I have a single depth-first search algorithm implemented to traverse my graph, given the iterator to a starting node.
Documentation summary:
GraphIter is a typedef for Graph::iterator
Graph class extends map<string, Node>
start->second.edges() returns set<string>
This code causes a segmentation fault if the size of start->second.edges() is 0:
(I've truncated the irrelevant pieces, including the recursive calls, for brevity.)
Bad Code
void Graph::dfs(GraphIter start)
{
cout << "EDGES SIZE: " << start->second.edges().size() << endl;
for (set<string>::iterator it = start->second.edges().begin();
it != start->second.edges().end(); ++it)
{
GraphIter iter = this->find(*it); // <--- SEGMENTATION FAULT
}
}
Now watch what happens when I pull start->second.edges() into a local variable: no more segfault!
Here's the code that doesn't generate a segfault:
Good Code
void Graph::dfs(GraphIter start)
{
set<string> edges = start->second.edges(); // <--- MAGIC TRICK
cout << "EDGES SIZE: " << edges.size() << endl;
for (set<string>::iterator it = edges.begin();
it != edges.end(); ++it)
{
GraphIter iter = this->find(*it);
}
}
So the difference is that in the good code, when the size of the set of strings (from the edges() method) is 0, the for loop is never entered in the second case. But in the first case, the for loop is still executed at least once until it realize that it can't dereference the it variable.
Why are these different? Don't they access the same parts of memory?
Because edges() returns a set by value, start->second.edges().begin() and start->second.edges().end() return iterators to different containers because each call to edges() results in a new set being returned.
By creating a single copy with a named variable you ensure that the iterators all come from the same container and you can validly iterator from begin() to end().
It could be that start->second.edges() returns an std::set<std::string> by value. That would render the iterators in the loop incompatible and lead to undefined behaviour.
Your "magic trick"
set<string> edges = start->second.edges();
ensures that you are iterating over the same container, "edges".
You could fix it by having Node::edges() return by reference:
const std::set<std::string>& edges() const { .... }
Related
Given a std:: list
std::list< int > myList
and a reference (or a pointer) to an element in that list
int& myElement | int* pElement
So, basically I know the address of that element
How can I get an std::list<int>::iterator to that element efficiently?
A slow yet working example is
const_iterator it
for( it = myList.begin(); it != &myElement; ++it)
{
// do nothing, for loop terminates if "it" points to "myElem"
}
Is there a quicker way? like
const_iterator it = magicToIteratorConverter( myList, myElem )
Case of vector (but I need list):
For a vector, you could do the following:
const int* pStart = &myVector[0] // address of first element
const int* pElement = &myElem; // address of my element
const idx = static_cast< int >( pElement- pStart ); // no need to divide by size of an elem
std::vector< int >::iterator it = myVector.begin() + idx;
Case of std::list:
a std::list<int>::iterator is not an int*, you need to access the element in the iterator and get its address. Also, std::find_if takes care of most of the boilerplate for you.
auto iter = std:find_if(myList.begin(), myList.end(),
[&myElement](const int & listElement)
{ return &myElement == &listElement; });
Writing the loop out yourself would look like:
auto iter = myList.end();
for(auto i = myList.begin(); i != myList.end(); ++i)
if(&*i == &myElement)
{
iter = i;
break;
}
This is actually a good question and IMHO a shame that there is no std way to do what OP has asked. If you understand that a list node looks essentially like this
struct list_node {
list_node* prev;
list_node* next;
T yourType;
}
it is bad that there is no default way to get to the node (the iterator is a pointer to the node) if you have a pointer to yourType without searching through the whole container.
Since std is not helping you have to get your hands dirty:
#include <list>
#include <iostream>
//This is essentially what you are looking for:
std::list<int>::iterator pointerToIter (int* myPointer) {
//Calculates the distance in bytes from an iterator itself
//to the actual type that is stored at the position the
//iterator is pointing to.
