why i++ instead of ++i in for loops

**Peter van Merkerk** · Jul 19 '05, 07:44 PM

Re: why i++ instead of ++i in for loops

* snip *
[color=blue]
> Could some please explain the need to use i++ over ++i.[/color]

Strictly speaking there is no real need for i++, since "int x = i;
++i;" yields the same result as "int x = i++;". As you have noticed in
for loops with int as loop variable using ++i or i++ doesn't make a
difference with most compilers. Note that with iterators the ++i version
may very well be more efficient than i++.

--
Peter van Merkerk
peter.van.merke rk(at)dse.nl

**Rolf Magnus** · Jul 19 '05, 07:44 PM

Re: why i++ instead of ++i in for loops

newtothis wrote:
[color=blue]
>
> I have been reading various texts in C/ C++ and Java. The for lops all
> run along the lines of :
>
>
>
> int i ;
>
> for(i = 0 ; i < 4 ; i++)
>
> {
>
> ....
>
> }
>
>
>
> I understand the difference between ++i and i++, but I can not see why
> i++ is used in these loops when, as I understand it, the steping
> expression would be more alined to i = i + 1 in this type of case. The
> logic of i++ is not the same. The final result might but the
> evaluation process is not.[/color]

For builtin types, it really doesn't matter. But in C++, you can write
and operator++ for your own class. And then it might matter, becaure
postfix ++ has to create a copy of the object so that the old value can
be returned. If you don't need the return value, that copy is
unnecessary. If the compiler doesn't do named return value
optimization, that copy might even need to be copied again, and all
that just to throw the result away. The postfix operator++ for an own
class might look something like this:

MyClass MyClass::operat or++(int)
{
MyClass retval(*this); // copy the object
// do whatever is needed to "increment" the object
reutrn retval; // return the copy by value
}

while prefix ++ might look like:

MyClass& MyClass::operat or++()
{
// do whatever is needed to "increment" the object
return *this; // return a refernce to the object
}

Therefore, it's considered a good habit to always use prefix ++ if the
return value is not needed.

**Artie Gold** · Jul 19 '05, 07:44 PM

Re: why i++ instead of ++i in for loops

newtothis wrote:[color=blue]
> I have been reading various texts in C/ C++ and Java. The for lops all
> run along the lines of :
>
> int i ;
>
> for(i = 0 ; i < 4 ; i++)
>
> {
> ....
> }
>
> I understand the difference between ++i and i++, but I can not see why
> i++ is used in these loops when, as I understand it, the steping
> expression would be more alined to i = i + 1 in this type of case. The
> logic of i++ is not the same. The final result might but the evaluation
> process is not.[/color]

Good call, "newtothis" !
[color=blue]
>
> Ok I realise at the end of the loop the value of i is the same which
> ever method you use. But that does not explaine why the preference.
>
> Also from what I can see at the assembly code level
>
> int i ;
>
> for(i = 0 ; i < 4 ; ++i)
> {
> ....
> }
>
> and
>
> int i ;
>
> for(i = 0 ; i < 4 ; i++)
> {
> ....
> }
>
> are exactly the same.
>[/color]
Which, by itself is irrelevant -- and only because a compiler can see
that the return value of `i++', i.e. the original value of `i' is
thrown
away.[color=blue]
>
> Could some please explain the need to use i++ over ++i.
>[/color]
Sorry. Can't be done. ;-) There is no `need'.
What there *is* is `force of habit'. Historical artifact. Custom. Idiom.

While it is true that there is likely to be no difference in the
generated code when dealing with basic types, there often *is* a
difference -- and a potentially significant one -- when dealing with
user defined types (where `++', prefix and postfix are overloaded
operators).

Again, good call. Using the prefix form in such loops is to be
preferred -- it's just a hard habit to break!

HTH,
--ag
[color=blue]
>
> --
> Posted via http://dbforums.com[/color]

--
Artie Gold -- Austin, Texas
Oh, for the good old days of regular old SPAM.

