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Using C++, and hopefully the standard library, I want to sort a sequence of samples in ascending order, but I also want to remember the original indexes of the new samples.
For example, I have a set, or vector, or matrix of samples A : [5, 2, 1, 4, 3]. I want to sort these to be B : [1,2,3,4,5], but I also want to remember the original indexes of the values, so I can get another set which would be:
C : [2, 1, 4, 3, 0 ] - which corresponds to the index of each element in 'B', in the original 'A'.
For example, in Matlab you can do:
[a,b]=sort([5, 8, 7])
a = 5 7 8
b = 1 3 2
Can anyone see a good way to do this?
Using C++ 11 lambdas:
#include <iostream>
#include <vector>
#include <numeric> // std::iota
#include <algorithm> // std::sort, std::stable_sort
using namespace std;
template <typename T>
vector<size_t> sort_indexes(const vector<T> &v) {
// initialize original index locations
vector<size_t> idx(v.size());
iota(idx.begin(), idx.end(), 0);
// sort indexes based on comparing values in v
// using std::stable_sort instead of std::sort
// to avoid unnecessary index re-orderings
// when v contains elements of equal values
stable_sort(idx.begin(), idx.end(),
[&v](size_t i1, size_t i2) {return v[i1] < v[i2];});
return idx;
}
Now you can use the returned index vector in iterations such as
for (auto i: sort_indexes(v)) {
cout << v[i] << endl;
}
You can also choose to supply your original index vector, sort function, comparator, or automatically reorder v in the sort_indexes function using an extra vector.
You could sort std::pair instead of just ints - first int is original data, second int is original index. Then supply a comparator that only sorts on the first int. Example:
Your problem instance: v = [5 7 8]
New problem instance: v_prime = [<5,0>, <8,1>, <7,2>]
Sort the new problem instance using a comparator like:
typedef std::pair<int,int> mypair;
bool comparator ( const mypair& l, const mypair& r)
{ return l.first < r.first; }
// forgetting the syntax here but intent is clear enough
The result of std::sort on v_prime, using that comparator, should be:
v_prime = [<5,0>, <7,2>, <8,1>]
You can peel out the indices by walking the vector, grabbing .second from each std::pair.
Suppose Given vector is
A=[2,4,3]
Create a new vector
V=[0,1,2] // indicating positions
Sort V and while sorting instead of comparing elements of V , compare corresponding elements of A
//Assume A is a given vector with N elements
vector<int> V(N);
std::iota(V.begin(),V.end(),0); //Initializing
sort( V.begin(),V.end(), [&](int i,int j){return A[i]<A[j];} );
vector<pair<int,int> >a;
for (i = 0 ;i < n ; i++) {
// filling the original array
cin >> k;
a.push_back (make_pair (k,i)); // k = value, i = original index
}
sort (a.begin(),a.end());
for (i = 0 ; i < n ; i++){
cout << a[i].first << " " << a[i].second << "\n";
}
Now a contains both both our values and their respective indices in the sorted.
a[i].first = value at i'th.
a[i].second = idx in initial array.
I wrote generic version of index sort.
template <class RAIter, class Compare>
void argsort(RAIter iterBegin, RAIter iterEnd, Compare comp,
std::vector<size_t>& indexes) {
std::vector< std::pair<size_t,RAIter> > pv ;
pv.reserve(iterEnd - iterBegin) ;
RAIter iter ;
size_t k ;
for (iter = iterBegin, k = 0 ; iter != iterEnd ; iter++, k++) {
pv.push_back( std::pair<int,RAIter>(k,iter) ) ;
}
std::sort(pv.begin(), pv.end(),
[&comp](const std::pair<size_t,RAIter>& a, const std::pair<size_t,RAIter>& b) -> bool
{ return comp(*a.second, *b.second) ; }) ;
indexes.resize(pv.size()) ;
std::transform(pv.begin(), pv.end(), indexes.begin(),
[](const std::pair<size_t,RAIter>& a) -> size_t { return a.first ; }) ;
}
Usage is the same as that of std::sort except for an index container to receive sorted indexes.
testing:
int a[] = { 3, 1, 0, 4 } ;
std::vector<size_t> indexes ;
argsort(a, a + sizeof(a) / sizeof(a[0]), std::less<int>(), indexes) ;
for (size_t i : indexes) printf("%d\n", int(i)) ;
you should get 2 1 0 3.
