用C ++排序向量

Question

I read a integer binary file to int vector. When I use the Sort function the vector is zeroing...

I know the vector is OK!

What could be wrong?

std::ifstream input("D:\\Amostra.txt", ios::binary);
vector<int> v (NumBlocks*ELEMENTS_PER_BLOCK*sizeof(int));

input.read(reinterpret_cast<char *>(&v[0]), NumBlocks*ELEMENTS_PER_BLOCK*sizeof(int));


sort(v.begin(), v.end());

for (int i=0; i<ELEMENTS_PER_BLOCK*NumBlocks; i++){
        cout << v[i] << endl;
    };
system("pause");

Answer 1

vector<int> v (NumBlocks*ELEMENTS_PER_BLOCK*sizeof(int));

The argument to that constructor is the number of elements you need, not the number of bytes those elements take. This will create sizeof(int)*N elements, where N is the number you need. After sorting the first (sizeof(int)-1)*N will be 0.

input.read(reinterpret_cast<char *>(&v[0]), NumBlocks*ELEMENTS_PER_BLOCK*sizeof(int));

The file has .txt extension, but you are reading it as if it was binary. If the file is a binary dump, then this read is... well... a code smell but not completely wrong. If the file is text then this is completely wrong.

You can read a text file that contains only space separated integers using the copy algorithm and a istream_iterator :

std::vector<int> v;
v.reserve(NumBlocks*ELEMENTS_PER_BLOCK);
std::copy( std::istream_iterator<int>(input), std::istream_iterator<int>(),
           std::back_inserter( v ) );

Answer 2

The error's in this line:

vector<int> v (NumBlocks*ELEMENTS_PER_BLOCK*sizeof(int));

The argument should be the number of elements, not the number of bytes, so take out the *sizeof(int) from the end. As it is, your vector has 4 times as many elements as you want. The ones you haven't read into are all zero, so when you call sort , they go to the front of the vector, and then you only print out the zero ones, not the ones with real data.