r - reorder second vector according to first

user_8552920 0
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Looking for a solution to do the following:

There are two vectors:

a <- c(2,1,3,4,7,6,5,9,8,10)
b <- c(3,2,1,6,5,4,7,8,9,10)

I would like to create a third vector that would give the order in which vector b needs to be reordered for it to be in the same order as a. In this case I would like to get:

c <- c(2,3,1,6,7,4,5,9,8,10)

so that:

> b[c] == a
[1] TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE

This question does not refer to cases in which a and b are double vectors, which is why the following question is also asked r - reorder second double vector according to first double vector

vector r
2 Answers

We can just use 'a' as index

c1 <- b[a]
b[c1] == a
#[1] TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE

If the values in 'a' are not integer i.e. numeric double 

b[order(a)]
user_37322710 
#include <iostream>
#include <vector>
#include <string>
#include <stdlib.h>
#include <string.h>

using namespace std;

int getnumber(string str)
{
    // For atoi, the input string has to start with a digit, so lets search for the first digit
    size_t i = 0;
    for ( ; i < str.length(); i++ ){ if ( isdigit(str[i]) ) break; }

    // remove the first chars, which aren't digits
    str = str.substr(i, str.length() - i );

    // convert the remaining text to an integer
    int id = atoi(str.c_str());

    return id;
}

int search_index_of_item_in_vector (vector<int> vec, int a)
{
    for (int i=0; i<vec.size(); i++)
    {
        if (vec.at(i) == a)
            return i;
    }
}

int main ()
{
    vector<string> vec1 = {"source55","source66","source33","source11","source22", "source115", "source219"};
    vector<string> vec2 = {"id115", "id219", "id55","id11","id22","id33","id66"};
    vector<string> vec3;
    vector<int> intvec1;
    vector<int> intvec2;
    vector<int> intvec3;

    cout << "Printing vec1" << endl;
    for (auto s = vec1.begin(); s != vec1.end(); s++)
        cout << *s << endl;

    cout << endl << endl;
    cout << "Printing vec2" << endl;
    for (auto s = vec2.begin(); s != vec2.end(); s++)
        cout << *s << endl;

    cout << endl << endl;
    //cout << "vector size = " << vec1.size() << endl;
    for (int i=0; i<vec1.size(); i++)
    {
        int v1 = getnumber(vec1.at(i)); 
        //cout << v1 << endl;
        intvec1.push_back (v1);
    }
    for (int i=0; i<vec2.size(); i++)
    {
        int v2 = getnumber(vec2.at(i)); 
        //cout << v2 << endl;
        intvec2.push_back (v2);
    }

#if 0
    for (auto a = intvec1.begin(); a != intvec1.end(); a++)
        cout << *a << endl;
    cout << endl << endl;
    for (auto a = intvec2.begin(); a != intvec2.end(); a++)
        cout << *a << endl;
#endif

    int index = -1;
    for (int i=0; i<vec1.size(); i++)
    {
        index = search_index_of_item_in_vector (intvec2, intvec1.at(i));
        if (index != i)
        {
            string temp = vec2.at(i);
            vec2.at(i) = vec2.at(index);
            vec2.at(index) = temp;

            int temp1 = intvec2.at(i);
            intvec2.at(i) = intvec2.at(index);
            intvec2.at(index) = temp1;
        }
    }

    cout << "Printing reordered vec2 " << endl;
    for (auto s = vec2.begin(); s != vec2.end(); s++)
        cout << *s << endl;

    cout << endl << endl;
}
user_61478900
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