Array Length for IndexOutOfRangeException int Array while

Do loop

The C# "do" keyword begins a loop. The loop body comes before its condition (which is specified in a while expression). Every looping construct has advantages.

This loop is rarely needed—it probably has more disadvantages than advantages. We use the do-while loop (but we probably should not).

Example program

First we use the do-while loop to sum the values of the elements in an int array. The array here is known to have 4 elements, so we can avoid checking its length.

Part 1 The code declares an integer array of 4 values. These are used inside the do-loop.

Part 2 A do-while loop is executed. The length of the array is not checked before adding the first element's value to the sum.

Part 3 This code would throw an exception if the array was empty. The array would be accessed out-of-range.

Info The while is executed after the code is run. No condition is tested before the statements in the loop are executed the first time.

class Program
{
    static void Main()
    {
        // Part 1: new int array.
        int[] ids = new int[] { 6, 7, 8, 10 };

        // Part 2: use do-while loop to sum numbers in 4-element array.
        int sum = 0;
        int i = 0;
        do
        {
            // Part 3: add to sum.
            sum += ids[i];
            i++;
        } while (i < 4);

        System.Console.WriteLine(sum);
    }
}
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Loops compared

In C# the most common type of loop is probably the for-loop. It allows you to specify the 3 conditions right at the start.

Here We modify a for-loop—we turn it into a do-while loop. We see how the loops compare to each other.

Version 1 This loop walks through the indexes 0, 1, 2, 3 and 4. The sum of these numbers is 10, which is the result of the SumFor method.

Version 2 This loop also walks through the indexes 0, 1, 2, 3 and 4. But it will evaluate the index 0 without checking any bounds.

Version 3 The body statements of this loop are the same as in the do-while loop. It has the iteration statement at the start.

using System;

class Program
{
    static int SumFor()
    {
        // Version 1: for loop.
        int sum = 0;
        for (int i = 0; i < 5; i++)
        {
            sum += i;
        }
        return sum;
    }

    static int SumDoWhile()
    {
        // Version 2: do-while loop.
        int sum = 0;
        int i = 0;
        do
        {
            sum += i;
            i++;
        } while (i < 5);
        return sum;
    }

    static int SumWhile()
    {
        // Version 3: while loop.
        int sum = 0;
        int i = 0;
        while (i < 5)
        {
            sum += i;
            i++;
        }
        return sum;
    }

    static void Main()
    {
        Console.WriteLine(SumFor());
        Console.WriteLine(SumDoWhile());
        Console.WriteLine(SumWhile());
    }
}10
10
10

Do-while, first iteration

Consider this program: it starts its iteration variable (value) at a constant. And it continues until the constant value 6 is reached.

So We know beforehand that the first iteration will always run. The do-while loop will avoid the first bounds check.

Note This theoretically could be faster than another kind of loop, but in practice the do-while does not help.

using System;

class Program
{
    static void Main()
    {
        // We start at a constant, so we know the first iteration will always be run.
        // ... This reduces the number of checks by 1.
        int value = 1;
        do
        {
            value++;
            Console.WriteLine("DO WHILE: " + value);
        }
        while (value <= 5);
    }
}DO WHILE: 2
DO WHILE: 3
DO WHILE: 4
DO WHILE: 5
DO WHILE: 6

A discussion

The critical difference between the for loop and the do-while loop is that in do-while, no conditions are checked before entering the loop.

Also The while-loop is the same as the do-while loop, but with the condition at the start.

Notes, first iteration

This difference in the first iteration is important. All other loops have a test before the first iteration. Think about the first iteration carefully.

Warning The do-while loop is much less common. This makes errors due to human factors more likely.

Summary

It is reasonable to avoid do-while loops—they are less familiar to developers. For most situations, using for-loops in all possible places is probably best.