在多维向量中插入元素 [英] Inserting elements in multidimensional Vector

问题描述

  vector< vector< int>> sort_a; 
 vector< int> v2; 
 vector< int> v3; 
 
 for（int i = 0; i <4; ++ i）{
 v2.push_back（i）; 
 
 for（int j = 0; j <4; ++ j）{
 v3.push_back（j）; 
 sort_a.push_back（v2）; 
 sort_a.push_back（v3）; 
} 
 
} 
  

向量sort_a应该是一个4x4数组，而是输出为31x1，有很多空元素，如何在多维向量中插入元素？

解决方案

认为它是一个多维向量，认为它是向量的向量。

  int n = 4; 
 std :: vector< std :: vector< int>> vec（n，std :: vector< int>（n））; 
 
 //通过外向量vec循环
 for（int i = 0; i  //循环通过内向量vec [i] 
 for（int j = 0; j （vec [i]）[j] = i * n + 
} 
} 
  

我在

b

如果你想通过 push_back 填充你的向量，你可以在内循环中创建一个临时向量，填充它，然后push_back它到您的向量：

  for（int i = 0; i  std :: vector ; int> temp_vec; 
 
 for（int j = 0; j< n; j ++）{
 temp_vec.push_back（j）; 
} 
 
 vec.push_back（temp_vec）; 
} 
  

但是， push_back 调用导致更慢的代码，因为不仅你需要重新分配你的向量所有的时间，而且你必须创建一个临时和复制它。

vector<vector<int>> sort_a;
vector<int> v2;
vector<int> v3;

for (int i=0; i<4; ++i) {
v2.push_back(i);

  for (int j=0; j<4; ++j) {
  v3.push_back(j);
  sort_a.push_back(v2);
  sort_a.push_back(v3);
  }

}

Vector sort_a should be a 4x4 array, instead the output is 31x1 with lots of empty elements, how do i insert elements in a multidimensional vector ?

解决方案

Don't think of it as a multidimentional vector, think of it as a vector of vectors.

int n = 4;
std::vector<std::vector<int>> vec(n, std::vector<int>(n));

// looping through outer vector vec
for (int i = 0; i < n; i++) {
  // looping through inner vector vec[i]
  for (int j = 0; j < n; j++) {
    (vec[i])[j] = i*n + j;
  }
}

I included parentheses in (vec[i])[j] just for understanding.

Edit:

If you want to fill your vector via push_back, you can create a temporary vector in the inner loop, fill it, and then push_back it to your vector:

for (int i = 0; i < n; i++) {
  std::vector<int> temp_vec;

  for (int j = 0; j < n; j++) {
    temp_vec.push_back(j);
  }

  vec.push_back(temp_vec);
}

However, push_back calls result in slower code, since not only you need to reallocate your vector all the time, but also you have to create a temporary and copy it.

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