for the compilers without c++0x support, replace the lamba expression as a class template:
template <class RAIter, class Compare>
class PairComp {
public:
Compare comp ;
PairComp(Compare comp_) : comp(comp_) {}
bool operator() (const std::pair<size_t,RAIter>& a,
const std::pair<size_t,RAIter>& b) const { return comp(*a.second, *b.second) ; }
} ;
and rewrite std::sort as
std::sort(pv.begin(), pv.end(), PairComp(comp)()) ;
I came across this question, and figured out sorting the iterators directly would be a way to sort the values and keep track of indices; There is no need to define an extra container of pairs of ( value, index ) which is helpful when the values are large objects; The iterators provides the access to both the value and the index:
/*
* a function object that allows to compare
* the iterators by the value they point to
*/
template < class RAIter, class Compare >
class IterSortComp
{
public:
IterSortComp ( Compare comp ): m_comp ( comp ) { }
inline bool operator( ) ( const RAIter & i, const RAIter & j ) const
{
return m_comp ( * i, * j );
}
private:
const Compare m_comp;
};
template <class INIter, class RAIter, class Compare>
void itersort ( INIter first, INIter last, std::vector < RAIter > & idx, Compare comp )
{
idx.resize ( std::distance ( first, last ) );
for ( typename std::vector < RAIter >::iterator j = idx.begin( ); first != last; ++ j, ++ first )
* j = first;
std::sort ( idx.begin( ), idx.end( ), IterSortComp< RAIter, Compare > ( comp ) );
}
as for the usage example:
std::vector < int > A ( n );
// populate A with some random values
std::generate ( A.begin( ), A.end( ), rand );
std::vector < std::vector < int >::const_iterator > idx;
itersort ( A.begin( ), A.end( ), idx, std::less < int > ( ) );
now, for example, the 5th smallest element in the sorted vector would have value **idx[ 5 ] and its index in the original vector would be distance( A.begin( ), *idx[ 5 ] ) or simply *idx[ 5 ] - A.begin( ).
Consider using std::multimap as suggested by #Ulrich Eckhardt. Just that the code could be made even simpler.
Given
std::vector<int> a = {5, 2, 1, 4, 3}; // a: 5 2 1 4 3
To sort in the mean time of insertion
std::multimap<int, std::size_t> mm;
for (std::size_t i = 0; i != a.size(); ++i)
mm.insert({a[i], i});
To retrieve values and original indices
std::vector<int> b;
std::vector<std::size_t> c;
for (const auto & kv : mm) {
b.push_back(kv.first); // b: 1 2 3 4 5
c.push_back(kv.second); // c: 2 1 4 3 0
}
The reason to prefer a std::multimap to a std::map is to allow equal values in original vectors. Also please note that, unlike for std::map, operator[] is not defined for std::multimap.
There is another way to solve this, using a map:
vector<double> v = {...}; // input data
map<double, unsigned> m; // mapping from value to its index
for (auto it = v.begin(); it != v.end(); ++it)
m[*it] = it - v.begin();
This will eradicate non-unique elements though. If that's not acceptable, use a multimap:
vector<double> v = {...}; // input data
multimap<double, unsigned> m; // mapping from value to its index
for (auto it = v.begin(); it != v.end(); ++it)
m.insert(make_pair(*it, it - v.begin()));
In order to output the indices, iterate over the map or multimap:
for (auto it = m.begin(); it != m.end(); ++it)
cout << it->second << endl;
Beautiful solution by #Lukasz Wiklendt! Although in my case I needed something more generic so I modified it a bit:
template <class RAIter, class Compare>
vector<size_t> argSort(RAIter first, RAIter last, Compare comp) {
vector<size_t> idx(last-first);
iota(idx.begin(), idx.end(), 0);
auto idxComp = [&first,comp](size_t i1, size_t i2) {
return comp(first[i1], first[i2]);
};
sort(idx.begin(), idx.end(), idxComp);
return idx;
}
Example: Find indices sorting a vector of strings by length, except for the first element which is a dummy.
vector<string> test = {"dummy", "a", "abc", "ab"};
auto comp = [](const string &a, const string& b) {
return a.length() > b.length();
};
const auto& beginIt = test.begin() + 1;
vector<size_t> ind = argSort(beginIt, test.end(), comp);
for(auto i : ind)
cout << beginIt[i] << endl;
prints:
abc
ab
a
Make a std::pair in function then sort pair :
generic version :
template< class RandomAccessIterator,class Compare >
auto sort2(RandomAccessIterator begin,RandomAccessIterator end,Compare cmp) ->
std::vector<std::pair<std::uint32_t,RandomAccessIterator>>
{
using valueType=typename std::iterator_traits<RandomAccessIterator>::value_type;
using Pair=std::pair<std::uint32_t,RandomAccessIterator>;
std::vector<Pair> index_pair;
index_pair.reserve(std::distance(begin,end));
for(uint32_t idx=0;begin!=end;++begin,++idx){
index_pair.push_back(Pair(idx,begin));
}
std::sort( index_pair.begin(),index_pair.end(),[&](const Pair& lhs,const Pair& rhs){
return cmp(*lhs.second,*rhs.second);
});
return index_pair;
}
ideone
Well, my solution uses residue technique. We can place the values under sorting in the upper 2 bytes and the indices of the elements - in the lower 2 bytes:
int myints[] = {32,71,12,45,26,80,53,33};
for (int i = 0; i < 8; i++)
myints[i] = myints[i]*(1 << 16) + i;
Then sort the array myints as usual:
std::vector<int> myvector(myints, myints+8);
sort(myvector.begin(), myvector.begin()+8, std::less<int>());
After that you can access the elements' indices via residuum. The following code prints the indices of the values sorted in the ascending order:
for (std::vector<int>::iterator it = myvector.begin(); it != myvector.end(); ++it)
std::cout << ' ' << (*it)%(1 << 16);
Of course, this technique works only for the relatively small values in the original array myints (i.e. those which can fit into upper 2 bytes of int). But it has additional benefit of distinguishing identical values of myints: their indices will be printed in the right order.
