Clash Royale CLAN TAG#URR8PPP

Some values in this array are turning to zero (0) for no particular reason

I take an input for an array, then run a completely unrelated function. Somehow, the values of the array are different after this point.
As far as I can tell, the first few values become zero (0).

Here is the opening segment of the main function:

int main()

int i, j, k, n;
Nod* root=NULL; //Nod class user defined elsewhere
cin>>n;
int A[n];
for(i=0; i<n; i++)

cin>>A[i];
cout<<A[i]<<"t";//TESTER

cout<<endl<<endl;
Nod no[n];
for(i=0; i<n; i++)
no[i].nowhere(n);
for(i=0; i<n; i++)
cout<<A[i]<<"t";//TESTER
cout<<endl<<endl;

...//rest of main()
...



Here is the class Nod with the nowhere() function:

class Nod

public: Nod* parent;
Nod* child;

void nowhere(int n)

parent=NULL;
for(int i=0; i<n; i++)
child[i]=NULL;

;


Here is a sample input and output:

Input:

5
4 -1 4 1 1


Output:

4 -1 4 1 1
0 0 4 1 1


As far as I can see, the nowhere() function should not affect the array A[] at all. Then, how are the values changing?

nowhere()

A[

Nod* child;

cin>>n; int A[n];

-std=c++17 -Wall -Werror -pedantic

2 Answers
2

Function nowhere causes undefined behavior by writing past the end of child array. The array has zero elements in your scenario, so accessing any index causes undefined behavior.

nowhere

child

Since you are allocating Nods in an array, you should change child to std::vector<Nod*> child, and initialize it to an appropriate size dynamically as needed:

Nod

child

std::vector<Nod*> child

std::vector<Nod*> child;

void nowhere(int n)
parent = NULL;
child = std::vector<Nod*>(n, NULL);



Note: Consider restructuring your code to not use "raw" pointers, or vectors of pointers. Switch to smart pointers, or use objects if possible.

Your problem:

You're getting zeros all over the first two cells of int A[n] because nowhere(int n) overwrites some of the contents of A[n]. The following loop does so:

int A[n]

nowhere(int n)

A[n]

for(int i=0; i<n; i++)
child[i]=NULL;


The problem is that child is not being allocated before the iterated assignments of its cells to the value of NULL, and so you effectively write NULL (numerical value of 0) to an unexpected place in the memory. This is called an Overflow/Overrun. In your case the program overruns the address of where child resides, in a way that gets you zeroes all the way onto the content of int A[n].

child

NULL

NULL

0

child

int A[n]

The solution:

Use correct C++ to define child, like this:

child

std::vector<Nod*> child;


then, rewrite nowhere(int n) like this:

nowhere(int n)

void nowhere(int n)

parent=NULL;
child.resize(n);
for(int i=0; i<n; i++)
child[i]=NULL;



BTW:

You can forgo the entire looping in nowhere(int n) and this one liner does the resize() for you and fills the values like you wanted:

nowhere(int n)

resize()

child = std::vector<Nod*>(n, NULL);

child = std::vector<Nod*>(n, NULL);

Nod* child; is quite likely not doing what you intended for it. The C++ syntax for native arrays of native pointers would in this case be either:

Nod* child;

Nod** child; if you're later allocating it dynamically like this child = new Nod*[n] (but don't forget to later free this memory with delete child)

Nod** child;

child = new Nod*[n]

delete child

or

Nod* child[n]; if n is a constant expression -- which is not the case here anyways as you are receiving n as input.

Nod* child[n];

n

n

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.