Why is the last value in this specific example user input not being taken for my while loop?-CodePudding

I'm facing a bug where, after taking in the user input from a while loop, my code does not accept the last value. This bug happens on ONE specific example, and I have no clue why this is happening.

So, for example, the user inputs:

7  
3 1 4 0 0 2 0

The output is:

HOWEVER, with the following user input (this is the specific example):

7  
3 0 1 0 0 2 0

The output should be:

BUT, the output is instead:

I can't figure this out at all. The code is attached below:

#include <iostream>
#include <vector>
#include <math.h>
using namespace std;

// Definition for a binary tree node.
struct TreeNode {
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode() : val(0), left(NULL), right(NULL)  {}
    TreeNode(int x) : val(x), left(NULL), right(NULL)  {}
};

TreeNode* construct_tree(){
    int n;
    cin >> n;
    int curr_inp;
    vector<TreeNode*> vec;
    for (int i = 0; i < n; i  ) {
        cin >> curr_inp;
        cout << curr_inp; // **this is the place of bug**
        if (curr_inp != 0) 
            vec.push_back(new TreeNode(curr_inp));
        else 
            vec.push_back(NULL);
    }

    for(int i = 0; i< floor(n/2);i   )
    {
        vec[i]->left = vec[2*i 1];
        vec[i]->right = vec[2*i 2];
    }
    cout << '\n';
    return vec[0];
}

int main() {
    TreeNode* root = construct_tree();
    return 0;
}

CodePudding user response：

The issue is not in cout << curr_inp; The issue is in loop which you used

    for(int i = 0; i< floor(n/2);i   )
    {
        vec[i]->left = vec[2*i 1];
        vec[i]->right = vec[2*i 2];
    }

You are trying to call left and right with null vector

After I added null check there is no segmentation fault

    for(int i = 0; i< floor(n/2);i   )
    {
        if (vec[i]) {
            vec[i]->left = vec[2*i 1];
            vec[i]->right = vec[2*i 2];
        } else {
            cout << "nullptr \n";
        }
    }

Now when I use

7
3 0 1 0 0 2 0

I got following ouput

7
3 0 1 0 0 2 0
3010020nullptr

Conclusion: I don't know what your logic is but the issue is because of dereferencing nullptr

CodePudding user response：

Your construct_tree() function is crashing inside of its 2nd for loop. That is preventing the output of the last character written to cout, because cout output is buffered by default, and the last character is still in the buffer and hasn't been flushed to the console yet when the crash occurs.

cin and cout are tie()'ed together by default, so reading input from cin will first implicitly flush cout, but there is no read from cin being done after the 1st for loop's last iteration writes to cout.

Try adding a call to cout << flush or cout.flush() after the 1st for loop is done, before entering the 2nd for loop. Or use cout << curr_inp << flush; inside the 1st for loop. Then you will see the last character displayed.

And then, you need to fix the crash.