If it's possible, you can build the position array using find function, and then sort the array.
Or maybe you can use a map where the key would be the element, and the values a list of its position in the upcoming arrays (A, B and C)
It depends on later uses of those arrays.
I recently stepped upon the elegant projection feature of C++20 <ranges> and it allows to write shorter/clearer code:
std::vector<std::size_t> B(std::size(A));
std::iota(begin(B), end(B), 0);
std::ranges::sort(B, {}, [&](std::size_t i){ return A[i]; });
{} refers to the usual std::less<std::size_t>. So as you can see we define a function to call on each element before any comparaison. This projection feature is actually quite powerful since this function can be, as here, a lambda or it can even be a method, or a member value. For instance:
struct Item {
float price;
float weight;
float efficiency() const { return price / weight; }
};
int main() {
std::vector<Item> items{{7, 9}, {3, 4}, {5, 3}, {9, 7}};
std::ranges::sort(items, std::greater<>(), &Item::efficiency);
// now items are sorted by their efficiency in decreasing order:
// items = {{5, 3}, {9, 7}, {7, 9}, {3, 4}}
}
If we wanted to sort by increasing price:
std::ranges::sort(items, {}, &Item::price);
Don't define operator< or use lambdas, use a projection!
Are the items in the vector unique? If so, copy the vector, sort one of the copies with STL Sort then you can find which index each item had in the original vector.
If the vector is supposed to handle duplicate items, I think youre better of implementing your own sort routine.
For this type of question
Store the orignal array data into a new data and then binary search the first element of the sorted array into the duplicated array and that indice should be stored into a vector or array.
input array=>a
duplicate array=>b
vector=>c(Stores the indices(position) of the orignal array
Syntax:
for(i=0;i<n;i++)
c.push_back(binarysearch(b,n,a[i]));`
Here binarysearch is a function which takes the array,size of array,searching item and would return the position of the searched item
One solution is to use a 2D vector.
#include <algorithm>
#include <iostream>
#include <vector>
using namespace std;
int main() {
vector<vector<double>> val_and_id;
val_and_id.resize(5);
for (int i = 0; i < 5; i++) {
val_and_id[i].resize(2); // one to store value, the other for index.
}
// Store value in dimension 1, and index in the other:
// say values are 5,4,7,1,3.
val_and_id[0][0] = 5.0;
val_and_id[1][0] = 4.0;
val_and_id[2][0] = 7.0;
val_and_id[3][0] = 1.0;
val_and_id[4][0] = 3.0;
val_and_id[0][1] = 0.0;
val_and_id[1][1] = 1.0;
val_and_id[2][1] = 2.0;
val_and_id[3][1] = 3.0;
val_and_id[4][1] = 4.0;
sort(val_and_id.begin(), val_and_id.end());
// display them:
cout << "Index \t" << "Value \n";
for (int i = 0; i < 5; i++) {
cout << val_and_id[i][1] << "\t" << val_and_id[i][0] << "\n";
}
return 0;
}
Here is the output:
Index Value
3 1
4 3
1 4
0 5
2 7
Assume I have a vector with the following elements {1, 1, 2, 3, 3, 4}
I want to write a program with c++ code to remove the unique values and keep only the duplicated once. So the end result will be something like this {1,3}.
So far this is what I've done, but it takes a lot of time,
Is there any way this can be more efficient,
vector <int> g1 = {1,1,2,3,3,4}
vector <int> g2;
for(int i = 0; i < g1.size(); i++)
{
if(count(g1.begin(), g1.end(), g1[i]) > 1)
g2.push_back(g1[i]);
}
v.erase(std::unique(g2.begin(), g2.end()), g2.end());
for(int i = 0; i < g2.size(); i++)
{
cout << g2[i];
}
My approach is to create an <algorithm>-style template, and use an unordered_map to do the counting. This means you only iterate over the input list once, and the time complexity is O(n). It does use O(n) extra memory though, and isn't particularly cache-friendly. Also this does assume that the type in the input is hashable.
#include <algorithm>
#include <iostream>
#include <iterator>
#include <unordered_map>
template <typename InputIt, typename OutputIt>
OutputIt copy_duplicates(
InputIt first,
InputIt last,
OutputIt d_first)
{
std::unordered_map<typename std::iterator_traits<InputIt>::value_type,
std::size_t> seen;
for ( ; first != last; ++first) {
if ( 2 == ++seen[*first] ) {
// only output on the second time of seeing a value
*d_first = *first;
++d_first;
}
}
return d_first;
}
int main()
{
int i[] = {1, 2, 3, 1, 1, 3, 5}; // print 1, 3,
// ^ ^
copy_duplicates(std::begin(i), std::end(i),
std::ostream_iterator<int>(std::cout, ", "));
}
This can output to any kind of iterator. There are special iterators you can use that when written to will insert the value into a container.
Here's a way that's a little more cache friendly than unordered_map answer, but is O(n log n) instead of O(n), though it does not use any extra memory and does no allocations. Additionally, the overall multiplier is probably higher, in spite of it's cache friendliness.