**Andrew Koenig** · Jul 19 '05, 07:44 PM

Re: why i++ instead of ++i in for loops

> I have been reading various texts in C/ C++ and Java. The for lops all[color=blue]
> run along the lines of :[/color]
[color=blue]
> int i ;[/color]
[color=blue]
> for(i = 0 ; i < 4 ; i++)[/color]
[color=blue]
> {[/color]
[color=blue]
> ....[/color]
[color=blue]
> }[/color]
[color=blue]
> I understand the difference between ++i and i++, but I can not see why
> i++ is used in these loops when, as I understand it, the steping
> expression would be more alined to i = i + 1 in this type of case. The
> logic of i++ is not the same. The final result might but the evaluation
> process is not.[/color]
[color=blue]
> Ok I realise at the end of the loop the value of i is the same which
> ever method you use. But that does not explaine why the preference.[/color]

You have hit on one of the reasons that "Accelerate d C++" always uses the
"++i" form unless something is going to be done with the value of the
expression. So, for example, we would write

++i;

instead of

i++;

but we would write

n = a[i++];

if it were appropriate to do so--the point being that the value of "i++" is
used in the second example.

Incidentally, instead of

for (i = 0; i < 4; i++)

we would write

for (i = 0; i != 4; ++i)

because that way, we can use the same general form of loop for all kinds of
iterators in addition to integers.

**jeffc** · Jul 19 '05, 07:44 PM

Re: why i++ instead of ++i in for loops

"newtothis" <member45182@db forums.com> wrote in message
news:3531211.10 67348081@dbforu ms.com...[color=blue]
>
> I understand the difference between ++i and i++, but I can not see why
> i++ is used in these loops when, as I understand it, the steping
> expression would be more alined to i = i + 1 in this type of case. The
> logic of i++ is not the same. The final result might but the evaluation
> process is not.
> Ok I realise at the end of the loop the value of i is the same which
> ever method you use. But that does not explaine why the preference.[/color]

I think you've answered your own question. In that case, the result is the
same so it really doesn't matter. If it doesn't matter, i++ seems more
aesthetically pleasing to most people, since logically you see the variable
first, and then you see what to do with it. If these were classes with
member functions, it would look like this.

Int i(1);
i.increment();
vs.
increment().i;

The first looks more natural.

**Howard** · Jul 19 '05, 07:44 PM

Re: why i++ instead of ++i in for loops

"Andrew Koenig" <ark@acm.org> wrote in message
news:3xvnb.1974 00$0v4.15332398 @bgtnsc04-news.ops.worldn et.att.net...
[color=blue]
> ...So, for example, we would write
>
> ++i;
>
> instead of
>
> i++;
>
> but we would write
>
> n = a[i++];
>
> if it were appropriate to do so--the point being that the value of "i++"[/color]
is[color=blue]
> used in the second example.
>[/color]

What do you mean that "the value of "i++" is used in the second example"?
If you're referring to n = a[i++], then the value that is used to index into
a is i, not i++. The increment is done afterwards. If, instead, you're
just saying that you should only use i++ when you need the value of i++,
again, I don't see how you can get that value, except in a following
statement, as in

x = a[i++];
n = a[i]; // here we use the incremented value of i

I think I see what you were getting at, but your phrasing is a little
confusing.

-Howard

**Rolf Magnus** · Jul 19 '05, 07:45 PM

Re: why i++ instead of ++i in for loops

Howard wrote:
[color=blue]
>
> "Andrew Koenig" <ark@acm.org> wrote in message
> news:3xvnb.1974 00$0v4.15332398 @bgtnsc04-news.ops.worldn et.att.net...
>[color=green]
>> ...So, for example, we would write
>>
>> ++i;
>>
>> instead of
>>
>> i++;
>>
>> but we would write
>>
>> n = a[i++];
>>
>> if it were appropriate to do so--the point being that the value of
>> "i++"[/color]
> is[color=green]
>> used in the second example.
>>[/color]
>
> What do you mean that "the value of "i++" is used in the second
> example"? If you're referring to n = a[i++], then the value that is
> used to index into a is i, not i++.[/color]

i and i++ have the same value. The value of postfix++ is the value that
the operand had before the increment.
[color=blue]
> The increment is done afterwards.[/color]

Right.
[color=blue]
> If, instead, you're just saying that you should only use i++ when you
> need the value of i++, again, I don't see how you can get that value,[/color]

int a = i++;

now a has the value of i++.
[color=blue]
> except in a following statement, as in
>
> x = a[i++];
> n = a[i]; // here we use the incremented value of i
>
> I think I see what you were getting at, but your phrasing is a little
> confusing.[/color]

You seem to be confused by the term "value of i++". The value of i++ is
what you get when you e.g. assign "i++" to something, not the value of
i after i++ has been executed.