size_t iterOffset = (size_t)&(*((std::list<void*>::iterator)nullptr));
//Subtract the offset from the passed pointer and make an
//iterator out of it
std::list<int>::iterator iter;
*(intptr_t*)&iter = (intptr_t)myPointer - iterOffset;
//You are done
return iter;
}
int main () {
std::list<int> intList;
intList.push_back (10);
int* i1 = &intList.back ();
intList.push_back (20);
intList.push_back (30);
int* i3 = &intList.back ();
intList.push_back (40);
intList.push_back (50);
int* i5 = &intList.back ();
std::cout << "Size: " << intList.size () << " | Content: ";
for (const int& value : intList)
std::cout << value << " ";
std::cout << std::endl;
intList.erase (pointerToIter (i1));
intList.erase (pointerToIter (i3));
intList.erase (pointerToIter (i5));
std::cout << "Size: " << intList.size () << " | Content: ";
for (const int& value : intList)
std::cout << value << " ";
std::cout << std::endl;
return 0;
}
Output (to prove that it works as intended):
Size: 5 | Content: 10 20 30 40 50
Size: 2 | Content: 20 40
This works perfectly even if an implementation of std::list would use a different layout for the list-node or add some more members to it. I also included the assembler code generated to see that the function is essentially reduced to myPointer - 0x10 (0x10 = 16 is the size of 2 pointers on a 64bit machine).
Assembler (with at least -O1):
std::list<int>::iterator pointerToIter (int* myPointer) {
0: 48 8d 47 f0 lea rax,[rdi-0x10]
}
4: c3 ret
Given a list, you can only get to anything by starting from one end walking through it, making sure you don't go beyond the end.
const_iterator it, end;
for( it = myList.begin(), end = myList.end(); it!=end && it != &myElement; ++it)
{
// do nothing, for loop terminates if "it" points to "myElem"
// or if we don't find your element.
}
Of course, you could use a standard algorithm, like std::find to look for it.
Alternatively, you could keep hold of the iterator when you insert, and under many conditions it will still be valid later on.
If you want lookup speed, you should probably use something other than a list.
If you have something like
int x = 42;
int * this_might_be_handy = &x;
myList.insert(x);
myList now has a COPY of the number - it has the value 42, but in a different memory location.
If you kept pointers to ints in the list, that would be different. getting the value from the front of the list and looking at the address will not give the same address as x.
But you would have to manage them smartly.
This is a dissected, formalized (C++11) and commented version of the very interesting solution of Xatian above. That solution is O(1) (that is one of the reasons it is so interesting), but it assumes some things that should be taken into account (Xatian's deep knowledge of the STL implementation of lists is the other reason why it is so interesting).
#include <iostream>
#include <list>
#include <cstdint>
int main(void)
{
// ASSUMPTIONS:
// 1.- nullptr == 0
// 2.- A std::list iterator is an object that contains just one thing: a pointer to the body of the iterator.
// 3.- A std::list iterator has a constructor that admits a pointer to a body provided by the user, which creates the iterator with that (assumed) existing body.
using DataType = int;
using DataList = std::list<DataType>;
std::cout << "Nullptr= " << reinterpret_cast<size_t>(nullptr) << std::endl;
std::cout << "Size of a pointer = " << sizeof(nullptr) << ", size of iterator = " << sizeof(DataList::iterator) << std::endl;
static_assert(reinterpret_cast<size_t>(nullptr) == 0,
"In this compiler, nullptr is not 0 and this will not work");
// we have a list filled with something.
DataList mylist{1,2,3,4};
// and an iterator pointing to some data.
DataList::iterator itaux{mylist.begin()};
++itaux;
++itaux;
// 1. calculate the offset of the data in a list iterator w.r.t. the beginning of the iterator body
DataList::iterator it{nullptr}; // call the iterator constructor. Nullptr becomes the address of the body where the iterator would store prev/next/data
// since nullptr is assumed to be 0, this is the same as to declare an iterator with its body at 0x00000000
DataType & itp = *it; // this is a reference to the user's data in the iterator body
// that iterator is a fake and does not contain any data, but since we are only dealing with addresses, no problem...
