C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  why i++ instead of ++i in for loops Goto page 1, 2  Next          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message newtothis Guest Posted: Tue Oct 28, 2003 1:34 pm    Post subject: why i++ instead of ++i in for loops 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. 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. Could some please explain the need to use i++ over ++i. -- Posted via http://dbforums.com Back to top Peter van Merkerk Guest Posted: Tue Oct 28, 2003 2:20 pm    Post subject: Re: why i++ instead of ++i in for loops * snip * Quote: Could some please explain the need to use i++ over ++i. 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.merkerk(at)dse.nl Back to top Artie Gold Guest Posted: Tue Oct 28, 2003 2:27 pm    Post subject: Re: why i++ instead of ++i in for loops newtothis wrote: Quote: 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. Good call, "newtothis"! Quote: 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. 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. Quote: Could some please explain the need to use i++ over ++i. 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 Quote: -- Posted via http://dbforums.com -- Artie Gold -- Austin, Texas Oh, for the good old days of regular old SPAM. Back to top Andrew Koenig Guest Posted: Tue Oct 28, 2003 3:01 pm    Post subject: Re: why i++ instead of ++i in for loops Quote: 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. 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. You have hit on one of the reasons that "Accelerated 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. Back to top jeffc Guest Posted: Tue Oct 28, 2003 3:16 pm    Post subject: Re: why i++ instead of ++i in for loops "newtothis" <member45182 (AT) dbforums (DOT) com> wrote Quote: 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. 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. Back to top Howard Guest Posted: Tue Oct 28, 2003 3:26 pm    Post subject: Re: why i++ instead of ++i in for loops "Andrew Koenig" <ark (AT) acm (DOT) org> wrote Quote: ...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++. 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 Back to top Andrew Koenig Guest Posted: Tue Oct 28, 2003 4:33 pm    Post subject: Re: why i++ instead of ++i in for loops Quote: ...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++. 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++. Back to top Howard Guest Posted: Tue Oct 28, 2003 5:38 pm    Post subject: Re: why i++ instead of ++i in for loops "Rolf Magnus" <ramagnus (AT) t-online (DOT) de> wrote Quote: Howard wrote: "Andrew Koenig" <ark (AT) acm (DOT) org> wrote in message news:3xvnb.197400$0v4.15332398 (AT) bgtnsc04-news (DOT) ops.worldnet.att.net... ...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++. i and i++ have the same value. The value of postfix++ is the value that the operand had before the increment. The increment is done afterwards. Right. 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, int a = i++; now a has the value of i++. 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. 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. 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 Back to top Howard Guest Posted: Tue Oct 28, 2003 7:22 pm    Post subject: Re: why i++ instead of ++i in for loops "Rolf Magnus" <ramagnus (AT) t-online (DOT) de> wrote Quote: Seems logical to me: Oh well. I find it confusing, you don't. We'll both live. :-) -Howard Back to top newtothis Guest Posted: Tue Oct 28, 2003 11:11 pm    Post subject: 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 Back to top Duane Hebert Guest Posted: Wed Oct 29, 2003 1:21 am    Post subject: Re: why i++ instead of ++i in for loops Quote: 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. 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. Back to top Andrew Koenig Guest Posted: Wed Oct 29, 2003 7:29 am    Post subject: Re: why i++ instead of ++i in for loops Quote: 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. 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. Quote: Why then dont the writeres of texts explain this instead of trying to lay down some form of undefinable law? Some of them do. Back to top Peter Koch Larsen Guest Posted: Wed Oct 29, 2003 9:01 am    Post subject: Re: why i++ instead of ++i in for loops newtothis <member45182 (AT) dbforums (DOT) com> wrote Quote: 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. Yes. Quote: This then is close to the arguments of underscore vs mixedcase vqariables. Not at all. In situations as the one you mentioned, you usually acquire a habit of either writing i++ or ++i. If you write ++i you are assured of optimal performance, if you write i++, you're not. Thus you should always prefer the ++i version. Quote: Personal preference. Why then dont the writeres of texts explain this instead of trying to lay down some form of undefinable law? I do believe that most good textbooks will tell you that you should prefer ++i. At least they ought to do so. Quote: Thanks for these comments as it clears up some programming "rules" people have tried to push on me. Kind regards Peter Koch Larsen Back to top Rolf Magnus Guest Posted: Wed Oct 29, 2003 1:31 pm    Post subject: Re: why i++ instead of ++i in for loops newtothis wrote: Quote: 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. 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::operator++(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::operator++() { // 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. Back to top newtothis Guest Posted: Wed Oct 29, 2003 1:32 pm    Post subject: Re: why i++ instead of ++i in for loops Thanks for all the comments. I did not intend to cause a general stir. This though has confirmed that my use of ++i, when appropiate, in such loops has been correct. I realise at times the use of i++ is appropiate and is dependent on the intent of the design and code. I now just have to get over the issues of mixed case or underscores in variables. That seems to have been discussed at some length anyway. 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