I don't understand why this function doesn't work.

Apr 29, 2022 at 12:03pm
nandikaka (3)
I am new to coding and I'm currently learning about Divide et Impera. I want to calculate the sum of the elements from an array, and I thought I should start with writing a function that builds the array, but for some reason it's not working and i can't figure out why.

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  void citire(int v[],int n)
{
    cin>>n;
    for(int i=0;i<n;i++)
    {
        cin>>v[i];
    }
}


If i do it without the arguments it works:
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void citire()
{
    cin>>n;
    for(int i=0;i<n;i++)
    {
        cin>>v[i];
    }
}


I'm just curious why. Can anyone explain?
Apr 29, 2022 at 12:07pm
seeplus (6627)
How is citire being used in both cases?

Apr 29, 2022 at 12:08pm
salem c (3715)
I'm assuming that the one without parameters is using global variables instead.

In order to make the former work, you need a reference parameter
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void citire(int v[],int &n)
{
    cin>>n;
    for(int i=0;i<n;i++)
    {
        cin>>v[i];
    }
}

So when you update n with the cin, the caller of the function gets it's value of n updated.
Apr 29, 2022 at 12:08pm
kigar64551 (837)
In your first code snippet, you pass n as argument, but then you immediately overwrite the value of n with the value read from std::cin. Why? And, in your second code snippet, where is variable n even defined?

If your intent was to "return" the value of n, you need a reference (or pointer).

Either way, how you do you ensure that your array v is large enough to hold (at least) n values ???


I think a correct implementation would be more like:

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int *citire(int &n)
{
    std::cin >> n; // ask number of elements to read
    if (n > 0)
    {
        int *v = new int[n]; // dynamically allocate array of the required size !!!
        for(int i = 0; i < n; i++)
        {
            std::cin >> v[i]; // fill array, element by element
        }
        return v; // return the array to caller; must be delete[]'ed by caller!
    }
    return NULL; // error
}

int main(void)
{
    int n;
    int *array = citire(n); // Note that 'n' is passed as a reference, so that it can be updated in the function!
    if (array)
    {
        for(int i = 0; i < n; i++)
        {
            std::cout << array[i];
        }
        delete[] array;
    }
}
Last edited on Apr 29, 2022 at 12:32pm
Apr 29, 2022 at 12:14pm
nandikaka (3)
in the first case the code would look like this:

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 int v[100], n;
void citire(int v[],int n)
{
    cin>>n;
    for(int i=0;i<n;i++)
    {
        cin>>v[i];
    }
}

int main()
{
    citire(v,n);

   //here comes function for the sum
    return 0;
}


In the second case:

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int v[100], n;

void citire()
{
    cin>>n;
    for(int i=0;i<n;i++)
    {
        cin>>v[i];
    }
}

int main()
{
     citire();
    //function for sum
    reuturn 0;


second case works, first doesn't :(
Apr 29, 2022 at 12:19pm
kigar64551 (837)
int v[100]


What if the user enters a value for n that is greater than or equal 100?

An out-of-bounds memory write will happen !!!

See my suggestion above, or at the very least make sure that the value of n is below 100...
Last edited on Apr 29, 2022 at 12:24pm
Apr 29, 2022 at 12:21pm
nandikaka (3)
Ok, thank you so much!!! Now I understand!!! I gave it as an argument but then immediately overwrote it. I'm sorry for such mistakes... Still learning. But really thank you so much!!!
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