for vector, using for(size_t i=0; i< size; i++) and for(auto& at:object) happen two different result

Question

int size = objects_op->size();
for (auto &at : objects_op->object) {
  EmObject& at = objects_op->objects().at(i);
  if (at.sub_type != ObjectType::VISION_PLD) {
     continue;
  }
  for (size_t i = 0; i < 4; i++) {
    std::cout << " c: " << at.object.polygon.pts.at(i).transpose()
    std::endl;
  }
  std::cout << BLUE << "after:" << &(at.object.polygon.pts) << RESET
  std::endl;
}

the above code is right result:

0 c: -3.38838 -4.42124 
1 c: -5.64087 -4.45888
2 c: -5.56526   -9.241
3 c: -3.32031 -9.20348 
after:0xf5d3760
0 c: -7.13594  3.63844 
1 c: -9.28928  3.66867 
2 c: -9.26098   8.2844 
3 c: -7.10295  8.25414 
after:0xf5d3880

but for the below

int size = objects_op->size();
for (size_t i=0; i< size; i++) {
   EmObject& at = objects_op->objects().at(i);
   if (at.sub_type != ObjectType::VISION_PLD) {
     continue;
   }
   for (size_t i = 0; i < 4; i++) {
      std::cout << " c: " << at.object.polygon.pts.at(i).transpose()
      std::endl;
   }
   std::cout << BLUE << "after:" << &(at.object.polygon.pts) << RESET
   std::endl;
}

the above code is right result:

0 c: -14.9e-18 1.9e+18 
1 c: -5.9e-18 -4.4e+17
2 c: -5.56526   -9.241
3 c: -3.32031 -9.20348 
after:0xf5d3720
0 c: -2.9e-18 1.2e+18 
1 c: -3.9e-18 -4.3e+17
2 c: -9.26098   8.2844 
3 c: -7.10295  8.25414 
after:0xf5d3870

I don not understand how it happen, please help..

Answer 1

The issue you’re encountering is due to the scope of the variable i in your second code snippet. In the inner loop, you’re redeclaring i for the loop variable, which shadows the i used in the outer loop for indexing the objects. This is why you’re seeing unexpected results.

You can change the variable in the inner loop from i to j. This way, the inner loop variable doesn’t interfere with the outer loop variable.