How to write a for loop that uses both an iterator and an index counter?

Question

I have a function that looks like this:

void myclass::myfunc()
{
    int i;
    for( std::vector<Foo>::iterator it = var.begin(), i = 0; it < var.end(); it++, i++ )
    {
        /* ... */
    }
}

I'm getting this error:

Cannot convert from int to std::_Vector_iterator<>

What is wrong with this code?

Use `++it` in order to increment iterator, because `it++` is the post-increment, and takes unnecessary time. — marbel82, Jun 10 '13 at 10:13

templatetypedef · Answer 1 · 2020-12-11T18:35:22.103

The issue is with this part of the for loop:

std::vector<Foo>::iterator it = var.begin(), i = 0

C++ is interpreting this not as two comma-separated statements, but as a variable declaration for a variable named it that's an iterator, and as a new declaration of a variable i that's an iterator and initialized to 0. The error is because you can't initialize a vector iterator to 0.

To fix this, you'll need to hoist the definition outside of the loop:

int i = 0;
std::vector<Foo>::iterator it = var.begin();
for(; it < var.end(); it++, i++ )
{
     // ...
}

Or move the initialization of i outside the loop:

int i = 0;
for( std::vector<Foo>::iterator it = var.begin(); it < var.end(); it++, i++ )
{
    // ...
}

Here's another option. If you need to keep track of the index into the vector you're currently looking at, you could consider just using a counting for loop (without the iterator), or using just the iterator and using iterator subtraction to recover the index:

for (auto it = var.begin(); it != var.end(); ++it) {
    // current position is it - var.begin();
}

And, finally, if you have a C++20-compliant compiler, you could eliminate the iterator entirely and use an enhanced for loop in the following way:

/* Requires C++20 */
for (int i = 0; Foo f: var) {
    /* Do something worthwhile with f. */

    i++;
}

Hope this helps!

I thought that since I already initialized the iterator it will be variable initialization. Guess I was wrong. I will be using the fix #2. Thank you. — Igor, Jun 10 '13 at 04:35

score 13 · Accepted Answer · edited Jun 10 '13 at 09:16

13

You can do it like this:

int i = 0;
for( std::vector<int>::iterator it = v.begin(); it < v.end(); ++it, ++i){}

edited Jun 10 '13 at 09:16

Emilio Garavaglia

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answered Jun 10 '13 at 03:12

Andrey Tuganov

351
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Marking this as an answer because it is the first one. Thank you. – Igor Jun 10 '13 at 04:36
2

why the inconsistency of `it++` and `++i`? – TemplateRex Jun 10 '13 at 06:39
7

Actually, it would be better to only use the pre-increment; while it matters little for `i`, it might matter for `it` depending on how *fat* the iterator is. – Matthieu M. Jun 10 '13 at 06:49
1

@TobiasKienzler, it's not easy for a language to be as unreadable as C++ is ;) – Shahbaz Jun 10 '13 at 14:23
1

@Shahbaz Indeed, it may even have to be specifically [designed](https://en.wikipedia.org/wiki/Malbolge) :-7 – Tobias Kienzler Jun 10 '13 at 14:53

Jerry Coffin · Answer 3 · 2013-06-10T03:15:13.383

2

Get rid of the i=0; part (at least inside the loop header).

Also, if you insist on doing this at all, consider using:

for (auto it : var)

or:

for (auto it = var.begin(); it != var.end(); ++it)

...instead. Since you're using a random access iterator anyway, what you have as i is equivalent to it - var.begin(). Conversely, you could just use:

for (int i=0; i<var.size(); i++)

...and get an iterator when needed as var.begin() + i.

Depending on what's in the body of the loop, you probably want to get rid of the loop entirely, and replace it with an algorithm though.

edited Jun 10 '13 at 03:15

answered Jun 10 '13 at 03:09

Jerry Coffin

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I'd love to use "auto", but I'm afraid that it's not compatible with OS X 10.6 XCode. Also I'm using iterators for efficiency, otherwise I'd definitely use just you 3rd form. Also what do you mean by "get rid of the loop". Could you clarify on how do I go thru the vector? – Igor Jun 10 '13 at 04:32

score 1 · Answer 4 · answered Jun 10 '13 at 04:49

Double iteration:

using std::begin; using std::end;
for (auto p = std::make_pair( begin(var), 0 ); p.first != end(var); ++p.first, ++p.second ) {
  /* ... */
}

double iteration with named indexes/iterators:

using std::begin; using std::end;
int i;
std::vector<Foo>::iterator it;
for (std::tie( it, i ) = std::make_pair( begin(var), 0 ); it != end(var); ++it, ++i ) {
  /* ... */
}

or bind the above pair on each iteration to better named variables:

using std::begin; using std::end;
for (auto p = std::make_pair( begin(var), 0 ); p.first != end(var); ++p.first, ++p.second ) {
  auto const& it = p.first;
  int i = p.second;
}

How to write a for loop that uses both an iterator and an index counter?

4 Answers4