c ++数组无法获得正确的数组

Question

I have Array A[9]= {1,2,3,4,5,6,7,8,9} and I need to delete the numbers which are not dividing by 2. The code I tried to do:

int main()
{
    int n;
    ifstream fd(Cdf);
    fd>>n; // read how many numbers are in the file.
    int A[n];
    for(int i = 0; i < n; i++)
    {
        fd >> A[i]; //read the numbers from file
    }
    for(int i = 0; i < n; i ++) // moving the numbers.
    {
        if(A[i] % 2 !=0)
        {
            for(int j = i; j < n; j++)
            {
                A[i] = A[i+1];
            }
        }
    }
    fd.close();
    return 0;
}

But I get numbers like 224466888 . what I need to do to get 2,4,6,8?

I need to delete numbers in the same array.

Answer 1

First you should use std::vector for dynamic size arrays. Second, for removing numbers that are even in a vector, you can do :

std::vector<int> inf = {12,0,5,6,8};
auto func = [](int i){return i % 2 != 0;}; 
inf.erase(std::remove_if(inf.begin(),inf.end(),func), inf.end());

EDIT :

Ok, so you can still do this without std::vectors, but it will be uglier :

#include <algorithm>

int res[] = {2,5,9,8,6,7};
int size = 6;
auto func = [](int i){return i % 2 != 0;};
int new_size = std::remove_if(res,res + size, func) - res;

All the data you want is in [0, new_size[ range, the other part of your array is now garbage.

Answer 2

An array can't be used for your purpose. It is allocated on stack and its size can't be changed dynamically (you can't change the size of an array in general, not only when it is allocated on stack).

You could allocate a second array and keep reallocating it with realloc everytime you add a new element but that's not the good way to do it. You are working with C++ so just use a std::vector<int> and your problems will be solved:

std::vector<int> evenArray;
evenArray.reserve(sizeof(A)/sizeof(A[0])/2);

if (number is even) {
  evenArray.pushBack(number);
}

Mind that vector stores elements contiguously so this is legal:

int *evenA = &evenArray[0];

Answer 3

For your inner for loop you should be referencing j , not i .

for(int j = i; j < n - 1; j++)
{
    A[j] = A[j+1];
}

Otherwise, what's the point of creating j ?

Of course, this also means if you read the whole array back you will display all the characters that were shifted (which will just be equal to the last number). So, you should probably keep track of the new length of the array and just iterate to that instead of the end of the array.

EDIT:
In the inner for loop you need to loop to n - 1 otherwise when you have A[j + 1] it will go off the end of the array when you to change it, which may or may not give you a runtime error.

Answer 4

Your removal loop is indexing with the wrong variable:

for(int j = i; j < n; j++)
{
    A[i] = A[i+1];
}

You're using i , which doesn't change in the loop.
Change it to j . You also need to subtract one from the upper limit, as you'd step outside of the array otherwise when accessing A[j + 1] .

for(int j = i; j < n - 1; j++)
{
    A[j] = A[j + 1];
}