#include <vector>
#include <algorithm>
void only_distinct_duplicates(::std::vector<int> &v)
{
::std::sort(v.begin(), v.end());
auto output = v.begin();
auto test = v.begin();
auto run_start = v.begin();
auto const end = v.end();
for (auto test = v.begin(); test != end; ++test) {
if (*test == *run_start) {
if ((test - run_start) == 1) {
*output = *run_start;
++output;
}
} else {
run_start = test;
}
}
v.erase(output, end);
}
I've tested this, and it works. If you want a generic version that should work on any type that vector can store:
template <typename T>
void only_distinct_duplicates(::std::vector<T> &v)
{
::std::sort(v.begin(), v.end());
auto output = v.begin();
auto test = v.begin();
auto run_start = v.begin();
auto const end = v.end();
for (auto test = v.begin(); test != end; ++test) {
if (*test != *run_start) {
if ((test - run_start) > 1) {
::std::swap(*output, *run_start);
++output;
}
run_start = test;
}
}
if ((end - run_start) > 1) {
::std::swap(*output, *run_start);
++output;
}
v.erase(output, end);
}
Assuming the input vector is not sorted, the following will work and is generalized to support any vector with element type T. It will be more efficient than the other solutions proposed so far.
#include <algorithm>
#include <iostream>
#include <vector>
template<typename T>
void erase_unique_and_duplicates(std::vector<T>& v)
{
auto first{v.begin()};
std::sort(first, v.end());
while (first != v.end()) {
auto last{std::find_if(first, v.end(), [&](int i) { return i != *first; })};
if (last - first > 1) {
first = v.erase(first + 1, last);
}
else {
first = v.erase(first);
}
}
}
int main(int argc, char** argv)
{
std::vector<int> v{1, 2, 3, 4, 5, 2, 3, 4};
erase_unique_and_duplicates(v);
// The following will print '2 3 4'.
for (int i : v) {
std::cout << i << ' ';
}
std::cout << '\n';
return 0;
}
I have 2 improvements for you:
You can change your count to start at g1.begin() + i, everything before was handled by the previous iterations of the loop.
You can change the if to == 2 instead of > 1, so it will add numbers only once, independent of the occurences. If a number is 5 times in the vector, the first 3 will be ignored, the 4th will make it into the new vector and the 5th will be ignored again. So you can remove the erase of the duplicates
Example:
#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
int main() {
vector <int> g1 = {1,1,2,3,3,1,4};
vector <int> g2;
for(int i = 0; i < g1.size(); i++)
{
if(count(g1.begin() + i, g1.end(), g1[i]) == 2)
g2.push_back(g1[i]);
}
for(int i = 0; i < g2.size(); i++)
{
cout << g2[i] << " ";
}
cout << endl;
return 0;
}
I'll borrow a principal from Python which is excellent for such operations -
You can use a dictionary where the dictionary-key is the item in the vector and the dictionary-value is the count (start with 1 and increase by one every time you encounter a value that is already in the dictionary).
afterward, create a new vector (or clear the original) with only the dictionary keys that are larger than 1.
Look up in google - std::map
Hope this helps.
In general, that task got complexity about O(n*n), that's why it appears slow. Does it have to be a vector? Is that a restriction? Must it be ordered? If not, it better to actually store values as std::map, which eliminates doubles when populated, or as a std::multimap if presence of doubles matters.
Using C++, and hopefully the standard library, I want to sort a sequence of samples in ascending order, but I also want to remember the original indexes of the new samples.
For example, I have a set, or vector, or matrix of samples A : [5, 2, 1, 4, 3]. I want to sort these to be B : [1,2,3,4,5], but I also want to remember the original indexes of the values, so I can get another set which would be:
C : [2, 1, 4, 3, 0 ] - which corresponds to the index of each element in 'B', in the original 'A'.
For example, in Matlab you can do:
[a,b]=sort([5, 8, 7])
a = 5 7 8
b = 1 3 2
Can anyone see a good way to do this?
Using C++ 11 lambdas:
#include <iostream>
#include <vector>
#include <numeric> // std::iota
#include <algorithm> // std::sort, std::stable_sort
using namespace std;
template <typename T>
vector<size_t> sort_indexes(const vector<T> &v) {
// initialize original index locations
vector<size_t> idx(v.size());
iota(idx.begin(), idx.end(), 0);
// sort indexes based on comparing values in v
// using std::stable_sort instead of std::sort
// to avoid unnecessary index re-orderings
// when v contains elements of equal values
stable_sort(idx.begin(), idx.end(),
[&v](size_t i1, size_t i2) {return v[i1] < v[i2];});
return idx;
}
Now you can use the returned index vector in iterations such as
for (auto i: sort_indexes(v)) {
cout << v[i] << endl;
}
You can also choose to supply your original index vector, sort function, comparator, or automatically reorder v in the sort_indexes function using an extra vector.
You could sort std::pair instead of just ints - first int is original data, second int is original index. Then supply a comparator that only sorts on the first int. Example:
Your problem instance: v = [5 7 8]
New problem instance: v_prime = [<5,0>, <8,1>, <7,2>]
Sort the new problem instance using a comparator like:
typedef std::pair<int,int> mypair;
bool comparator ( const mypair& l, const mypair& r)
{ return l.first < r.first; }
// forgetting the syntax here but intent is clear enough
The result of std::sort on v_prime, using that comparator, should be:
v_prime = [<5,0>, <7,2>, <8,1>]
You can peel out the indices by walking the vector, grabbing .second from each std::pair.
