do-while

150
-8
63

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Enter marks out of 100:
Accepted 63 after 3 attempt(s).

Read the shape, because it is the one to reuse: ask, check, and repeat while the answer is unusable. The scanf return is checked separately, so that a loop cannot spin for ever on input that has ended.

That last point matters. A validation loop with no escape when the input runs out is an infinite loop, and it is the commonest way a student's program hangs in a practical examination.

The menu shape

MU's Practical 2(b) asked for a menu-driven calculator, and chapter 26 gave the switch. A real menu repeats, and a menu must be shown at least once, so it is a do-while round a switch.

#include <stdio.h>

int main(void)
{
    int choice;

    do {
        printf("1 greet\n2 count to three\n0 quit\n");
        printf("Choice: ");
        if (scanf("%d", &choice) != 1) {
            printf("\nInput ended.\n");
            return 1;
        }
        printf("\n");
        switch (choice) {
            case 1:
                printf("Hello.\n");
                break;
            case 2:
                for (int i = 1; i <= 3; i++) {
                    printf("%d ", i);
                }
                printf("\n");
                break;
            case 0:
                printf("Goodbye.\n");
                break;
            default:
                printf("%d is not on the menu.\n", choice);
                break;
        }
        printf("\n");
    } while (choice != 0);
    return 0;
}

1
2
9
0

1 greet
2 count to three
0 quit
Choice:
Hello.

1 greet
2 count to three
0 quit
Choice:
1 2 3

1 greet
2 count to three
0 quit
Choice:
9 is not on the menu.

1 greet
2 count to three
0 quit
Choice:
Goodbye.

case 0 prints its goodbye and the loop condition then ends the loop. Using break in case 0 to try to leave the loop would not work: a break inside a switch leaves the switch, not the loop around it. Chapter 30 is exactly that trap.

The do { } while (0) idiom

You will meet this and it is worth recognising once:

#define SWAP(a, b) do { int t = (a); (a) = (b); (b) = t; } while (0)

A macro that expands to several statements has to behave as one statement, so that if (x) SWAP(p, q); else ... works. A bare block would leave the semicolon at the call site dangling; do { ... } while (0) runs once and swallows the semicolon. It is not a loop. It is the only way to write a multi-statement macro safely, and chapter 22's advice still holds: prefer a function.

What this does NOT mean

A do-while is not a while written backwards. The difference is real: the body runs before the first test, so it always runs at least once.

The semicolon is not optional. It is the only control structure in C that ends with one.

while (condition); on its own is not a do-while. It is a while loop with an empty body, which is chapter 23's stray-semicolon bug.

A do-while does not guarantee valid input. It guarantees one pass. Making the input valid is the condition's job, and handling input that has ended is yours.

break inside a switch inside a do-while does not leave the loop. It leaves the switch.

Quick revision

• do statement while (condition); tests after the body, so the body always runs at least once.
• The trailing semicolon is required. It is the only loop that needs one.
• while and for may run zero times; do-while may not.
• Use it when something must happen once: asking for input, showing a menu, a retry.
• Validation shape: ask, check, repeat while the answer is unusable, with an escape for input that ends.
• A menu is a do-while round a switch, with 0 meaning quit.
• do { ... } while (0) in a macro is not a loop; it makes several statements behave as one.

Test yourself

1. How many times does this body run?

int i = 10;
do { printf("%d ", i); i++; } while (i < 5);

Once. The body runs before the condition is tested, and the condition is false the first time.

2. What is the one syntactic feature that distinguishes do-while from the other loops?

The semicolon after the while clause.

3. Write a loop that asks for a positive number and keeps asking until it gets one.

do {
    printf("Enter a positive number: ");
    if (scanf("%d", &n) != 1) return 1;
} while (n <= 0);

4. Why is a do-while the right loop for a menu?

Because the menu must be shown at least once before the user can choose anything, and then repeated for as long as they do not choose to quit.

5. In a do-while containing a switch, how do you leave the loop from inside a case?

Set the loop's condition variable so the test fails, or use a flag, or return from the function. A break leaves only the switch.

6. What is do { ... } while (0) used for?

To make a multi-statement macro behave as a single statement, so that it can be used as the body of an if without the semicolon at the call site causing a syntax error. It is not a loop.

What can be asked on this, and how to answer it

"Explain the do-while loop with syntax and an example." Give the syntax including the semicolon, say the condition is tested after the body so it runs at least once, and give a program where the condition is false from the start to prove it. Contrast with while in one line.

"Distinguish between while and do-while." while tests before the body and may run zero times; do-while tests after and always runs at least once. do-while ends with a semicolon. while suits a loop that may not need to run; do-while suits input validation and menus.

"Write a program to accept marks and validate them using a do-while loop." Give this chapter's validation program. Keep the scanf return check and say why: without it, input that has ended makes the loop spin for ever.

"Which loop is a post-test loop?" do-while. while and for are pre-test loops.