Annoying crash..

**hauzer** · 10-27-2008

I am wondering why does this program crash:

Code:

#include <stdlib.h>
#include <iostream>

using namespace std;

int main(int argc,char **argv)
{
    int *num=NULL; //Array for storing numbers..
    int i=0;       //Counter

    cout<<"--Program for finding number average--\n"<<
          "------------Input 0 to stop-----------\n\n";

    while(1)
    {
        int n; //Dummy variable

        cout<<"Enter number "<<i+1<<": ";
        cin>>n;

        if(n==0) break;

        num=new int; //Allocate space for number
        num[i]=n;
        ++i;
    }

    int aver=0;

    for(int x=0;x<i;++x) aver+=num[x];

    cout<<"Average of inputed numbers is: "<<aver/i;
    cin.get();

    for(int x=0;x<i;++x) delete num;

    return 1;
}

**matsp** · 10-27-2008

Becuase you only allocate space for one number (many times, but still only one number at a time), and then use the pointer as if you have space for multiple numbers.

--
Mats

**hauzer** · 10-27-2008

So that is not how C++ new works? I thought it can "detect" if it needs to malloc or realloc, or something.. Can you give me an example?

**laserlight** · 10-27-2008

Originally Posted by hauzer

I thought it can "detect" if it needs to malloc or realloc, or something.

new certainly cannot do that, and since you want a dynamically allocated array, you should be using new[] instead of new.

But wait. The standard containers have just want you wanted, and in this case you can use std::vector, e.g.,

Code:

std::vector<int> numbers;
while (1)
{
    int n; //Dummy variable

    cout << "Enter number " << (numbers.size() + 1) << ": ";
    cin >> n;

    if (n == 0) break;

    numbers.push_back(n);
}

**hauzer** · 10-27-2008

I see that vectors are useful but can you show me how can I do it with new? Also, I tried compiling with vectors but it seems that they are not a member of std.

EDIT: I researched a bit and I see that vectors are commonly used. Thank you both.

**hauzer** · 10-27-2008

Okay, I made it this time and it works great, except one thing. cin.get() doesn't work at the end:

Code:

#include <stdlib.h>
#include <iostream>
#include <vector>

using namespace std;

int main(int argc,char **argv)
{
    vector<int> num;     //Vector for storing numbers..
    unsigned int i=0;   //Counter

    cout<<"--Program for finding number average--\n"<<
          "------------Input 0 to stop-----------\n\n";

    while(1)
    {
        int n=0; //Dummy variable

        cout<<"Enter number "<<i+1<<": ";
        cin>>n;

        if(n==0) break;

        num.push_back(n);
        ++i;
    }

    int aver=0;

    for(unsigned int x=0;x<i;++x) aver+=num.at(x);

    cout<<"Average of inputed numbers is: "<<aver/i;
    cin.get();

    num.clear();

    return 1;
}

EDIT: problem solved, just use cin.get() twice or _getch()

**Elysia** · 10-27-2008

That is because sometimes there is crap left in the input buffer and so cin.get() "eats" that something and returns immediately.
You should not really use it to stop the console from disappearing. A better approach is using a proper IDE or a debugger to break.
But these are platform dependant... what OS are you using?

Also, every new must be matched with a delete; otherwise you get a memory leak.

**hauzer** · 10-27-2008

It's windows. I will just use _getch().

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