CODEWITHSUNDEEP
c++arraysdynamiccrashboolean
user2807429
user2807429

Reputation: 11

C++ Dynamic bool array causes crash

Today I tried to program the Sieve of Eratosthenes and it works as far as it provides me with the prime numbers. But I have a problem with the dynamic array I don't understand.

First problem: As soon as I try to enter a "big" value for n (for example 120), the program crashes, it doesn't even allocate the memory.

Second problem: If I enter a value like 50 it is able to give out the correct prime numbers but crashes before it deletes the array.

Third problem: If I enter a very small value like 5 it is able to execute the entire program, it gives out the correct numbers and deletes the memory.

But I don't understand why it acts so differently. 120 boolean values can't crash my memory, at least I think so. And why isn't it able to delete an array of 50 values but is actually able to delete an array of 5 values? Can anyone tell me what's the problem? 

int n;
cin >> n;
n=n+1;
bool *feld = new bool[n];
for(int i=2;i<n;i++)
{
    int j=i*i;
    feld[j]=true;
    for(;j<n;j+=i)
        feld[j]=true;
}
for(int i=2;i<n;i++)
    if(!feld[i])
        cout << i << endl;
    else;
delete[] feld;
feld = NULL;

Upvotes: 1

Views: 1761

Answers (3)

hasan
hasan

Reputation: 24205

Actually It's not just that feld[j]=true; is causing the error.

Also, you don't need that line at all before the loop.

because, it's the first case inside the loop.

Upvotes: 0

john
john

Reputation: 87959

This code is wrong

bool *feld = new bool[n];
for(int i=2;i<n;i++)
{
    int j=i*i;
    feld[j]=true;
    ...
}

Suppose n == 10 and i == 9, then j == 81 but you only have 10 elements in your bool array.

This is how it works when you write bugged programs, sometimes it seems to work, it might even give the right answer, other times it will crash. This is a very important lesson, and you're actually lucky to have learned it early.

Upvotes: 5

Paul R
Paul R

Reputation: 212979

Your problem is here:

int j=i*i;
feld[j]=true;

there is no check as to whether j < n so you are stomping over unallocated memory when j >= n.

Upvotes: 6

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