    discount = amount * (10.0 / 100);   

                    netPayable = amount - discount;

                    cout << "The discount at the rate 10 % is Rupees     " << discount << endl ; 

        cout << "The payable amount is Rupees    " << netPayable ; 

                }

  }

  In the program we declared the variables as double. We do this to get the correct results

  (results may be in decimal points) of the calculations. Look at the statement which

  calculates the discount. The statement is 

  discount = amount * (15.0  / 100) ; 

  Here in the above statement we write 15.0 instead of 15. If we write here 15  then the

  division 15 / 100 will be evaluated as integer division and the result of division (0.15)

  will be truncated and we get 0 and this will result the whole calculation to zero. So it is

  necessary to write at least one operand in decimal form to get the correct result by

  division and we should also declare the variables as float or double. We do the same in

  the line discount = amount * (10.0 / 100);

  A sample execution of the program is given below

  Please enter the amount of the bill         6500

  The discount at the rate 15 % is Rupees         975

  The payable amount is Rupees      5525

  Tips

  •  Always put the braces in an if/else structure

  •  Type the beginning and ending braces before typing inside them

  •  Indent both body statements of an if and else structure

  •  Be careful while combining the conditions with logical operators

  •  Use if/else structure instead of a number of single selection if statements