C++ different output using global variables vs passing variable thorugh function

Question

I was solving this question.

When the min and max variables are set as global variable, I'm getting the correct output, but when I pass them in the functions it messes up the output.

I can't figure out the reason. Can someone tell me, how are these two code snippets different.

global:

    int min = 1,max = 0;

    //dfs in which left subtree is travelled before right
    void travLeft(Node * root, int i,vector<int> &left)
    {
        if(root==NULL) return;
        
        if(i<min)
        {
            min = i;
            left.push_back(root->data);
        }
        
        travLeft(root->left,i-1,left);
        travLeft(root->right,i+1,left);
    }
    
     void travRight(Node * root, int i,vector<int> &right)
    {
        if(root==NULL) return;
        
        if(i>max)
        {
            max = i;
            right.push_back(root->data);
        }
        
        travRight(root->right,i+1,right);
        travRight(root->left,i-1,right);
    }
    
    void topView(Node * root) {
        
        vector<int> left,right;
        
        travLeft(root,0,left);
        travRight(root,0,right);
        
        for(int i=left.size()-1;i>=0;i--)
        {
            cout<<left[i]<<" ";
        }
        
        for(int i=0;i<right.size();i++)
        {
            cout<<right[i]<<" ";
        }
        
    }

passing in the function :


    //dfs in which left subtree is travelled before right
    void travLeft(Node * root,int min, int i,vector<int> &left)
    {
        if(root==NULL) return;
        
        if(i<min)
        {
            min = i;
            left.push_back(root->data);
        }
        
        travLeft(root->left,min,i-1,left);
        travLeft(root->right,min,i+1,left);
    }
    
     void travRight(Node * root,int max, int i,vector<int> &right)
    {
        if(root==NULL) return;
        
        if(i>max)
        {
            max = i;
            right.push_back(root->data);
        }
        
        travRight(root->right,max,i+1,right);
        travRight(root->left,max,i-1,right);
    }
    
    void topView(Node * root) {
        
        vector<int> left,right;
        
        travLeft(root,INT_MAX,0,left);
        travRight(root,0,0,right);
        
        for(int i=left.size()-1;i>=0;i--)
        {
            cout<<left[i]<<" ";
        }
        
        for(int i=0;i<right.size();i++)
        {
            cout<<right[i]<<" ";
        }
        
    }

score 1 · Accepted Answer · answered Jun 17 '21 at 17:23

The global variable is global. The changes to the value will retain among all calls of travLeft and travRight.

On the other hand, the argument is local to the function. The new value of min and max are passed to the next level of recursion, but the update of min in the first recursion

travLeft(root->left,min,i-1,left);

will not passed to the second recursion

travLeft(root->right,min,i+1,left);

Because the same value of min, which is updated (or not updated and come from the previous level) at this level, is passed for the two calls. The same thing will also happen with max and travRight.

This is the difference.

score 0 · Answer 2 · answered Jun 17 '21 at 18:37

When using as global variables, the changes will take place(and retain the changes) through out all the layers. But, if you pass them to the function, for every recursion stage(layer) the variables will have certain values, which will remain unchanged for that particular layer.

score 0 · Answer 3 · answered Jun 17 '21 at 19:48

@MikeCat and @SoumitraChatterjee has already stated the reason why your version that passes in the variable fail to work.

To make it work, you can alternatively set the parameter to take in min and max as references, so the changes will be reflexed to the original value:

void travLeft(Node* root, int& min, int i, vector<int> &left) { //... }
                          ^^^^

Note that it might be tempting to set a default value to parameter max and min like:

//                                                   this doesn't work
//                                                      ↓ ↓ ↓ ↓ ↓ ↓
// void travLeft(Node* root, int i, vector<int> &left, int& min = 0)

However, this will not work because you are attempting to set min to a the reference of a temporary value, which does not work.

For more about that, consider: Default value to a parameter while passing by reference in C++

C++ different output using global variables vs passing variable thorugh function

3 Answers3