3.5.6. `calc.cpp`

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The next example creates a simple four-function calculator to demonstrate do-while loops. Unlike for- and do-loops that perform the loop test at the loop's top, do-while loops evaluate the loop test at the loop's bottom. So, the program always executes the loop's body at least once. This behavior makes do-while loops a good choice where the loop test depends, at least in part, on a value calculated or input in the body. The example also nests a switch statement in the loop, causing two potentially confusing features. As you study the program, try to understand the case 'e' and the loop test: choice != 'e' && choice != 'E'.

Program Listing

#include <iostream>
using namespace std;

int main()
{
    char    choice;						// choice must be defined outside the do-while
    double  left;						// left and right may be defined inside the do-while
    double  right;

    do
    {								// creates a new block with a new scope
        cout << "A\tAdd\n";					// prints a menu of options
        cout << "S\tSub\n";
        cout << "M\tMult\n";
        cout << "D\tDiv\n";
        cout << "E\tExit\n";
        cout << "\nChoice?: ";

        cin >> choice;						// reads the user's choice
        cin.ignore();						// discards the new line - see the ignore function

        switch (choice)
        {
            case 'A':						// either 'A' or 'a' selects addition
            case 'a':
                cout << "enter the first operand: ";
                cin >> left;
                cout << "enter the second operand: ";
                cin >> right;
                cout << left + right << endl;
                break;

            case 'S':						// either 'S' or 's' selects subtraction
            case 's':
                cout << "enter the first operand: ";
                cin >> left;
                cout << "enter the second operand: ";
                cin >> right;
                break;

            case 'M':						// either 'M' or 'm' selects multiplication
            case 'm':
                cout << "enter the first operand: ";
                cin >> left;
                cout << "enter the second operand: ";
                cin >> right;
                cout << left * right << endl;
                break;

            case 'D':						// either 'D' or 'd' selects division
            case 'd':
                cout << "enter the first operand: ";
                cin >> left;
                cout << "enter the second operand: ";
                cin >> right;
                cout << left / right << endl;
                break;

            case 'E':						// these cases don't do anything, but without
            case 'e':						// them the program prints the default error message
                break;

            default :
                cout << "Unrecognized choice: " <<		// prints an error message if the user's input
                    choice << endl;				// doesn't match a valid choice
                break;
        }
    }								// closes the do-while block and its scope
    while (choice != 'e' && choice != 'E');			// AND or OR?

    return 0;
}

do-while example: calc.cpp. Several program features warrant close inspection:

The braces displayed in red form the do-while body and create a new scope. The do-while test, appearing outside the body, relies on the variable choice, which would be out of scope if defined inside the do-while body.
The 'E' and 'e' cases don't perform any operations but are still necessary. Imagine what happens without them: The user enters one of the letters to exit the program, but without a matching case, the default statement runs, printing a spurious and confusing error message.
If the logical-AND used in the test choice != 'e' && choice != 'E' looks confusing, consider the test's behavior if we replace it with a logical-OR. Imagine the user entering an 'e,' making choice != 'e' false, so choice != 'E' must be true. If the user enters 'E,' the result remains one true and one false value. The truth table for logical-OR (b) indicates that false OR true is true and true OR false is true. Either way, the test always evaluates to true, and the loop continues without end!

char    choice;
double  left;
double  right;
double  result;

case 'A':
case 'a':
    cout << "enter the first operand: ";
    cin >> left;
    cout << "enter the second operand: ";
    cin >> right;
    result = left + right;
    cout << result << endl;
    break;

(a)

(b)

A calc.cpp variation. The first version of calc.cpp evaluates the selected arithmetic expression in the output statement. This behavior solves the problem of creating a simple calculator but doesn't allow us to extend it by reusing a calculated value as one of the operands. The second version saves the calculated value in a new variable, the first step for implementing the proposed extension.

The program defines a new variable in function-scope. The program could also define the variable in the do-while loop but not in the switch.
The program now saves the operation's result in the new variable, where the program can use it in subsequent operations.