Suppose Given vector is
A=[2,4,3]
Create a new vector
V=[0,1,2] // indicating positions
Sort V and while sorting instead of comparing elements of V , compare corresponding elements of A
//Assume A is a given vector with N elements
vector<int> V(N);
std::iota(V.begin(),V.end(),0); //Initializing
sort( V.begin(),V.end(), [&](int i,int j){return A[i]<A[j];} );
vector<pair<int,int> >a;
for (i = 0 ;i < n ; i++) {
// filling the original array
cin >> k;
a.push_back (make_pair (k,i)); // k = value, i = original index
}
sort (a.begin(),a.end());
for (i = 0 ; i < n ; i++){
cout << a[i].first << " " << a[i].second << "\n";
}
Now a contains both both our values and their respective indices in the sorted.
a[i].first = value at i'th.
a[i].second = idx in initial array.
I wrote generic version of index sort.
template <class RAIter, class Compare>
void argsort(RAIter iterBegin, RAIter iterEnd, Compare comp,
std::vector<size_t>& indexes) {
std::vector< std::pair<size_t,RAIter> > pv ;
pv.reserve(iterEnd - iterBegin) ;
RAIter iter ;
size_t k ;
for (iter = iterBegin, k = 0 ; iter != iterEnd ; iter++, k++) {
pv.push_back( std::pair<int,RAIter>(k,iter) ) ;
}
std::sort(pv.begin(), pv.end(),
[&comp](const std::pair<size_t,RAIter>& a, const std::pair<size_t,RAIter>& b) -> bool
{ return comp(*a.second, *b.second) ; }) ;
indexes.resize(pv.size()) ;
std::transform(pv.begin(), pv.end(), indexes.begin(),
[](const std::pair<size_t,RAIter>& a) -> size_t { return a.first ; }) ;
}
Usage is the same as that of std::sort except for an index container to receive sorted indexes.
testing:
int a[] = { 3, 1, 0, 4 } ;
std::vector<size_t> indexes ;
argsort(a, a + sizeof(a) / sizeof(a[0]), std::less<int>(), indexes) ;
for (size_t i : indexes) printf("%d\n", int(i)) ;
you should get 2 1 0 3.
for the compilers without c++0x support, replace the lamba expression as a class template:
template <class RAIter, class Compare>
class PairComp {
public:
Compare comp ;
PairComp(Compare comp_) : comp(comp_) {}
bool operator() (const std::pair<size_t,RAIter>& a,
const std::pair<size_t,RAIter>& b) const { return comp(*a.second, *b.second) ; }
} ;
and rewrite std::sort as
std::sort(pv.begin(), pv.end(), PairComp(comp)()) ;
I came across this question, and figured out sorting the iterators directly would be a way to sort the values and keep track of indices; There is no need to define an extra container of pairs of ( value, index ) which is helpful when the values are large objects; The iterators provides the access to both the value and the index:
/*
* a function object that allows to compare
* the iterators by the value they point to
*/
template < class RAIter, class Compare >
class IterSortComp
{
public:
IterSortComp ( Compare comp ): m_comp ( comp ) { }
inline bool operator( ) ( const RAIter & i, const RAIter & j ) const
{
return m_comp ( * i, * j );
}
private:
const Compare m_comp;
};
template <class INIter, class RAIter, class Compare>
void itersort ( INIter first, INIter last, std::vector < RAIter > & idx, Compare comp )
{
idx.resize ( std::distance ( first, last ) );
for ( typename std::vector < RAIter >::iterator j = idx.begin( ); first != last; ++ j, ++ first )
* j = first;
std::sort ( idx.begin( ), idx.end( ), IterSortComp< RAIter, Compare > ( comp ) );
}
as for the usage example:
std::vector < int > A ( n );
// populate A with some random values
std::generate ( A.begin( ), A.end( ), rand );
std::vector < std::vector < int >::const_iterator > idx;
itersort ( A.begin( ), A.end( ), idx, std::less < int > ( ) );
now, for example, the 5th smallest element in the sorted vector would have value **idx[ 5 ] and its index in the original vector would be distance( A.begin( ), *idx[ 5 ] ) or simply *idx[ 5 ] - A.begin( ).
Consider using std::multimap as suggested by #Ulrich Eckhardt. Just that the code could be made even simpler.
Given
std::vector<int> a = {5, 2, 1, 4, 3}; // a: 5 2 1 4 3
To sort in the mean time of insertion
std::multimap<int, std::size_t> mm;
for (std::size_t i = 0; i != a.size(); ++i)
mm.insert({a[i], i});
To retrieve values and original indices
std::vector<int> b;
std::vector<std::size_t> c;
for (const auto & kv : mm) {
b.push_back(kv.first); // b: 1 2 3 4 5
c.push_back(kv.second); // c: 2 1 4 3 0
}
The reason to prefer a std::multimap to a std::map is to allow equal values in original vectors. Also please note that, unlike for std::map, operator[] is not defined for std::multimap.
