code: public final int getVariableByName(String variableName) {
      int index = getVariableIndex(variableName);
      if (index == -1) {
        return index;
      } else {
        return nums[index];
      }
    }
docs: Gets the index of the variable named {@code variableName}. Returns
  {@code -1} if no such variable exists.
  
  @param variableName
  @return