A program can test multiple cases by placing if…else statements inside other if…else statements to create nested if …else statements. For example, the following pseudocode represents a nested if…else that prints A for exam grades greater than or equal to 90, B for grades 80 to 89, C for grades 70 to 79, D for grades 60 to 69 and F for all other grades:

If student’s grade is greater than or equal to 90
      Print “A”
else
      If student’s grade is greater than or equal to 80
            Print “B”
      else
            If student’s grade is greater than or equal to 70
                  Print “C”
            else
                  If student’s grade is greater than or equal to 60
                        Print “D”
                  else
                        Print “F”

We use shading to highlight the nesting. This pseudocode may be written in C++ as

if (studentGrade >= 90) {
   cout << "A";
}
else {
   if (studentGrade >= 80) {
      cout << "B";
   }
   else {
      if (studentGrade >= 70) {
         cout << "C";
      }
      else {
         if (studentGrade >= 60) {
            cout << "D";
         }
         else {
            cout << "F";
         }
      }
   }
}

If variable studentGrade is greater than or equal to 90, the first four conditions in the nested if…else statement will be true, but only the statement in the if part of the first if…else statement will execute. After that statement executes, the else part of the “outermost” if…else statement is skipped. Many programmers prefer to write the preceding nested if…else statement in the following form, which is identical except for the spacing and intentation that the compiler ignores:

if (studentGrade >= 90) {
   cout << "A";
}
else if (studentGrade >= 80) {
   cout << "B";
}
else if (studentGrade >= 70) {
   cout << "C";
}
else if (studentGrade >= 60) {
   cout << "D";
}
else {
   cout << "F";
}

The latter form avoids deep indentation of the code to the right. Such indentation often leaves little room on a line of code, forcing lines to wrap.

Throughout the text, we always enclose control statement bodies in braces ({ and }). This avoids a logic error called the “dangling-else” problem. We investigate this problem in Exercises 4.23–4.25.

The if statement normally expects only one statement in its body. To include several statements in the body of an if (or the body of an else for an if…else statement), enclose the statements in braces. As we’ve done throughout the text, it’s good practice to always use the braces. Statements contained in a pair of braces (such as the body of a control statement or function) form a block. A block can be placed anywhere in a function that a single statement can be placed.

The following example includes a block of multiple statements in the else part of an if…else statement:

if (grade >= 60) {
   cout << "Passed";
}
else
{
   cout << "Failed
";
   cout << "You must take this course again.";
}

In this case, if grade is less than 60, the program executes both statements in the body of the else and prints

Failed
You must take this course again.

Without the braces surrounding the two statements in the else clause, the statement

cout << "You must take this course again.";

would be outside the body of the else part of the if…else statement and would execute regardless of whether the grade was less than 60.

C++ provides the conditional operator (?:) that can be used in place of an if…else statement. This can make your code shorter and clearer. The conditional operator is C++’s only ternary operator (i.e., an operator that takes three operands). Together, the operands and the ?: symbol form a conditional expression. For example, the statement

cout << (studentGrade >= 60 ? "Passed" : "Failed");

prints the value of the conditional expression. The first operand (to the left of the ?) is a condition, the second operand (between the ? and :) is the value of the conditional expression if the condition is true and the third operand (to the right of the :) is the value of the conditional expression if the condition is false. The conditional expression in this statement evaluates to the string "Passed" if the condition

studentGrade >= 60

is true and to the string "Failed" if it’s false. Thus, this statement with the conditional operator performs essentially the same function as the first if…else statement in Section 4.6. The precedence of the conditional operator is low, so the entire conditional expression is normally placed in parentheses. We’ll see that conditional expressions can be used in some situations where if…else statements cannot.

The values in a conditional expression also can be actions to execute. For example, the following conditional expression also prints "Passed" or "Failed":

grade >= 60 ? cout << "Passed" : cout << "Failed";

The preceding is read, “If grade is greater than or equal to 60, then cout << "Passed"; otherwise, cout << "Failed".” This is comparable to an if…else statement. Conditional expressions can appear in some program locations where if…else statements cannot.