Dynamic array crashed for more than 8 elements

Question

#include<iostream>
using namespace std;
void arrayin(int x[], int n);
void arrayout(int x[], int n);
main()
{
    int n, x[n];
    cout << "Please enter the number of elements in the array: " << endl;
    cin >> n;
    cout << "Please enter the elements: " << endl;
    arrayin(x,n);
    cout << "Array is of " << n << " elements."<< endl;
    cout << "Elements are as follow :" << endl;
    arrayout(x,n); 
}
void arrayin(int x[],int n)
{
    for (int i = 0; i < n; i ++)
    {
        cin >> x[i];
    }
}   
void arrayout(int x[], int n)
{
    for (int i = 0; i < n; i++)
    {
        cout << x[i] << "\t";
    }
}

I'm new to programming. It crashes for more than 8 elements, if n > 8 crashes.. but for n<8 works fine.. Dont know why!

I don't know, C++ supports VLA? I thought it was more of a C thing, but I may be wrong. — Sourav Ghosh
– Sourav Ghosh, Commented Dec 14, 2016 at 11:06
Are you compiling it as C++? (Not that that really matters since you declare x[n] before you know n). Variable length arrays are a C thing - in C++ I'd use a vector. — doctorlove
– doctorlove, Commented Dec 14, 2016 at 11:08
C and C++ are quite different with respect to arrays. You should decide which one you have before asking here. From the view of it you are trying to use C concepts within C++. Don't do that, you'll never know what you get. — Jens Gustedt
– Jens Gustedt, Commented Dec 14, 2016 at 11:09

Humam Helfawi · Accepted Answer · 2016-12-14 11:08:23Z

5

Here is the problem:

 int n, x[n]; // It is undefined behaviour
 cout << "Please enter the number of elements in the array: " << endl;
 cin >> n;

The correct way is (on your compiler with the variable-size-array extension):

 int n;
 cout << "Please enter the number of elements in the array: " << endl;
 cin >> n;
 int x[n];

The correct way using C++ is to use std::vector instead:

 int n;
 cout << "Please enter the number of elements in the array: " << endl;
 cin >> n;
 std::vector<int> x(n);

and you have to make some other changes to adapt std::vector.

answered Dec 14, 2016 at 11:08

Humam Helfawi

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Comments

Jabberwocky · Accepted Answer · 2016-12-14 11:10:05Z

3

The problem is here:

int n, x[n];  // <-- n is not yet initialized
cout << "Please enter the number of elements in the array: " << endl;
cin >> n;
cout << "Please enter the elements: " << endl;
arrayin(x,n);

You need this:

int n;
cout << "Please enter the number of elements in the array: " << endl;
cin >> n;
int x[n];   // << now n has been initialized
cout << "Please enter the elements: " << endl;
arrayin(x,n);

BTW: VLA (or dynamic arrays as you call them) are non standard in C++, but gcc (and possibily clang) has them as extension.

answered Dec 14, 2016 at 11:10

Jabberwocky

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Comments

LPs · Accepted Answer · 2016-12-14 11:27:48Z

2

int n, x[n]; is the problem

You are declaring n that will have an indeterminate value. With this value you are declaring an array that will have an indeterminate size.

You are using C++, so use new keyword to create your array, after user input:

cout << "Please enter the number of elements in the array: " << endl;
cin >> n;
int *x = new int[n];
// your stuff
delete x;

edited Dec 14, 2016 at 11:27

answered Dec 14, 2016 at 11:15

LPs

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Comments

shiv · Accepted Answer · 2016-12-14 11:19:54Z

0

Declare your array after taking input for n.

#include<iostream>
using namespace std;
void arrayin(int x[], int n);
void arrayout(int x[], int n);
main()
{
    int n;
    cout << "Please enter the number of elements in the array: " << endl;
    cin >> n;
    int x[n];
    cout << "Please enter the elements: " << endl;
    arrayin(x,n);
    cout << "Array is of " << n << " elements."<< endl;
    cout << "Elements are as follow :" << endl;
    arrayout(x,n);
}
void arrayin(int x[],int n)
{
    for (int i = 0; i < n; i ++)
    {
        cin >> x[i];
    }
}
void arrayout(int x[], int n)
{
    for (int i = 0; i < n; i++)
    {
        cout << x[i] << "\t";
    }
}

This code works. The problem is when you declare array with no value for n the array will be initialized with whatever is on stack from past. Variable length arrays require the value. So declare the array later.

Someone downvoted without any reason.

Edit: Variable length arrays are not part of ISO C++. To write standard compliant C++ code you should use -pedantic flag with g++ or clang.

You have two simple choice. Either use std::array for fixed length arrays or std::vector for dynamic arrays.

edited Dec 14, 2016 at 11:19

answered Dec 14, 2016 at 11:06

shiv

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4 Comments

WhozCraig Over a year ago

Or.. use std::vector<int>

Humam Helfawi Over a year ago

I downvoted WITH a reason.. your question, before editing, consisted of "Declare your array after taking input for n.". this is not a real answer, it is a comment. After your edit I will withdraw my downvote. But IMO this answer is not useful since you did not mentioned about the nonstandard extensions. or even a real C++ solution

shiv Over a year ago

That is okay. There is no need to strain your voicebox. A programmer is supposed to use compiler well. The answer should also contain that for g++ one should use -pedantic -Wall for seeing this.

RoadRunner Over a year ago

Yeah calm down @HumamHelfawi, you sound pretty stressed.

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