New ISO scoping rule for " for LOOP"

Question

I have a piece of code that was written by someone else before the New ISO come into effect.

The for LOOP in for (pa=a.begin(), i=0; pa != a.end(); ++pa) has a little trouble executing because of the i=0 part of the syntax. Also, I had to prefix the other for loop syntaxes to read for ( int i .....) with the int before the i. However, I don't know how to fix the int i=0 in this line: for (pa=a.begin ( ), i=0; pa != a.end ( ); ++pa). Please help me out.

  for ( int i = 0; pa != a.end(); ++pa)
      *pa = ++i;

  for (int i=0; i<10; i++)
      std::cout << "a[" << i << "]=" << a[i] << std::endl;

  // int i;  // note that this will work, but I do not want this extra line.

  for (pa=a.begin(), i=0; pa != a.end(); ++pa)
      std::cout << "a[" << i++ << "]=" << *pa << std::endl;

It was written before the first standard was finished in 1997. And I doubt that just because you don't want that extra line, people will change the language to not require it... — PlasmaHH, Jul 04 '12 at 12:44
You could use a `std::pair` or a struct, but that line is a lot easier to read. — chris, Jul 04 '12 at 12:45
points well taken, but I was wondering if there is a more "compact" syntax, something to the effects of : for (pa=a.begin(), int i=0; .......), but the int i=0; declaration here won't work because of the comma preceding the "int". Any further clarification and help will be greatly appreciated. — Zzz Zz, Jul 04 '12 at 12:49
See here: http://stackoverflow.com/questions/2687392/is-it-possible-to-declare-two-variables-of-different-types-in-a-for-loop?lq=1 — jrok, Jul 04 '12 at 12:50
Check this stack overflow link : http://stackoverflow.com/questions/2687392/is-it-possible-to-declare-two-variables-of-different-types-in-a-for-loop — Santhosh, Jul 04 '12 at 13:12
That *New ISO* has something like 14 years. The *new new ISO* was approved last year. — David Rodríguez - dribeas, Jul 04 '12 at 13:17
possible duplicate of [Can I declare variables of different types in the initialization of a for loop?](http://stackoverflow.com/questions/8644707/can-i-declare-variables-of-different-types-in-the-initialization-of-a-for-loop) — Mechanical snail, Aug 17 '12 at 05:23

Mike Seymour · Accepted Answer · 2012-07-04T13:03:12.503

An extra declaration outside the for loop is the only sensible way to have two iteration variables of unrelated types in C++98 and later versions of the language. The initialiser can either be a single expression or a single declaration, and a declaration can't declare variables of multiple unrelated types.

If you really want a one-liner in this situation, then you could use this monstrosity:

for (int i = ((pa = a.begin()), 0); pa != a.end(); ++pa, ++i)

If you do that sort of thing regularly, then make sure that no-one who maintains your code knows where you live.

score 3 · Answer 2 · edited May 23 '17 at 11:56

No, you can declare only variables of one type inside the for. If your problem is scope then you may enclose the loop inside a block (for some nice examples look boost source code for loop macros):

for ( int i = 0; pa != a.end(); ++pa)
    *pa = ++i;

for (int i=0; i<10; i++)
    std::cout << "a[" << i << "]=" << a[i] << std::endl;

{
    int i = 0;
    for (pa=a.begin(); pa != a.end(); ++pa)
        std::cout << "a[" << i++ << "]=" << *pa << std::endl;
}

If you simply want to make it more nice then the answer is no, you can't.

EDIT
The best trick I saw to do what you need and to keep it clear is on this answer on SO. Instead of multiple variables you can use an unnamed struct declared inline:

for (struct { int i; iterator it; } d = { 0, pa.begin() }; d.it != a.end(); ++d.i, ++d.it )
    std::cout << "a[" << d.i << "]=" << *d.it << std::endl;

It's a little bit more prolix (so I wonder if what you save with the extra line is re-payed) but you make clear your intent and you keep code readable (moreover you can use it to pack any number and any type of variables).

Thank you all for all the nice thought provocative answers ! — Zzz Zz, Jul 04 '12 at 13:38

New ISO scoping rule for " for LOOP"

2 Answers2