There is another way to solve this, using a map:
vector<double> v = {...}; // input data
map<double, unsigned> m; // mapping from value to its index
for (auto it = v.begin(); it != v.end(); ++it)
m[*it] = it - v.begin();
This will eradicate non-unique elements though. If that's not acceptable, use a multimap:
vector<double> v = {...}; // input data
multimap<double, unsigned> m; // mapping from value to its index
for (auto it = v.begin(); it != v.end(); ++it)
m.insert(make_pair(*it, it - v.begin()));
In order to output the indices, iterate over the map or multimap:
for (auto it = m.begin(); it != m.end(); ++it)
cout << it->second << endl;
Beautiful solution by #Lukasz Wiklendt! Although in my case I needed something more generic so I modified it a bit:
template <class RAIter, class Compare>
vector<size_t> argSort(RAIter first, RAIter last, Compare comp) {
vector<size_t> idx(last-first);
iota(idx.begin(), idx.end(), 0);
auto idxComp = [&first,comp](size_t i1, size_t i2) {
return comp(first[i1], first[i2]);
};
sort(idx.begin(), idx.end(), idxComp);
return idx;
}
Example: Find indices sorting a vector of strings by length, except for the first element which is a dummy.
vector<string> test = {"dummy", "a", "abc", "ab"};
auto comp = [](const string &a, const string& b) {
return a.length() > b.length();
};
const auto& beginIt = test.begin() + 1;
vector<size_t> ind = argSort(beginIt, test.end(), comp);
for(auto i : ind)
cout << beginIt[i] << endl;
prints:
abc
ab
a
Make a std::pair in function then sort pair :
generic version :
template< class RandomAccessIterator,class Compare >
auto sort2(RandomAccessIterator begin,RandomAccessIterator end,Compare cmp) ->
std::vector<std::pair<std::uint32_t,RandomAccessIterator>>
{
using valueType=typename std::iterator_traits<RandomAccessIterator>::value_type;
using Pair=std::pair<std::uint32_t,RandomAccessIterator>;
std::vector<Pair> index_pair;
index_pair.reserve(std::distance(begin,end));
for(uint32_t idx=0;begin!=end;++begin,++idx){
index_pair.push_back(Pair(idx,begin));
}
std::sort( index_pair.begin(),index_pair.end(),[&](const Pair& lhs,const Pair& rhs){
return cmp(*lhs.second,*rhs.second);
});
return index_pair;
}
ideone
Well, my solution uses residue technique. We can place the values under sorting in the upper 2 bytes and the indices of the elements - in the lower 2 bytes:
int myints[] = {32,71,12,45,26,80,53,33};
for (int i = 0; i < 8; i++)
myints[i] = myints[i]*(1 << 16) + i;
Then sort the array myints as usual:
std::vector<int> myvector(myints, myints+8);
sort(myvector.begin(), myvector.begin()+8, std::less<int>());
After that you can access the elements' indices via residuum. The following code prints the indices of the values sorted in the ascending order:
for (std::vector<int>::iterator it = myvector.begin(); it != myvector.end(); ++it)
std::cout << ' ' << (*it)%(1 << 16);
Of course, this technique works only for the relatively small values in the original array myints (i.e. those which can fit into upper 2 bytes of int). But it has additional benefit of distinguishing identical values of myints: their indices will be printed in the right order.
If it's possible, you can build the position array using find function, and then sort the array.
Or maybe you can use a map where the key would be the element, and the values a list of its position in the upcoming arrays (A, B and C)
It depends on later uses of those arrays.
I recently stepped upon the elegant projection feature of C++20 <ranges> and it allows to write shorter/clearer code:
std::vector<std::size_t> B(std::size(A));
std::iota(begin(B), end(B), 0);
std::ranges::sort(B, {}, [&](std::size_t i){ return A[i]; });
{} refers to the usual std::less<std::size_t>. So as you can see we define a function to call on each element before any comparaison. This projection feature is actually quite powerful since this function can be, as here, a lambda or it can even be a method, or a member value. For instance:
struct Item {
float price;
float weight;
float efficiency() const { return price / weight; }
};
int main() {
std::vector<Item> items{{7, 9}, {3, 4}, {5, 3}, {9, 7}};
std::ranges::sort(items, std::greater<>(), &Item::efficiency);
// now items are sorted by their efficiency in decreasing order:
// items = {{5, 3}, {9, 7}, {7, 9}, {3, 4}}
}
If we wanted to sort by increasing price:
std::ranges::sort(items, {}, &Item::price);
Don't define operator< or use lambdas, use a projection!
Are the items in the vector unique? If so, copy the vector, sort one of the copies with STL Sort then you can find which index each item had in the original vector.
If the vector is supposed to handle duplicate items, I think youre better of implementing your own sort routine.
For this type of question
Store the orignal array data into a new data and then binary search the first element of the sorted array into the duplicated array and that indice should be stored into a vector or array.
input array=>a
duplicate array=>b
vector=>c(Stores the indices(position) of the orignal array
Syntax:
for(i=0;i<n;i++)
c.push_back(binarysearch(b,n,a[i]));`
Here binarysearch is a function which takes the array,size of array,searching item and would return the position of the searched item
One solution is to use a 2D vector.
#include <algorithm>
#include <iostream>
#include <vector>
using namespace std;
int main() {
vector<vector<double>> val_and_id;
val_and_id.resize(5);
for (int i = 0; i < 5; i++) {
val_and_id[i].resize(2); // one to store value, the other for index.
