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.
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);
}
}31
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.
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.
Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.
Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
This page was last updated on Jan 20, 2024 (edit).
