Programming in D – Tutorial and Reference
Ali Çehreli

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while Loop

The while loop is similar to the if statement and essentially works as a repeated if statement. Just like if, while also takes a logical expression and evaluates the block when the logical expression is true. The difference is that the while statement evaluates the logical expression and executes the expressions in the block repeatedly, as long as the logical expression is true, not just once. Repeating a block of code this way is called looping.

Here is the syntax of the while statement: 

    while (a_logical_expression) {
        // ... expression(s) to execute while true
    }

For example, the code that represents eat cookies as long as there is cookie can be coded like this: 

import std.stdio;

void main() {
    bool existsCookie = true;

    while (existsCookie) {
        writeln("Take cookie");
        writeln("Eat cookie");
    }
}

That program would continue repeating the loop because the value of existsCookie never changes from true.

while is useful when the value of the logical expression changes during the execution of the program. To see this, let's write a program that takes a number from the user as long as that number is zero or greater. Remember that the initial value of int variables is 0: 

import std.stdio;

void main() {
    int number;

    while (number >= 0) {
        write("Please enter a number: ");
        readf(" %s", &number);

        writeln("Thank you for ", number);
    }

    writeln("Exited the loop");
}

The program thanks for the provided number and exits the loop only when the number is less than zero.

The continue statement

The continue statement starts the next iteration of the loop right away, instead of executing the rest of the expressions of the block.

Let's modify the program above to be a little picky: instead of thanking for any number, let's not accept 13. The following program does not thank for 13 because in that case the continue statement makes the program go to the beginning of the loop to evaluate the logical expression again: 

import std.stdio;

void main() {
    int number;

    while (number >= 0) {
        write("Please enter a number: ");
        readf(" %s", &number);

        if (number == 13) {
            writeln("Sorry, not accepting that one...");
            continue;
        }

        writeln("Thank you for ", number);
    }

    writeln("Exited the loop");
}

We can define the behavior of that program as take numbers as long as they are greater than or equal to 0 but skip 13.

continue works with do-while, for, and foreach statements as well. We will see these features in later chapters.

The break statement

Sometimes it becomes obvious that there is no need to stay in the while loop any longer. break allows the program to exit the loop right away. The following program exits the loop as soon as it finds a special number: 

import std.stdio;

void main() {
    int number;

    while (number >= 0) {
        write("Please enter a number: ");
        readf(" %s", &number);

        if (number == 42) {
            writeln("FOUND IT!");
            break;
        }

        writeln("Thank you for ", number);
    }

    writeln("Exited the loop");
}

We can summarize this behavior as take numbers as long as they are greater than or equal to 0 or until a number is 42.

break works with do-while, for, foreach, and switch statements as well. We will see these features in later chapters.

Unconditional loop

Sometimes the logical expression is intentionally made a constant true. The break statement is a common way of exiting such unconditional loops. (Infinite loop is an alternative but not completely accurate term that means unconditional loop.)

The following program prints a menu in an unconditional loop; the only way of exiting the loop is a break statement: 

import std.stdio;

void main() {
    /* Unconditional loop, because the logical expression is always
     * true */
    while (true) {
        write("0:Exit, 1:Turkish, 2:English - Your choice? ");

        int choice;
        readf(" %s", &choice);

        if (choice == 0) {
            writeln("See you later...");
            break;   // The only exit of this loop

        } else if (choice == 1) {
            writeln("Merhaba!");

        } else if (choice == 2) {
            writeln("Hello!");

        } else {
            writeln("Sorry, I don't know that language. :/");
        }
    }
}

Note: Exceptions can terminate an unconditional loop as well. We will see exceptions in a later chapter.

Exercises
  1. The following program is designed to stay in the loop as long as the input is 3, but there is a bug: it doesn't ask for any input: 
    import std.stdio;

void main() {
    int number;

    while (number == 3) {
        write("Number? ");
        readf(" %s", &number);
    }
}

    Fix the bug. The program should stay in the loop as long as the input is 3.

  2. Make the computer help Anna and Bill play a game. First, the computer should take a number from Anna in the range from 1 to 10. The program should not accept any other number; it should ask again.

    Once the program takes a valid number from Anna, it should start taking numbers from Bill until he guesses Anna's number correctly.

    Note: The numbers that Anna enters obviously stay on the terminal and can be seen by Bill. Let's ignore this fact and write the program as an exercise of the while statement.

... the solutions