如何摆脱细分错误？

Question

My goal is to find the maximum value of (minimum of p consecutive number)*p. Here 1<=p<=N and 1<=N<=100000 and 1<=number<=1000000.I have partially solved the problem.But for some cases I am getting this runtime error.

" terminate called after throwing an instance of 'std::bad_alloc' what(): std::bad_alloc This application has requested the Runtime to terminate it in an unusual way. Please contact the application's support team for more information."

I have found that the error is because program is trying to access memory that is not allowed. But I am not getting which part of code is trying to access unallocated memory.Please help me.

#include <iostream>

using namespace std;

int main(){
int N=0;
cin>>N;
int **A,*B,i,j,num;
A = new int*[N];
for(i=0;i<N;i++)
    A[i] = new int[N]();
B = new int[N]();

for(i=0;i<N;i++){
    cin>>num;
    A[0][B[0]] = num;
    for(j=0;j<B[0];j++){
        A[j+1][B[j+1]] = min(A[j][B[j+1]],A[j][B[j+1]+1]);
        B[j+1]++;
    }
    B[0]++;
}
long long maxim = 0;
for(i=0;i<N;i++){
   for(j=0;j<B[i];j++){
      if(A[i][j]*(i+1) > maxim)
        maxim = A[i][j]*(i+1);
   }
}

cout<<maxim;
delete []A;
delete []B;
return 0;
}

Answer 1

std::bad_alloc is thrown when failing to allocate memory .

You are trying to allocate space for 100000 * 100000 integers. That's ~40GB of RAM, which you probably do not have.

Answer 2

Part of becoming a software engineer is learning to break problems into smaller chunks. To solve this issue you should start by breaking the problem down to the smallest unit.

int main()
{
    size_t dimension = 100000;
    int** outer = new int[dimension];
    for (size_t i = 0; i < dimension; ++i) {
        outer[i] = new int[dimension];
    }
}

Compile and load into a debugger, step through and find where the exception occurs. Or you could write thusly:

#include <iostream>

int main()
{
    size_t dimension = 100000;
    size_t allocated = 0;
    int** outer = new int*[dimension];
    allocated += sizeof(int*) * dimension;
    for (size_t i = 0; i < dimension; ++i) {
        try {
            outer[i] = new int[dimension];
            allocated += sizeof(int) * dimension;
        } catch (std::bad_alloc&) {
            std::cerr << "bad alloc at iteration " << i << " with " << allocated << " bytes allocated\n";
            return -1;
        }
    }
}

Or

#include <iostream>

int main()
{
    size_t dimension = 100000;
    std::cout << "Allocating outer: " << (dimension * sizeof(int*)) << " bytes\n";
    int** outer = new int[dimension];
    for (size_t i = 0; i < dimension; ++i) {
        std::cout << "Allocating outer[" << i << "] (" << (dimension * sizeof(int)) << " bytes\n";
        outer[i] = new int[dimension];
    }
}