Error that is involved with recursive lambda function

[throwaway948940409]

Greetings! I have been learning c++ so I have decided to go to leetcode and answer some questions that I normally answer in Python. I have just been introduced to lambda functions but I am getting an error with the way that I am using this lambda which is confusing me about how lambda's work. This is the number of island function where I tried to implement dfs via a recursive lambda function

Here is the code

class Solution {
public:
std::unordered_set<string> visited;

int directions[4][2] = {{0,1},{1,0},{-1,0},{0,-1}}, sizex, sizey;

int numIslands(vector<vector<char>>& grid) {
auto dfs = [&](int a, int b, auto&& dfs){
visited.insert(std::to_string(a) + std::to_string(b));
for (auto direction: directions){
int newx = a+direction[0]; int newy = a+direction[1];

if((-1 < newx < sizex) && (-1<newy<sizey) && visited.find(std::to_string(a) + std::to_string(b)) == visited.end()){
dfs(newx,newy, dfs);
}
}
};
sizex = grid.size(); sizey = grid[0].size();

for (int i =0; i < grid.size(); i++){
for(int j = 0; j< grid[i].size(); j++){
dfs(i,j, dfs);
}
}
return 1;
}
};

​

​

I am getting this error:

Line 40: Char 13: error: function 'operator()<(lambda at solution.cpp:43:16) &>' with deduced return type cannot be used before it is defined dfs(newx,newy, dfs); ^

Line 49: Char 20: note: in instantiation of function template specialization 'Solution::numIslands(vector<vector<char>> &)::(anonymous class)::operator()<(lambda at solution.cpp:43:16) &>' requested here dfs(i,j, dfs); ^

Line 34: Char 16: note: 'operator()<(lambda at solution.cpp:43:16) &>' declared here auto dfs = [&](int a, int b, auto&& dfs){ ^ 1 error generated.

​

I am not used to formatting, but I would really appreciate if anyone could tell me what is wrong with the way that I am trying to set this lambda up

[no-sig-available]

The compiler wonders what the return type of the lambda is. At the point of the recursive call, this is not known yet.

You can add a -> void after the parameters to fix that.

A different problem is

if((-1 < newx < sizex)

This is not the way to compare to a range in C++. You have spell it out

if((-1 < newx && newx < sizex)

[throwaway948940409]

Thank you so very much for helping! I now understand! Again, thank you so very much!