}
// Store value in dimension 1, and index in the other:
// say values are 5,4,7,1,3.
val_and_id[0][0] = 5.0;
val_and_id[1][0] = 4.0;
val_and_id[2][0] = 7.0;
val_and_id[3][0] = 1.0;
val_and_id[4][0] = 3.0;
val_and_id[0][1] = 0.0;
val_and_id[1][1] = 1.0;
val_and_id[2][1] = 2.0;
val_and_id[3][1] = 3.0;
val_and_id[4][1] = 4.0;
sort(val_and_id.begin(), val_and_id.end());
// display them:
cout << "Index \t" << "Value \n";
for (int i = 0; i < 5; i++) {
cout << val_and_id[i][1] << "\t" << val_and_id[i][0] << "\n";
}
return 0;
}
Here is the output:
Index Value
3 1
4 3
1 4
0 5
2 7
How can I get the maximum or minimum value in a vector in C++?
And am I wrong in assuming it would be more or less the same with an array?
I need an iterator, right? I tried it with max_element, but I kept getting an error.
vector<int>::const_iterator it;
it = max_element(cloud.begin(), cloud.end());
error: request for member ‘begin’ in ‘cloud’, which is of non-class type ‘int [10]’
Using C++11/C++0x compile flags, you can
auto it = max_element(std::begin(cloud), std::end(cloud)); // C++11
Otherwise, write your own:
template <typename T, size_t N> const T* mybegin(const T (&a)[N]) { return a; }
template <typename T, size_t N> const T* myend (const T (&a)[N]) { return a+N; }
See it live at http://ideone.com/aDkhW:
#include <iostream>
#include <algorithm>
template <typename T, size_t N> const T* mybegin(const T (&a)[N]) { return a; }
template <typename T, size_t N> const T* myend (const T (&a)[N]) { return a+N; }
int main()
{
const int cloud[] = { 1,2,3,4,-7,999,5,6 };
std::cout << *std::max_element(mybegin(cloud), myend(cloud)) << '\n';
std::cout << *std::min_element(mybegin(cloud), myend(cloud)) << '\n';
}
Oh, and use std::minmax_element(...) if you need both at once :/
If you want to use the function std::max_element(), the way you have to do it is:
double max = *max_element(vector.begin(), vector.end());
cout<<"Max value: "<<max<<endl;
Let,
#include <vector>
vector<int> v {1, 2, 3, -1, -2, -3};
If the vector is sorted in ascending or descending order then you can find it with complexity O(1).
For a vector of ascending order the first element is the smallest element, you can get it by v[0] (0 based indexing) and last element is the largest element, you can get it by v[sizeOfVector-1].
If the vector is sorted in descending order then the last element is the smallest element,you can get it by v[sizeOfVector-1] and first element is the largest element, you can get it by v[0].
If the vector is not sorted then you have to iterate over the vector to get the smallest/largest element.In this case time complexity is O(n), here n is the size of vector.
int smallest_element = v[0]; //let, first element is the smallest one
int largest_element = v[0]; //also let, first element is the biggest one
for(int i = 1; i < v.size(); i++) //start iterating from the second element
{
if(v[i] < smallest_element)
{
smallest_element = v[i];
}
if(v[i] > largest_element)
{
largest_element = v[i];
}
}
You can use iterator,
for (vector<int>:: iterator it = v.begin(); it != v.end(); it++)
{
if(*it < smallest_element) //used *it (with asterisk), because it's an iterator
{
smallest_element = *it;
}
if(*it > largest_element)
{
largest_element = *it;
}
}
You can calculate it in input section (when you have to find smallest or largest element from a given vector)
int smallest_element, largest_element, value;
vector <int> v;
int n;//n is the number of elements to enter
cin >> n;
for(int i = 0;i<n;i++)
{
cin>>value;
if(i==0)
{
smallest_element= value; //smallest_element=v[0];
largest_element= value; //also, largest_element = v[0]
}
if(value<smallest_element and i>0)
{
smallest_element = value;
}
if(value>largest_element and i>0)
{
largest_element = value;
}
v.push_back(value);
}
Also you can get smallest/largest element by built in functions
#include<algorithm>
int smallest_element = *min_element(v.begin(),v.end());
int largest_element = *max_element(v.begin(),v.end());
You can get smallest/largest element of any range by using this functions. such as,
vector<int> v {1,2,3,-1,-2,-3};
cout << *min_element(v.begin(), v.begin() + 3); //this will print 1,smallest element of first three elements
cout << *max_element(v.begin(), v.begin() + 3); //largest element of first three elements
cout << *min_element(v.begin() + 2, v.begin() + 5); // -2, smallest element between third and fifth element (inclusive)
cout << *max_element(v.begin() + 2, v.begin()+5); //largest element between third and first element (inclusive)
I have used asterisk (*), before min_element()/max_element() functions. Because both of them return iterator. All codes are in c++.
You can print it directly using the max_element or min_element function.
