Iterate multiple maps with single loop

Question

I am trying to iterate two maps with one loop. it works fine with one map. when I add second map(see code below) it gives me error "Cannot deduce 'auto' type"

both variables are of same type.

what is the best way to iterate two maps.

 for ( auto& insertEntry = insertedInstances.begin(),
       auto& updateEntry = toUpdateInstances.begin(); 
       insertEntry != insertedInstances.end(); insertEntry++,updateEntry++
     )
 {
    //do something
 }

How is iteration in parallel supposed to work - is there a guarantee that `insertedInstances` and `toUpdateInstances` have the same number of elements? Also, the code doesn't seem to be advancing the `updateEntry` iterator anywhere. — user4815162342, Jun 19 '14 at 07:27

Rakib · Accepted Answer · 2014-06-19T07:50:57.947

4

it is equivalent to

for(int i=0, j=0; i<10;i++) { //do something }

so remove the extra auto&

for ( auto insertEntry = insertedInstances.begin(),
      updateEntry = toUpdateInstances.begin(); 
      insertEntry != insertedInstances.end(); 
      ++insertEntry , ++updateEntry //pre-increment might produce better code
    )
{
  //do something
}

you can not use same type twice with comma operator.

edited Jun 19 '14 at 07:50

answered Jun 19 '14 at 07:25

Rakib

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@MuhammadZaighum, look [here](http://stackoverflow.com/questions/24901/is-there-a-performance-difference-between-i-and-i-in-c) and [here](http://stackoverflow.com/questions/2020184/preincrement-faster-than-postincrement-in-c-true-if-yes-why-is-it) – Rakib Jun 19 '14 at 07:35
@RakibulHasan, `auto& insertEntry = ...` is not good. Try to compile some code with that construct. It should be `auto insertEntry = ...`. – R Sahu Jun 19 '14 at 07:42

score 3 · Answer 2 · answered Jun 19 '14 at 07:27

3

You can't have multiple type declarators inside the loop. In your case you use auto& twice. As long as both maps have the same type you could used

for (auto insertEntry = insertInstances.begin(), updatedEntry = insertInstances.begin();
     insertEntry != insertInstances.end(); ++insertEntry, ++updateEntry) {
    ...
}

(you probably don't want to use auto& in this context but auto anyway).

answered Jun 19 '14 at 07:27

Dietmar Kühl

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I think you need to increment the second iterator at some point. – juanchopanza Jun 19 '14 at 07:29
@juanchopanza: there wasn't an increment for the second iterator anywhere in the original code. I don't know what these iterators are being used for, i.e., I don't speculate what is done in the `...` part. OK, just saw the update to the question and updated my answer correspondingly – Dietmar Kühl Jun 19 '14 at 07:31
Fair enough! But I suspect it is an oversight or a but in OP's code. – juanchopanza Jun 19 '14 at 07:32
is there a better way to Iterate two maps ? I have two maps with same number of content but one of them contains ids that I want to copy to the other and do some operations – Muhammad Zaighum Jun 19 '14 at 07:35
@MuhammadZaighum This way seems perfectly fine to me. This kind of flexibility is what iterators and the generic `for` loop are for. – user4815162342 Jun 19 '14 at 07:56
@MuhammadZaighum "one of them contains ids that I want to copy to the other" - you should think about whether those insertions will always be before the node you've iterated to in that destination container - if not, you'll have to design your code to handle visiting the newly inserted node, and also do something to ensure the check for end doesn't trigger prematurely or too late.... – Tony Delroy Jun 19 '14 at 08:06

Iterate multiple maps with single loop

2 Answers2