C# While (i++ ...) when increments?

Question

Why does While loop post-increments (or post-decrements) a variable right after examination of the condition and not after the whole loop?

As in:

int x = 0;
while (x++ < 5)
Console.WriteLine(x);

The output is: 1 2 3 4 5, when I believe it should be 0 1 2 3 4. It seems it checks the condition - true - inrements right away. Is that normal behaviour, becase I've recently been practising Plain C and it's completely different.

Should I rather go with this option for more clarity?

while (x < 5)
Console.WriteLine(x);
++x;

https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/increment-operator — SLaks, Mar 12 '18 at 15:32
You're incrementing `x` before you write the line.. What do you expect? — Brandon Miller, Mar 12 '18 at 15:33
if you want to test and increment at the end then use `do, while` — pm100, Mar 12 '18 at 15:34
The pre- and post- increments are done in one operation, and have nothing to do with a loop. In the `while` condition, `x++` returns the current value of `x` (`0` in the first iteration) and increments the value. In the next line, `x` has been incremented, so you see `1` output to the console. — Rufus L, Mar 12 '18 at 15:53

score 8 · Answer 1 · answered Mar 12 '18 at 15:35

8

If you would use a for loop your expected answer would be correct: the while does not have any special inner workings like that.

for (x = 0; x < 5; x++)

is the way to write this with the most clarity.

answered Mar 12 '18 at 15:35

ikdekker

425
3
11

score 2 · Answer 2 · answered Mar 12 '18 at 15:34

The behavior is correct and ANSI-C does it in the same way, if you execute the same code.
x++ is "one command" and the increment is already done as soon as you move on to the next command. The difference between x++ and ++x is, that the later one returns the "new value" and the first one returns the "old value".
May the following be what you want:

int x = 0;
do
{
    Console.WriteLine(x++);
}
while (x < 5);

But I also think in general this specific case of "do it 5-times" is best solved with a for-loop as suggested by @ikdekker.

score 2 · Answer 3 · answered Mar 12 '18 at 15:40

Your loop is equivalent to:

int x = 0;
while (x < 5)
{
  x++;
  Console.WriteLine(x);
}

Since the incrementation will be done immeditely after the condition line (x<5).

So your second loop is the correct one (you should add braces though), or if you really want condition and increment in the same time you can use the do..while loop:

int x = 0;
do
{
  Console.WriteLine(x);
} while (++x < 5);

Josh Brown · Answer 4 · 2018-03-12T18:34:12.707

0

You can use a for loop for this as has been mentioned. You can also declare the x variable inside the for loop.

for (var x = 0; x < 5; x++)
{
    Console.WriteLine(x);
}

edited Mar 12 '18 at 18:34

answered Mar 12 '18 at 17:01

Josh Brown

1
3

score 0 · Answer 5 · answered Mar 12 '18 at 18:43

0

you are using x++ in the wrong place

Console.WriteLine(x++)

instead of

while(x++ < 5)

mark as answer if it helps

answered Mar 12 '18 at 18:43

C# While (i++ ...) when increments?

5 Answers5