About using range based for loop over a vector of vectors

Question

I am working on some graph problem. I have:

vector<vector<int>> e

that I populate as:

for(vector<int> edge: edges) { e[edge[0]].push_back(edge[1]); e[edge[1]].push_back(edge[0]); }

Now when I try to access e using a range based for loop like:

for(vector<int> v: e[node])

I get an error:

no viable conversion from int to vector<int>

Which I guess means I should use:

for(int i: e[node])

How - isn't each element of e a vector?

Open ended - sometimes I find it difficult to visualize complex data-structures like vector of unordered_maps, etc. Any resources that would teach me to visualize them? — J. Doe
– J. Doe, Commented May 13, 2020 at 4:41
You're not iterating over e, you're iterating over one element of e (a vector), and each element of that element of e is an int. — 1201ProgramAlarm
– 1201ProgramAlarm, Commented May 13, 2020 at 4:43
Yes, so each element of e is a vector, so shouldn't I have to use vector<int> instead of just int? — J. Doe
– J. Doe, Commented May 13, 2020 at 4:44
Teachyou to visualize, I dunno. I just use a pencil (a 0.5mm Papermate G-Force) and paper (no special requirements). — user4581301
– user4581301, Commented May 13, 2020 at 4:46
for (const std::vector<int>& edge : edges)or for (const auto& edge : edges) would avoid unnecessary copies over your for(vector<int> edge : edges). — Jarod42
– Jarod42, Commented May 13, 2020 at 8:14

lilucpp · Accepted Answer · 2020-05-13 04:53:26Z

1

each element of e is a vector, each element of e[node] is int.

answered May 13, 2020 at 4:53

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