# V831. Decreased performance. Consider replacing the call to the 'at()' method with the 'operator[]'.

The analyzer has detected a block of code where an element of a sequence container (std::array, std::vector, or std::deque) is accessed using the 'at' method, with the index known to be valid.

The 'at' method returns a reference to the container element specified by the index. Before doing so, the method checks if the index is within the container's bounds and generates an 'std::out_of_range' exception if it is not. If the index is already known to be within the container's bounds, the call to the 'at' method can be safely replaced with a call to 'operator[]', which does not perform such additional checks. Removing the unnecessary check will help increase the code's performance.

Consider the following example:

    std::vector<std::string> namelessIds;
    ....
    if (!namelessIds.empty()) {
      LIST<char> userIds(1);
      for (std::string::size_type i = 0; i < namelessIds.size(); i++) {
        userIds.insert(mir_strdup(namelessIds.at(i).c_str()));           // <=
      }
      ....
    }

When iterating over the vector elements in the loop, the incrementing index cannot become larger than the number of the last element. Thus, the code can be simplified by replacing 'at' with square brackets:

    std::vector<std::string> namelessIds;
    ....
    if (!namelessIds.empty()) {
      LIST<char> userIds(1);
      for (std::string::size_type i = 0; i < namelessIds.size(); i++) {
        userIds.insert(mir_strdup(namelessIds[i].c_str()));
      }
      ....
    }