**Andrew Koenig** · Jul 19 '05, 07:45 PM

Re: why i++ instead of ++i in for loops

> > ...So, for example, we would write[color=blue][color=green]
> >
> > ++i;
> >
> > instead of
> >
> > i++;
> >
> > but we would write
> >
> > n = a[i++];
> >
> > if it were appropriate to do so--the point being that the value of "i++"
> > is used in the second example.
> >[/color]
>
> What do you mean that "the value of "i++" is used in the second example"?
> If you're referring to n = a[i++], then the value that is used to index[/color]
into[color=blue]
> a is i, not i++.[/color]

I meant exactly what I said: The value of i++ is used in the second
example. Now, as it happens, if i is an integer, then the value of i++ is a
copy of what the value of i was before i was incremented. If i is a
user-defined type, the value of i++ might be something different. Either
way, evaluating the expression a[i++] involves using the value of i++.

**Howard** · Jul 19 '05, 07:45 PM

Re: why i++ instead of ++i in for loops

"Rolf Magnus" <ramagnus@t-online.de> wrote in message
news:bnm57f$sbn $07$1@news.t-online.com...[color=blue]
> Howard wrote:
>[color=green]
> >
> > "Andrew Koenig" <ark@acm.org> wrote in message
> > news:3xvnb.1974 00$0v4.15332398 @bgtnsc04-news.ops.worldn et.att.net...
> >[color=darkred]
> >> ...So, for example, we would write
> >>
> >> ++i;
> >>
> >> instead of
> >>
> >> i++;
> >>
> >> but we would write
> >>
> >> n = a[i++];
> >>
> >> if it were appropriate to do so--the point being that the value of
> >> "i++"[/color]
> > is[color=darkred]
> >> used in the second example.
> >>[/color]
> >
> > What do you mean that "the value of "i++" is used in the second
> > example"? If you're referring to n = a[i++], then the value that is
> > used to index into a is i, not i++.[/color]
>
> i and i++ have the same value. The value of postfix++ is the value that
> the operand had before the increment.
>[color=green]
> > The increment is done afterwards.[/color]
>
> Right.
>[color=green]
> > If, instead, you're just saying that you should only use i++ when you
> > need the value of i++, again, I don't see how you can get that value,[/color]
>
> int a = i++;
>
> now a has the value of i++.
>[color=green]
> > except in a following statement, as in
> >
> > x = a[i++];
> > n = a[i]; // here we use the incremented value of i
> >
> > I think I see what you were getting at, but your phrasing is a little
> > confusing.[/color]
>
> You seem to be confused by the term "value of i++". The value of i++ is
> what you get when you e.g. assign "i++" to something, not the value of
> i after i++ has been executed.
>[/color]

That's exactly where I was confused. Which is why I asked what he meant by
"the value of i++".

It seems very strange to refer to the "value of i++" unless you were
refering to the value you get by performing the ++ operation upon the i
variable, which is obviously not the intent. To me, it would be like
referring to the "value of 7+1", and intending to refer to the value 7
(before adding one), whereas in normal conversation, you'd be talking about
the value 8, which is what 7+1 is equal to. See why it's confusing (at
least to me)?

No argument here on the "truth" of what Andrew said...just confusion on the
terminology.

