BAD_ACCES iterator on map c++ - c++

Question

I'm trying to implement a function using a map in a map like that :
typedef std::map<int, float> inner_map;
typedef std::map<int, inner_map> outer_map;
I'm filling the map this way. The idea is the first map contains a int key and an other map which contains another key and a float. If the pair I obtain is known, I increment the value otherwise I fill the map.
for (int y = 2; y < imgSize.y - 2; y++){
for (int x = 2; x < imgSize.x - 2; x++) {
thisPos = x + y*imgSize.x ;
{SOME CODE}
lj = labelMap[thisPos] ;
if (condition) {
// get the 2 label
li = pro_label_mod[thisPos] ;
// look if it lj is in the map
is_lj = pi_matrix.count(lj) ;
// this lj has already been observed
if (is_lj == 1) {
is_li = pi_matrix[lj].count(li);
// this pair is known -> I increment
if (is_li==1) {
pi_matrix[lj][li] += 1.0f ;
}
else {
pi_matrix.at(lj).insert(pi_matrix[lj].end(), std::make_pair(li, 1.0f)) ;
}
}
else {
inner_map inter ; inter.insert(std::make_pair(li, 1.0f)) ;
pi_matrix.emplace(lj, inter) ;
}
}
numb_lj[lj] += 1.0f ;
}
}
And then, I would like to iterate throught the map in my code. I did that with iterators such as :
std::vector<std::pair<int,float> > max_pi(size_lj) ;
float maxValue, thisValue ; int label_max ;
for (outer_map::iterator it_lj = pi_matrix.begin(), it_lj_end = pi_matrix.end(); it_lj != it_lj_end; ++it_lj) {
maxValue = 0.0f ;
label_max = 0 ;
inner_map &innerMap = it_lj->second;
for(inner_map::iterator it_li = innerMap.begin(), it_li_end = innerMap.end(); it_li != it_li_end; ++it_li) {
thisValue = it_li->second / numb_lj[it_lj->first] ;
if (thisValue >= maxValue) {
maxValue = thisValue ;
label_max = it_li->first ;
}
}
max_pi[it_lj->first] = std::make_pair(label_max, maxValue) ;
i+=1;
}
However, i'm getting a segmentation fault at the lines of the for loop (either the first one or the second one). BUT ! Not every time. I'm calling this function at each frame and I can have 5 calls without a BAD_ACCESS and suddenly, it crashes.. Sometimes after the first call and then the 10th.
I really can't see why and how to solve it..
Thank you in advance for any clues/comments which could be helpful !

Answer 1

It appears that your design of a nested map is causing all sorts of unnecessary complications. More natural appears a single map with a pair<int,int> as key:
using key_type = std::pair<int,int>;
using map_type = std::map<key_type,float>;
void add_to_map(map_type&map, key_type const&key)
{
map[key] += 1.f; // also works as desired if map.count(key)==0
}
// find key and value of first element with maximum value
map_type::value_type find_max(map_type const&map)
{
auto max = map.begin();
for(auto it=max; it!=map.end(); ++it)
if(it.second > max.second)
max = it;
return *max;
}
no need to juggle with map::count or map::iterator.