Decision Statements

• C++ programs are a collection of functions each having a declaration part and a computational part which are executed sequentially, that is statement by statement.
• In practice, it becomes necessary to make some changes in the order of execution and you need the in-built decision making mechanism. This is accomplished by a set of control constructs. In accordance with the decision, control will be transferred to another part of the computational structure, inside the function.
• This decision may be, 1. One way branching. 2. Two way branching.

  
  One way branching.

• One way branching means evaluating a condition and then branching. In accordance with the test advocated, set of statements are executed. This type of decision making technique takes the form:

      If (test expression)
      {
      statement-block
      }
      statement-n;
  

• This syntax is for a simple if statement. That is, the decision is made based on a condition,
• Braces { and } are used to group declarations and statement together into a compound statement. This set of compound statement together enclosed within parenthesis is called a block.

  
  One way Branching

  
• he statement block can be a single statement or group of statements. If the logical expression is true then statement block is executed otherwise the statement block will be skipped and the statement-n will be executed. When the condition is TRUE that is when logical expression evaluates to true both the statement and the statement-n are executed.
• The test expression is made up of relational and logical operator.

  example
  # include < iostream.h > 
  void main()
  {
  int n1 ,n2;
  cout << "Enter 2 random numbers \t"; 
  cin >> n1							 
  cin >> n2;
  if (n1 == n2)
  cout << "Numbers are equal" << endl;
  }
  

  Two way branching

• Two way branching is used in situations, wherein you need to trace two mutually exclusive sets of actions. This is accomplished by if ..then ..else construct of C++, which takes the form:

      If (test expression)
        {
        statement(s) for the condition being true
        }
       else
       {
       statement(s) for the condition being false
       }
        statement-n;
  

  Two way branching

  
  
• If the test expression is true then the block of statement, immediately following the if statement is executed; otherwise the second block of statement is executed.
• Note that, in either case, either the true-block of statements or the false-block of the statement alone is executed and not both.

  Example
         void main()
          {
         int a, b;
        cout << Enter Two Number; 
        cin >> a >> b;
       if(a==b)
        {
         cout << "Both number is equal";
        }
       else
        {
         cout << "Both number is not equal";
        }
      }
  

  Nested if-then-else

• The else clause, like the if clause, may contain a compound statement. This is a more flexible control construct. Further, a clause of if statement may itself contain another if statement. This is known as nested if statement.
• The following example illustrates the nested if construct.

      If (x == 3)
        {
         if (x> 5)
          cout << "The numbers lies between 3 and 5" << endl;
         }
       else
          cout << "the number does not lie between 3 and 5" << endl;
  

  Multi-way branching

• The multiway branching can be achieved by a nested if-then-else clause.
• Thereby any number of mutually exclusive statement blocks can be accommodated.

  Example:
     if (marks> 80)
       grade = 'A';
        else if (marks > 60)
         grade = 'B';
          else if (marks> 50)
           grade = 'C';
           else
            grade = 'F';
  

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