l值必須是可修改的錯誤

Question

這是修復紅黑樹的函數，但編譯器一直給出l值必須是可修改的，while循環條件中的'z'導致問題（z！=根）時，而應r.un直到Z指針是指向特定鍵達到「z」的指針指向

void Tree::fixup(int z) 
{ 
fixuppri(root, z); 
} 

void Tree::fixuppri(node* root, int a) 
{ 
node* z ; 
z = returnnode(a); 

while (z != root && z->parent->color = 'r') 
{ 
    node* y; 

    if (z->parent == z->parent->parent->left) 
    { 
     y = z->parent->parent->right; 
    } 
    else 
    { 
     y = z->parent->parent->left; 
    } 

    if (y->color = 'r') 
    { 
     z->parent->parent->color = 'r'; 
     z->parent->color = 'b'; 
     y->color = 'b'; 
     z = z->parent->parent; 
    } 
    else 
    { 
     if (z->parent == z->parent->parent->left && z == z->parent->left) 
     { 
      char tempcolor = z->parent->color; 
      z->parent->color = z->parent->parent->color; 
      z->parent->parent->color = tempcolor; 
      rightrotate(z->parent->parent); 
     } 
     if (z->parent == z->parent->parent->left && z == z->parent->right) 
     { 
      char tempcolor = z->color; 
      z->color = z->parent->parent->color; 
      z->parent->parent->color = tempcolor; 
      leftrotate(z->parent); 
      rightrotate(z->parent->parent); 
     } 
     if (z->parent == z->parent->parent->right && z == z->parent->right) 
     { 
      char tempcolor = z->parent->color; 
      z->parent->color = z->parent->parent->color; 
      z->parent->parent->color = tempcolor; 
      leftrotate(z->parent->parent); 
     } 
     if (z->parent == z->parent->parent->right && z == z->parent->left) 
     { 
      char tempcolor = z->color; 
      z->color = z->parent->parent->color; 
      z->parent->parent->color = tempcolor; 
      rightrotate(z->parent); 
      leftrotate(z->parent->parent); 
     } 
    } 


} 

root->color = 'b'; 
} 

Answer 1

關鍵這是一個簡單的拼寫錯誤引起，因爲與相互作用的一個令人困惑的錯誤運算符優先級。

你寫了=你應該寫的地方==。

&&的優先級低於==，但優先級高於=。

這意味着

z != root && z->parent->color = 'r' 

相當於

(z != root && z->parent->color) = 'r' 

和z != root && z->parent->color不是一個可修改的L值。