Multiway if-else-Statement

1. The syntax of the multi-way if-else-statement is:
   
   if (Boolean_Expression_1)
       Statement_1
   else if (Boolean_Expression_2)
       Statement_2
   .
   .
   .
   else if (Boolean_Expression_n)
       Statement_n
   else
       Statement_for_all_other_possibilities
   
   The Boolean expression are checked one after the other until the first true Boolean expression is encountered, then the corresponding statement is executed. Program control is then passed to the statement after the multiway if-else-statement. If none of the Boolean expression is true then the statement after the last else is executed.
   
   
2. The else and the statement associated with it are optional (not required to use them).
   
   
3. There can be any number (including zero) of if-else blocks.
   
   
4. Again any statement can be replaced by a compound statement or a block of statements.
   
   
5. For example, consider writing a computerized number guessing game in which the program picks a random number between 1 and 10, and the player has 3 chances to take a guess at the number. Each time the player's guess is higher than the answer, the program will respond "too high", and if the number is lower than the answer, the program will respond with "too low", and if the guess is the same as the answer, the program responds with "correct!".
   
   Suppose the player's choice is stored in the variable guess, and the correct number is stored in the variable answer. You could code that as follows:
   
   if (guess > answer)
       cout << "Too high!\n";
   if (guess < answer)
       cout << "Too low!\n";
   if (guess == answer)
       cout << "Correct! You've just won a million dollars!\n";
   
   The code will do the job, but in an inefficient way. There are three separate if statements involving three separate conditions that must always be tested.
   
   A better way is to use a multiway if-else-statement to write:
   
   if (guess > answer)
       cout << "Too high!\n";
   else if (guess < answer)
       cout << "Too low!\n";
   else
       cout << "Correct! You've just won a million dollars!\n";
   
   Note that not all of the three conditions need to be tested. Since the three choices are mutually exclusive (i.e. one and only one of the conditions can be true), the last condition never needs to be tested.
   
   
6. For better efficiency, try to arrange the cases so that the most probably ones are tested first. For example if the median score on the first exam is 52, meaning that half of the student has a score higher than 52 and the other half has a score lower than 52, but the average score to pass the course is fixed at 70, then it is better to write:
   
   if (score < 60)
       cout << "You need to do better the next exam!\n";
   else if (score < 75)
       cout << "You are barely passing so far.\n";
   else if (score < 90)
       cout << "You are doing well so far.\n";
   else
       cout << "Excellent work! Keep it up!\n";
   
   
   
7. /* Given that the lowest scores for an 'A', a 'B', a 'C', and a 'D' are 91, 78, 64, 51, respectively, write a program to determine the letter grade of a student. The program needs to input the first name, the last name, the gender, and the score for a student. The program outputs the name of the student together with the letter grade and an appropriate comment. */
   
   #include <iostream>
   #include <string>
   
   using namespace std;
   
   // Typically variable names for constants are written in all upper cases.
   const int lowestScoreForA(91), lowestScoreForB(78), lowestScoreForC(64), lowestScoreForD(52);
   const string commentA("You did a superb job!"),
           commentB("You did a good job!"),
           commentC("You did an average work."),
           commentD("You did a poor job."),
           commentF("You failed the course.");
   
   int main( )
   {
       string lastName, firstName, comment;
       char gender, letterGrade;
       int score;
   
       // input last name and first name
       cout << "Enter your first name: ";
       cin >> firstName;
       cout << "Enter your last name: ";
       cin >> lastName;
   
       // input gender
       cout << "Enter \'M\' for male or \'F\' for female: ";
       cin >> gender;
   
       // input score
       cout << "Enter your score: ";
       cin >> score;
   
       // compute the letter grade
       if (score >= lowestScoreForA)
           {
               letterGrade = 'A';
               comment = commentA;
           }
       else if (score >= lowestScoreForB)
           {
               letterGrade = 'B';
               comment = commentB;
           }
       else if (score >= lowestScoreForC)
           {
               letterGrade = 'C';
               comment = commentC;
           }
       else if (score >= lowestScoreForD)
           {
               letterGrade = 'D';
               comment = commentD;
           }
       else
           {
               letterGrade = 'F';
               comment = commentF;
           }
   
       // output results
       if (gender == 'M')
           cout << "Mr. ";
       else
           cout << "Ms. ";
   
       cout << firstName << ' ' << lastName << endl;
       cout << "Your score of " << score <<
                   " gives you a letter grade of " << letterGrade << endl;
       cout << comment << endl << endl;
   
   }