Thread: finding element in std::map

Thank you all very much for help !

To sum up: according to what has been said this version is correct:

Code:

#include <cstdio>
#include <map>
#include <algorithm>
using namespace std;

typedef map<int,int>::iterator iter;
iter find(map<int,int> &s, int x) {
  iter it=s.lower_bound(x);
  return it==s.begin() ? s.end() : --it;
}

int main() {
  map<int,int> t;
  int x=0;
  for(int i=4;i<=8;i+=2) t[i]=++x;
  for(int i=2;i<=9;++i) {
    iter it=find(t,i);
    printf("find(%d):", i);
    if(it==t.end()) printf(" NONE\n");
    else printf("(%d,%d)\n",it->first,it->second);
  }
  return 0;
}

Why ? If map is empty, then lower_bound(x)==s.end()==s.begin() and result is s.end(), and if map is not empty and is the lower_bound(x) is s.begin(), then result is s.end(), and if the lower_bound(x)!=s.begin() then --lower_bound(x) is the answer (even if lower_bound(x) is s.end(), since performing --s.end() is correct in this case).