For example:
cout << *max_element(v.begin(), v.end());
cout << *min_element(v.begin(), v.end());
Assuming cloud is int cloud[10] you can do it like this:
int *p = max_element(cloud, cloud + 10);
In C++11, you can use some function like that:
int maxAt(std::vector<int>& vector_name) {
int max = INT_MIN;
for (auto val : vector_name) {
if (max < val) max = val;
}
return max;
}
If you want to use an iterator, you can do a placement-new with an array.
std::array<int, 10> icloud = new (cloud) std::array<int,10>;
Note the lack of a () at the end, that is important. This creates an array class that uses that memory as its storage, and has STL features like iterators.
(This is C++ TR1/C++11 by the way)
You can use max_element to get the maximum value in vector.
The max_element returns an iterator to largest value in the range, or last if the range is empty. As an iterator is like pointers (or you can say pointer is a form of iterator), you can use a * before it to get the value.
So as per the problem you can get the maximum element in an vector as:
int max=*max_element(cloud.begin(), cloud.end());
It will give you the maximum element in your vector "cloud".
Hope it helps.
Answer on the behalf of the author
for (unsigned int i = 0; i < cdf.size(); i++)
if (cdf[i] < cdfMin)
cdfMin = cdf[i];
where cdf is a vector.
Just this:
// assuming "cloud" is:
// int cloud[10];
// or any other fixed size
#define countof(x) (sizeof(x)/sizeof((x)[0]))
int* pMax = std::max_element(cloud, cloud + countof(cloud));
I have a vector of strings:
std::vector<std::string> fName
which holds a list of file names <a,b,c,d,a,e,e,d,b>.
I want to get rid of all the files that have duplicates and want to retain only the files that do not have duplicates in the vector.
for(size_t l = 0; l < fName.size(); l++)
{
strFile = fName.at(l);
for(size_t k = 1; k < fName.size(); k++)
{
strFile2 = fName.at(k);
if(strFile.compare(strFile2) == 0)
{
fName.erase(fName.begin() + l);
fName.erase(fName.begin() + k);
}
}
}
This is removing a few of the duplicate but still has a few duplicates left, need help in debugging.
Also my input looks like <a,b,c,d,e,e,d,c,a> and my expected output is <b> as all other files b,c,d,e have duplicates they are removed.
#include <algorithm>
template <typename T>
void remove_duplicates(std::vector<T>& vec)
{
std::sort(vec.begin(), vec.end());
vec.erase(std::unique(vec.begin(), vec.end()), vec.end());
}
Note: this require that T has operator< and operator== defined.
Why it work?
std::sort sort the elements using their less-than comparison operator
std::unique removes the duplicate consecutive elements, comparing them using their equal comparison operator
What if i want only the unique elements?
Then you better use std::map
#include <algorithm>
#include <map>
template <typename T>
void unique_elements(std::vector<T>& vec)
{
std::map<T, int> m;
for(auto p : vec) ++m[p];
vec.erase(transform_if(m.begin(), m.end(), vec.begin(),
[](std::pair<T,int> const& p) {return p.first;},
[](std::pair<T,int> const& p) {return p.second==1;}),
vec.end());
}
See: here.
If I understand your requirements correctly, and I'm not entirely sure that I do. You want to only keep the elements in your vector of which do not repeat, correct?
Make a map of strings to ints, used for counting occurrences of each string. Clear the vector, then copy back only the strings that only occurred once.
map<string,int> m;
for (auto & i : v)
m[i]++;
v.clear();
for (auto & i : m)
if(i.second == 1)
v.push_back(i.first);
Or, for the compiler-feature challenged:
map<string,int> m;
for (vector<string>::iterator i=v.begin(); i!=v.end(); ++i)
m[*i]++;
v.clear();
for (map<string,int>::iterator i=m.begin(); i!=m.end(); ++i)
if (i->second == 1)
v.push_back(i->first);
#include <algorithms>
template <typename T>
remove_duplicates(std::vector<T>& vec)
{
std::vector<T> tvec;
uint32_t size = vec.size();
for (uint32_t i; i < size; i++) {
if (std::find(vec.begin() + i + 1, vec.end(), vec[i]) == vector.end()) {
tvec.push_back(t);
} else {
vec.push_back(t);
}
vec = tvec; // : )
}
}
You can eliminate duplicates in O(log n) runtime and O(n) space:
std::set<std::string> const uniques(vec.begin(), vec.end());
vec.assign(uniques.begin(), uniques.end());
But the O(log n) runtime is a bit misleading, because the O(n) space actually does O(n) dynamic allocations, which are expensive in terms of speed. The elements must also be comparable (here with operator<(), which std::string supports as a lexicographical compare).
If you want to store only unique elements:
template<typename In>
In find_unique(In first, In last)
{
if( first == last ) return last;
In tail(first++);
int dupes = 0;
while( first != last ) {
if( *tail++ == *first++ ) ++dupes;
else if( dupes != 0 ) dupes = 0;
else return --tail;
}
return dupes == 0 ? tail : last;
}
The algorithm above takes a sorted range and returns the first unique element, in linear time and constant space. To get all uniques in a container you might use it like so:
auto pivot = vec.begin();
for( auto i(find_unique(vec.begin(), vec.end()));
i != vec.end();
i = find_unique(++i, vec.end())) {
std::iter_swap(pivot++, i);
}
vec.erase(pivot, vec.end());