c++ – I am not sure why I get this result

#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

void myfunction(int i)
{
    cout<<" "<<i;
};

void multiply(int a)
{
    a*2;
}

int main()
{
     int t[]={10,5,9,6,2,4,7,8,3,1};
    vector<int> v1(t,t+10);
    for_each(v1.begin(),v1.end(),multiply);
    iter_swap(v1.begin(), t+9);
    for_each(v1.begin(),v1.end(),myfunction);
    
    return 0;
}

Hello, I actually have question about this code.
when I run this, I see the result is 1 5 9 6 2 4 7 8 3 1 and I am not sure why the last element is still 1 even though I assigned the statement iter_swap(v1.begin(), t+9); in the middle.
I am the kind of beginner so I appreciate someone answer this question.
Thank you.