How to iterate through a std::map and std::vector using single for loop?

Question

How to iterate through a std::map<string,int> and std::vector<int> using single for loop ?

I have seen this questions but could not solve my problem.

I am trying like this

map<string,int> data;
data["Shravan"] = 1;
data["Mama"] = 2;
data["Sa1"] = 3;
data["Jhandu"] = 4;

vector<int> values = {1,2,3,4};

for(const auto& it1: data,it2 : values) {
    // Do something 
}

Edit : I can not go through one by one. Because i am using the key of std::map and value of std::vector in the same function. Which will be called inside for loop.

Both the container of same size.

Why?? And what behaviour do you want; go through one container then the other? Or can they be interleaved? — BoBTFish, Feb 10 '17 at 07:50
have a look at http://www.cplusplus.com/reference/map/map/begin/ and http://www.cplusplus.com/reference/map/map/end/ , but it is to be noted, that this is not a good practice... Similar functionality also applies to vectors — Kupto, Feb 10 '17 at 07:52
@BoBTFish : I can not go through one by one. Because i am using the key of `std::map` and value of `std::vector` in the same function. — Shravan40, Feb 10 '17 at 07:54
There are may good answers, I wish i could accept all. But I have to obey the SO rules, hence accepting one. — Shravan40, Feb 10 '17 at 08:56

Paul Evans · Accepted Answer · 2017-02-10T08:07:16.743

3

If you know there's both the same length, use something like:

auto vit = begin(value);
auto mit = begin(data);
for (; vit != end(value); ++mit, ++vit) {
    // Use mit and vit
}

edited Feb 10 '17 at 08:07

answered Feb 10 '17 at 07:53

Paul Evans

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Your code doesn't compile. `mit` is a reference to _const_ and `++` operator is not const. – frogatto Feb 10 '17 at 08:05
Modified answer appropriately – Paul Evans Feb 10 '17 at 08:07

frogatto · Answer 2 · 2017-02-10T08:24:28.860

2

How about a do-while? Provided that your containers aren't empty.

auto iv = std::begin(value);
auto id = std::begin(data);

do {
    // Use those iterators
} while(++iv != std::end(value) && ++id != std::end(data))

Or use while if you'd like to handle empty containers too.

auto iv = std::begin(value);
auto id = std::begin(data);

while(iv != std::end(value) && id != std::end(data)) {
    // Use those iterators

    iv++; id++;
}

edited Feb 10 '17 at 08:24

answered Feb 10 '17 at 08:00

frogatto

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3

A simple `while` handle empty containers. – Jarod42 Feb 10 '17 at 08:14
@Jarod42 Yes, my code fails on empty ones. Added it as a pre-condition :) – frogatto Feb 10 '17 at 08:18

score 1 · Answer 3 · edited May 23 '17 at 12:17

1

Consider boost::zip_iterator discussed in this answer https://stackoverflow.com/a/8513803/2210478

edited May 23 '17 at 12:17

Community

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answered Feb 10 '17 at 07:54

Chen OT

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@MikeMB Yes, you're right. The code I suggested is wrong. I've removed it. Maybe OP can check the zip_iterator still. – Chen OT Feb 10 '17 at 08:03
1

Ok, removed the down vote. If you add a usage example you get an upvote. – MikeMB Feb 10 '17 at 08:07

score 1 · Answer 4 · answered Feb 10 '17 at 08:14

You can iterate over both the map and the vector and make sure to check the iterators against the end of the corresponding container.

auto map_iter = data.begin();
auto vec_iter = value.begin();
for (; map_iter != data.end() && vec_iter != value.end();
       ++map_iter, ++vec_iter) {
    // Use map_iter and vec_iter
}

How to iterate through a std::map and std::vector using single for loop?

4 Answers4

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