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POHow to iterate through map, modify map but restore at each iteration?
 

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  1. POHow to iterate through map, modify map but restore at each iteration?
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    <p>I have a <code>std::map&lt;int,int&gt;</code> lets call it <code>my_map</code></p> <p>I iterate through this map using iterators and a for loop.</p> <p>Within each iteration I want to modify many elements in this map but restore it again to its original values for next iteration of the loop.</p> <p>I thought I could create a temporary copy of the iterator <code>my_temp_map</code> , but then I wouldn't be able to use the iterator to find the element I ought to be working on.</p> <p>I then thought I could create a temporary copy, work on the origin <code>my_map</code> and at the end of each loop restore the original back to the temporary copy. However I believe this would invalidate the iterators as an assignment deletes all elements</p> <p>How does one solve this problem?</p> <p><strong>Code added</strong></p> <p>So each inner loop will modify current_partition (and there is some more absent code that will store the result of the modified current_partition), but after each inner_loop I need current_loop to be restored to its former self.</p> <pre><code>std::map&lt;int,int&gt; current_partition = bitset_to_map(centre-&gt;second-&gt;bit_partitions); int num_parts = (std::max_element(current_partition.begin(), current_partition.end(),value_comparer))-&gt;second; for (std::map&lt;int,int&gt;::iterator itr = current_partition.begin(); itr != current_partition.end(); ++itr) { for (int next_part = 0; next_part&lt;num_parts+1; ++next_part) { if (next_part != itr-&gt;second) { int current_part = itr-&gt;second; itr-&gt;second = next_part; std::vector&lt;int&gt; first_changed_part, last_changed_part; for (std::map&lt;int,int&gt;::iterator new_itr = current_partition.begin(); new_itr != current_partition.end(); ++new_itr) { if (new_itr-&gt;second == current_part) first_changed_part.push_back(new_itr-&gt;first); if (new_itr-&gt;second == next_part) last_changed_part.push_back(new_itr-&gt;first); } } } } </code></pre>
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      1. POHow to iterate through map, modify map but restore at each iteration?
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    1. COYour question doesn't make a whole lot of sense. The whole premise is a little weird (you're modifying a bunch of data and then discarding it?), but the main issue is that you keep saying "the iterators" and it's not clear which iterators you mean. In your first example, you could certainly use an iterator into `my_temp_map` to do what you want -- are you talking about some other iterator? In the second example, which iterators are you worried about invalidating? Some code (or pseudo-code) would probably be helpful here.
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      1. POHow to iterate through map, modify map but restore at each iteration?
      1. USTyler McHenry
 

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