-Howard

**Rolf Magnus** · Jul 19 '05, 07:45 PM

Re: why i++ instead of ++i in for loops

Howard wrote:
[color=blue]
>
> "Rolf Magnus" <ramagnus@t-online.de> wrote in message
> news:bnm57f$sbn $07$1@news.t-online.com...[color=green]
>> Howard wrote:
>>[color=darkred]
>> >
>> > "Andrew Koenig" <ark@acm.org> wrote in message
>> > news:3xvnb.1974 0[/color][/color][/color]
$0v4.15332398@b gtnsc04-news.ops.worldn et.att.net...[color=blue][color=green][color=darkred]
>> >
>> >> ...So, for example, we would write
>> >>
>> >> ++i;
>> >>
>> >> instead of
>> >>
>> >> i++;
>> >>
>> >> but we would write
>> >>
>> >> n = a[i++];
>> >>
>> >> if it were appropriate to do so--the point being that the value of
>> >> "i++"
>> > is
>> >> used in the second example.
>> >>
>> >
>> > What do you mean that "the value of "i++" is used in the second
>> > example"? If you're referring to n = a[i++], then the value that is
>> > used to index into a is i, not i++.[/color]
>>
>> i and i++ have the same value. The value of postfix++ is the value
>> that the operand had before the increment.
>>[color=darkred]
>> > The increment is done afterwards.[/color]
>>
>> Right.
>>[color=darkred]
>> > If, instead, you're just saying that you should only use i++ when
>> > you need the value of i++, again, I don't see how you can get that
>> > value,[/color]
>>
>> int a = i++;
>>
>> now a has the value of i++.
>>[color=darkred]
>> > except in a following statement, as in
>> >
>> > x = a[i++];
>> > n = a[i]; // here we use the incremented value of i
>> >
>> > I think I see what you were getting at, but your phrasing is a
>> > little confusing.[/color]
>>
>> You seem to be confused by the term "value of i++". The value of i++
>> is what you get when you e.g. assign "i++" to something, not the
>> value of i after i++ has been executed.
>>[/color]
>
> That's exactly where I was confused. Which is why I asked what he
> meant by "the value of i++".
>
> It seems very strange to refer to the "value of i++" unless you were
> refering to the value you get by performing the ++ operation upon the
> i
> variable, which is obviously not the intent. To me, it would be like
> referring to the "value of 7+1", and intending to refer to the value 7
> (before adding one), whereas in normal conversation, you'd be talking
> about
> the value 8, which is what 7+1 is equal to. See why it's confusing
> (at least to me)?[/color]

Seems logical to me:

int x = 3 + 1;

the value of 3 + 1 is 4, so x is set to 4.

int y = x;

the value of x is 4, so y is set to 4.

int f(int i)
{
return i+1;
}

y = f(x);

the value of f(x) is 5, so y is set to 5 now.

int g(int& i)
{
int retval = i;
++i;
return retval;
}

y = g(x);

the value of g(x) is 4, so y is set to 4.

z = x++;

the value of x++ is 5, so z is set to 5 now.

**Howard** · Jul 19 '05, 07:46 PM

Re: why i++ instead of ++i in for loops

"Rolf Magnus" <ramagnus@t-online.de> wrote in message
news:bnmaat$ir6 $06$1@news.t-online.com...
[color=blue]
> Seems logical to me:
>[/color]

Oh well. I find it confusing, you don't. We'll both live. :-)

-Howard

**newtothis** · Jul 19 '05, 07:46 PM

Re: why i++ instead of ++i in for loops

So from what has been said, except for may be the situation of an
oveloaded operator, the only reason for ++i vs i++ in the for loop is
programmer preference.

This then is close to the arguments of underscore vs mixedcase
vqariables.

Personal preference.

Why then dont the writeres of texts explain this instead of trying to
lay down some form of undefinable law?

Thanks for these comments as it clears up some programming "rules"
people have tried to push on me.

--
Posted via http://dbforums.com

**Duane Hebert** · Jul 19 '05, 07:46 PM

Re: why i++ instead of ++i in for loops

[color=blue]
> I think you've answered your own question. In that case, the result is[/color]
the[color=blue]
> same so it really doesn't matter. If it doesn't matter, i++ seems more
> aesthetically pleasing to most people, since logically you see the[/color]
variable[color=blue]
> first, and then you see what to do with it. If these were classes with
> member functions, it would look like this.[/color]

Actually that's not true. i++ has to create a temporary and ++i doesn't.
This may
not be a big deal when i is an int, but it gets pretty inefficient when i is
a larger type or
an iterator etc.

**Andrew Koenig** · Jul 19 '05, 07:47 PM

Re: why i++ instead of ++i in for loops

> So from what has been said, except for may be the situation of an[color=blue]
> oveloaded operator, the only reason for ++i vs i++ in the for loop is
> programmer preference.[/color]

Not quite. There's no difference in practice if i is an integer variable,
but if it's an iterator, there may be a performance difference. There is
also the philosophical question of whether it's a good thing to ask the
machine to do work that you know is going to be discarded.
[color=blue]
> Why then dont the writeres of texts explain this instead of trying to
> lay down some form of undefinable law?[/color]

Some of them do.

why i++ instead of ++i in for loops

why i++ instead of ++i in for loops

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