std::map key value pairs not iterating in sequence

Question

I am inserting key value pairs in a std::map as

key = //something;
value = //something;
demoMap[key] = value;

Printing the key and value here gives me correct results. However, when I iterate this map as:

for( std::map<std::string, std::string>::iterator it = demoMap.begin();
     it != demoMap.end(); it++ ) {
    std::cout << it->first + "," << it->second;
}

Using above iteration, I get the second key value pair printed before the first one. Why is it so? The first key value pair should be printed first since the iterator of the map is set to begin() for the map.

Since you do not show the data the you are storing or the order you fill the map I can only guess that you are seeing that the map sorts by key but your filling of the map is not in key order. — NathanOliver, Jun 14 '16 at 17:30
Note that you don't print new line, or any separator between `second` and next `first` — Jarod42, Jun 14 '16 at 17:30
std::map doesn't preserve the order of element insertion. Maybe you want std::vector ? — nephtes, Jun 14 '16 at 17:30
@πάνταῥεῖ ah yes, forgot about std::pair. Good call. — nephtes, Jun 14 '16 at 20:09

score 4 · Accepted Answer · edited Jun 14 '16 at 17:35

4

A std::map is not ordered by insertion order, but through ordering of the keys which is by default obtained with operator< on the keys.

For std::string the default operator< uses lexicographical ordering, so keys will be iterated in that order only.

edited Jun 14 '16 at 17:35

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answered Jun 14 '16 at 17:32

Jack

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std::map key value pairs not iterating in sequence

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