Error when trying to print out array in a loop using cout

Question

#include <cstdio>
#include <iostream>

int main (){
    int n;
    std::cin>>n;
    int*a;
    for (int i=0;i<n;i++){
        std::cin>>a[i];
    }
    for(int i=0;i<n;i++){
        std::cout<<a[i];
    }

    return 0;
}

I just started working on a problem and I wanted to check if I knew how to read and array and make a sample output array. When I include the second loop program crashes as soon as I enter n and the first number With the following message

3 1

Process returned -1073741819 (0xC0000005) execution time : 4.943 s Press any key to continue.

You don't have any arrays anywhere. `a` is a pointer that doesn't point to anything; trying to index off it causes undefined behaviour. I suggest getting [a book](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) — M.M, Oct 11 '16 at 07:50
Don't bother with a pointer here, if you want to determine the size at runtime, just use `std::vector `. — George, Oct 11 '16 at 07:59

Rafael Werlang · Answer 1 · 2020-06-16T08:59:36.323

int *a; is a pointer to an integer, it is just a pointer to some memory, it has no memory allocated on its own. Since you are dereferencing this pointer a[i] without setting it up first, your compiler should even give you some warning telling that you are using a variable that has not been initialized.

0xC0000005 error code in Windows means access violation. In this case, you are trying to write to some memory which you don't have access to.

You need to allocate memory before you can read or write to it.

If you know beforehand how many entries you will have, you can do static memory allocation, if you don't, then you need to do dynamic memory allocation.

For instance, if you knew that you would need only 20 entries at max, you could easily swap int *a; for int a[20];.

But since you are only getting to know how many entries there will be when the program runs, then you need to go for dynamic memory allocation: int *a = new int[n];.

So your code should be

#include <cstdio>
#include <iostream>

int main (){
    int n;
    std::cin>>n;
    int *a = new int[n];
    for (int i=0;i<n;i++){
        std::cin>>a[i];
    }
    for(int i=0;i<n;i++){
        std::cout<<a[i];
    }
    delete[] a; // Release allocated memory.
    return 0;
}

score 0 · Answer 2 · answered Oct 11 '16 at 07:49

0

You need to allocate memory for a, otherwise the behaviour of the program is undefined. Writing

int* a = new int[n];

would to it, followed by delete[] a; when you're all done (put this just before return 0;).

Alternatively, use a std::vector<int> a(n); and all the memory allocation will be taken care of for you.

answered Oct 11 '16 at 07:49

Bathsheba

231,907
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Oh god no, don't get this guy to start newing stuff before he's read up on it. – George Oct 11 '16 at 07:57

Andreas DM · Answer 3 · 2016-10-11T08:14:16.177

0

int* is a pointer to int, not an array.
To create an array of int, example: int a[100]; - where 100 is the size

Moreover, you should use a std::vector<int> instead:

vector<int> vec;
for (int i = 0; i != n; ++i) {
    int temp;
    cin >> temp;
    vec.emplace_back(temp);
}

edited Oct 11 '16 at 08:14

answered Oct 11 '16 at 08:01

Andreas DM

10,685
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score 0 · Answer 4 · answered Oct 11 '16 at 08:05

0

Try : int a[20]; rather than int *a;

answered Oct 11 '16 at 08:05

manianis

109
3
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why 20? He requests a number then inputs that many integers. He should use vector. – CashCow Oct 11 '16 at 08:06
I think the last solution above is even better then a vector – Treycos Oct 11 '16 at 08:08
20 to get a working program as he won't enter more than 20 integer by hand to test his program. – manianis Oct 11 '16 at 08:08

score 0 · Answer 5 · answered Oct 11 '16 at 08:05

Your a doesn't point to any valid memory, resulting in undefined behaviour.

What you need is an std::vector :

#include <vector>
int n;
std::cin>>n;
std::vector<int> numbers;
for (int i=0;i<n;i++){
    int val;
    std::cin>>val;
    numbers.push_back(val);
}
for(int i=0;i<n /* or numbers.size()*/ ;i++){
    std::cout<< numbers[i];
}

This takes care of dynamic allocation for you so that you don't have to do the dirty stuff yourself.

score 0 · Answer 6 · answered Oct 11 '16 at 08:11

There are a few issues with your code. Primarily, you request the number 'n' then need to allocate enough space to store that many integers.

The best way to do that is to use vector. You can create this with:

std::vector< int > numbers( n );

You can also create it allocating the memory but waiting until you have data:

std::vector< int > numbers;
numbers.reserve( n );

You also should probably validate your input, for example your input stream (cin) will be invalid if the user enters something that is not an integer, and the original 'n' should be positive if you are going to try to create a vector of that size, and you may need to set a limit or you will suffer a bad_alloc. If you do not mind suffering a bad_alloc you should catch that exception and print an error such as "There is insufficient space to allocate as many numbers".

Of course if you enter a high number like 10 million, you may find the compiler is able to allocate that many but your user will get bored in your loop when you ask him to enter integers 10 million times.

You do not need <cstdio> as a header. You will need <vector> and <iostream>.

Error when trying to print out array in a loop using cout

6 Answers6