Looping through an array via iterator in C++

Question

Supposed I have a graph defined in such way,

unordered_map<int, unordered_set<int>> graph = {
    { 0, { 1, 2, 3 } },
    { 1, { 3, 4, 5 } },
    { 2, { 6, 5, 4 } }
};

And I want to empty the hash set of each entry in graph. I do two options here,

A. for(auto v = graph.begin(); v != graph.end(); ++v) v->second.clear();

B. for(auto v : graph) v.second.clear();

I see A works but B does not. I do not quite understand. My theory is, the way B is doing, v is a copy of the actual element. So it cannot clear the actual hash set.

Need help. Thanks!

eerorika · Accepted Answer · 2015-09-27 22:52:01Z

4

My theory is, the way B is doing, v is a copy of the actual element

Your theory is correct.

Since you want to modify the original, iterate with a reference instead of a copy:

for(auto& v : graph) v.second.clear();

answered Sep 27, 2015 at 22:52

eerorika

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4 Comments

CodingFanSteve Over a year ago

Thanks, it works! And I have another question. According to my test, the running time of for(auto &v : graph) is longer than for(auto v : graph). I am confused here. The first way does not require making a copy of graph, v is just a reference. It will make more sense to me that the second one runs longer.

CodingFanSteve Over a year ago

I see probably the longer time is due to the server is busier at the moment.

eerorika Over a year ago

@CodingForFun15 Perhaps the compiler can deduce that clearing the temporary copies has no effect on the program, so it decides to remove the loop entirely.

CodingFanSteve Over a year ago

@ user2079303 Make sense. Thanks!

Francis Cugler · Accepted Answer · 2015-09-29 15:21:40Z

0

If you are working exclusively on a Windows Platform and have VS2012 or newer this method is also available to use.

typedef unordered_map<int, unordered_set<int>> Graph;

Graph myGraph = {
    { 0, { 1, 2, 3 } },
    { 1, { 3, 4, 5 } },
    { 2, { 6, 5, 4 } }
};

for each ( Graph& graph in myGraph ) {
    // Do Work Here!
    graph.second.clear();
}

answered Sep 29, 2015 at 15:21

Francis Cugler

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