for loop

for loop for modern C++

#include <iostream>
#include <vector>
int main() {
  
  // In the standard library, a std::vector is an array with automatic size.
  // Let's make a vector of ints and loop over the contents.
  // The syntax for std::vector<> is discussed further in the lecture on template types.
  
  std::vector<int> int_list;
  int_list.push_back(1);
  int_list.push_back(2);
  int_list.push_back(3);
  
  // Automatically loop over each item, one at a time:
  for (int x : int_list) {
    // This version of the loop makes a temporary copy of each
    // list item by value. Since x is a temporary copy,
    // any changes to x do not modify the actual container.
    x = 99;
  }
  
  for (int x : int_list) {
    std::cout << "This item has value: " << x << std::endl;
  }
  
std::cout << "If that worked correctly, you never saw 99!" << std::endl;

  return 0;
}

-----------------------------------------------------------------
#include <iostream>
#include <vector>
int main() {
  
  std::vector<int> int_list;
  int_list.push_back(1);
  int_list.push_back(2);
  int_list.push_back(3);
  
  for (int& x : int_list) {
    // This version of the loop will modify each item directly, by reference!
    x = 99;
  }
  
  for (int x : int_list) {
    std::cout << "This item has value: " << x << std::endl;
  }
  
  std::cout << "Everything was replaced with 99!" << std::endl;

  return 0;
}


--------------------------------------------------------------------------
#include <iostream>
#include <vector>
int main() {
  
  std::vector<int> int_list;
  int_list.push_back(1);
  int_list.push_back(2);
  int_list.push_back(3);
  
  for (const int& x : int_list) {
    // This version uses references, so it doesn't make any temporary copies.
    // However, they are read-only, because they are marked const!
    std::cout << "This item has value: " << x << std::endl;
    // This line would cause an error:
    //x = 99;
  }

  return 0;
}

-----------------------------------------------------------

singed int와 unsigned int를 서로 비교 하지 않도록 주의! 보통 컨테이너의 
사이즈 값은 unsigned int인 경우가 많음. 

std::vector<int> v = {1,2,3,4};
for (int i=0; i < v.size(); i++) {
    std::cout << v[i] << std::endl;
}

정상 동작 가능. 하지만..

std::vector<int> v = {1,2,3,4};
for (int i=0; i <= v.size()-1; i++) {
    std::cout << v[i] << std::endl;
}

v의 사이즈가 0 인 경우 계산 값은 어마어마하게 큰 값. segfault유발. 
std::vector<int> v;
for (int i=0; i <= v.size()-1; i++) {
    std::cout << v[i] << std::endl;
}



-해법---------------------------------------------------

// Casting to signed int first helps to ensure that the result
// of subtraction will truly be a signed negative value when size is 0:
for (int i=0; i <= (int)v.size()-1; i++) {
    // ...
}

// Rewriting the algebra to perform addition instead of subtraction
// helps to avoid going below 0:
for (int i=0; i+1 <= v.size(); i++) {
    // ...
}