Issues using dynamic boolean array

Question

I want to dynamically declare a boolean array for my C++ program and set its value to false.I am having issues with memory allocation I guess how and getting a dangling pointer I am trying to find prime numbers by Sieve of Eratosthenes I am trying to use boolean array dynamically but facing issues.

I am not trying to implement it using memset .

int  limit = 2000000 ;
int crossLimit = sqrt(limit) ;


void  SievePrime(int limit)
{

    bool* boolArray = new bool[false] ;

    for(int i =4;i<=limit ;i = i + 2)
    {
        boolArray[i] = true ; //getting error hereThread 1:                       
                         //EXC_BAD_ACCESS (code=2, address=0x100786000)
    }

    for(int j= 3 ;j<=crossLimit ; j= j+2){
        if (not boolArray[j])

        {
            for(int k =j*j;k<=limit;k*=2)
        {
            boolArray[k] = true ;
        }
    }
}

    double sum = 0 ;

    for(int i =2 ; i<=limit ;i++)
    {
        if(!boolArray[i])
        {
            sum = sum + i ;
        }
    }

    cout<<sum<<endl ;
}

expected output: 142913828922

error:Thread 1: EXC_BAD_ACCESS (code=2, address=0x100786000)

bool* boolArray = new bool[false] ; What do you think you're doing here ? — Denys Séguret
– Denys Séguret, Commented Jul 10, 2019 at 15:39
The value that you gives in new bool[...] should be the size of your array, it doesn't correspond to the value of your array elements. So the problem is that you are iterating through an array that isn't well allocated — Adrien Givry
– Adrien Givry, Commented Jul 10, 2019 at 15:45

GlenL · Accepted Answer · 2019-07-12 15:31:14Z

2

I sense that you're new to C++.

"false" is generally zero so you're not allocating anything.

bool* boolArray = new bool[false] ;

Try:

bool* boolArray = new bool[limit];


for(int i =0; i<limit; i+)
{
    boolArray[i] = false;                       
}

And put a delete at the end:

delete boolArray;

Also, that loop will overrun the bounds. If you have N items, the last valid entry is N-1 as the first is 0. So use k<limit in the for loop.

edited Jul 12, 2019 at 15:31

answered Jul 10, 2019 at 15:46

GlenL

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3 Comments

Najmus Saqib Makhdoomi Over a year ago

ok that means array size has to be specified at declaration time.But what will be the value of items in those 2 million indexes .is it set to false by default?

WhozCraig Over a year ago

@NajmusSaqibMakhdoomi Not unless you value-initialize them (and you can, via bool *boolArray = new bool[limit](); (note the parens at the end). Related, though the performance is less-than-stellar, std::vector<bool> ar(limit+1); would also work, free you from having to dynamically free at the end, and be much more inline with modern C++, where new and delete are almost never necessary in user programs. I added the +1 because your loops run through limit; not just one-short of it, and therefore you'll breach without the increased size.

user10957435 Over a year ago

This answer would be even better with an example of how an initialization loop would work.

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