找到两个向量之间的交点

Question

And I'm doing pretty good in fact I found the intersection and think I have the right code. Only problem is that it doesn't seem to print out the last value.

So if I have two sets:

9 12 7 8 1 19 11 2 14

15 10 8 2 5 16 14 7 19 0 11 3 13 18 9 17 1 12

My code will produce the following output:

1
2
7
8
9
11
12
14

But the right intersection of the sets should be:

1
2
7
8
9
11
12
14
19

So, my code doesn't print out the last value and I can't find out why.

void findIntersection(vector<int> A, vector<int> B)
{
    vector<int> intersection;
    int n1 = A.size();
    int n2 = B.size();
    int i = 0, j =0;
    while(i <= n1 && j <= n2)
    {
        if(A[i] > B[j])
        {
            j++;
        }
        else if( B[j] > A[i])
        {
            i++;
        }
        else
        {
            intersection.push_back(A[i]);
            i++;
            j++;
        }
    }

    for(unsigned int i = 0; i <= intersection.size(); i++)
    {
        cout << intersection[i] << endl;
    }
}

void slowintersect(vector<vector<int> > v)
{
    vector<int> vec;
    vector<int> c;
    int store_0;

    int row = 0;
    for(size_t j =0; j < v.at(row).size(); j++)
    {
        store_0 = v[row][j];
        c.push_back(store_0);
    }

    for(size_t i = 0; i < v.size(); i++)
    {
        for(size_t k = 0; k < v.at(i).size(); k++)
        {
            vec.push_back(k);
        }
    }

    findIntersection(c, vec);
}

Answer 1

#include <algorithm>
#include <set>

set<int> intersect;
set_intersection(s1.begin(),s1.end(),s2.begin(),s2.end(), // sorted!
                  std::inserter(intersect,intersect.begin()));

example

if you don't want std::algorithm , there are several errors in your code. ie:

for(unsigned int i = 0; i <= intersection.size(); i++)
                           ^
                        should be <

here is an important error:

int i = 0, j =0;
while(i <= n1 && j <= n2)
{        ^          ^
         <          <
    if(A[i] > B[j])
    {
        j++;

you will skip all n-1 A elements if A[0] is greater than each B element, and end the loop

Answer 2

I'm assuming you do not want to use the C++ standard algorithm.

Couple of things.
1. Your algorithm shall work only if both vectors are initially sorted. Are they?
2. You shouldn't access vector[vector.size()] element as it is out of bounds.

My second point means:

while(i <= n1 && j <= n2)

Change this to

while(i < n1 && j < n2)

And change the following

for(unsigned int i = 0; i <= intersection.size(); i++)

to

for(unsigned int i = 0; i < intersection.size(); i++)

Also, MOST IMPORTANT ERROR!!!!

for(size_t k = 0; k < v.at(i).size(); k++)
            {
                vec.push_back(k);

            }

Change it to:

for(size_t k = 0; k < v[i].size(); k++)
            {
                vec.push_back(v[i][k]);

            }

Answer 3

Your while co dition is broken. You must continue until both indexes are at the end. Currently you stop when one of the indexes has reached the end.

When you fix that, you also must ensure that you don't increase i beyond n1 ; same for j and n2 .

While you are developing you should rather use vector.at(i) instead of vector[i] to enable safety checks.