DataType * aitp = & itp; // this gets the address equivalent to the reference, which is at some point in memory from 0
size_t iteroffset = reinterpret_cast<size_t>(aitp); // That address becomes, actually, the offset of the data w.r.t. the beginning of the iterator body
std::cout << "Offset from iterator body start to data = " << iteroffset << std::endl;
// 2. we can get the pointer to the data from our existing iterator
DataType * mypointer = &(*itaux);
// 3. we can create a valid iterator from the pointer to the data
DataList::iterator iter;
*(reinterpret_cast<intptr_t*>(&iter)) = reinterpret_cast<intptr_t>(mypointer) - iteroffset; // the address of the beginning of the body (mypointer-iteroffset) is stored into
// the pointer to the body that the iterator actually is
std::cout << "pointed element: " << (*iter) << std::endl;
return(0);
}
Following one of the "deleting while iterating" patterns on a vector, I don't understand why this code works, or if it's making use of undefined behavior:
The Code:
#include <vector>
#include <iostream>
int main(int argc, char* argv[], char* envz[])
{
std::vector<std::string> myVec;
myVec.push_back("1");
myVec.push_back("2");
myVec.push_back("3");
for (std::vector<std::string>::iterator i = myVec.begin();
i != myVec.end();
++i)
{
if ("1" == *i)
{
std::cout << "Erasing " << *i << std::endl;
i = myVec.erase(i);
--i;
continue;
}
std::cout << *i << std::endl;
}
return 0;
}
The Output:
>g++ -g main.cpp
>./a.out
Erasing 1
2
3
Question:
Consider the first iteration of the for-loop:
i is myVec.begin(), which "points to" 1.
We enter the conditional block.
1 is erased and i is set to one past the erased element, i.e. 2, which is now also pointed to by myVec.begin()
I decrement i, so now it points to...one prior to myVec.begin() ???
I'm confused by why this seems to work, as evidenced by the output, but something feels fishy about decrementing the iterator. This code is easy enough to rationalize if the conditional is if ("2" == *i), because the iterator decrement still places it at a valid entry in the vector. I.e. if we conditionally erased 2, i would be set to point to 3, but then manually decremented and thus point to 1, followed by the for-loop increment, setting it to point back to 3 again. Conditionally erasing the last element is likewise easy to follow.
What Else I Tried:
This observation made me hypothesize that decrementing prior to vector::begin() was idempotent, so I tried addition an additional decrement, like so:
#include <vector>
#include <iostream>
int main(int argc, char* argv[], char* envz[])
{
std::vector<std::string> myVec;
myVec.push_back("1");
myVec.push_back("2");
myVec.push_back("3");
for (std::vector<std::string>::iterator i = myVec.begin();
i != myVec.end();
++i)
{
if ("1" == *i)
{
std::cout << "Erasing " << *i << std::endl;
i = myVec.erase(i);
--i;
--i; /*** I thought this would be idempotent ***/
continue;
}
std::cout << *i << std::endl;
}
return 0;
}
But this resulted in a segfault:
Erasing 1
Segmentation fault (core dumped)
Can someone explain why the first code bock works, and specifically why the single decrement after erasing the first element is valid?
No, your code has undefined behaviour: if i == myVec.begin(), then i = myVec.erase(i); results in i again being (the new value of) myVec.begin(), and --i has undefined behaviour since it goes outside the valid range for the iterator.
If you don't want to use the erase-remove idiom (i.e. myVec.erase(std::remove(myVec.begin(), myVec.end(), "1"), myVec.end())), then the manual loop-while-mutating looks like this:
for (auto it = myVec.begin(); it != myVec.end(); /* no increment! */) {
if (*it == "1") {
it = myVec.erase(it);
} else {
++it;
}
}
Regardless, the crucial point both here and in your original code is that erase invalidates iterators, and thus the iterator must be re-assigned with a valid value after the erasing. We achieve this thanks to the return value of erase, which is precisely that new, valid iterator that we need.
This might work in some compilers, but might fail in others (e.g. the compiler might actually check in runtime that you are not decrementing under begin() and throw exception in such case - I believe that at least one compiler does it but don't remember which one).
In this case the general pattern is to not increment in the for but inside the loop:
for (std::vector<std::string>::iterator i = myVec.begin();
i != myVec.end();
/* no increment here */)
{
if ("1" == *i)
{
std::cout << "Erasing " << *i << std::endl;
i = myVec.erase(i);
continue;
}
std::cout << *i << std::endl;
++i;
}
With vector the wrong iteration might actually work in more cases, but you'd have very bad time if you try that e.g. with std::map or std::set.
The key here is the continue right after decrementing.
By calling it, ++i will be triggered by the loop iteration before dereferencing i.
How do I remove from a map while iterating it? like:
std::map<K, V> map;
for(auto i : map)
if(needs_removing(i))
// remove it from the map
If I use map.erase it will invalidate the iterators
The standard associative-container erase idiom:
for (auto it = m.cbegin(); it != m.cend() /* not hoisted */; /* no increment */)
{
if (must_delete)
{
m.erase(it++); // or "it = m.erase(it)" since C++11
}
else
{
++it;
}
}
Note that we really want an ordinary for loop here, since we are modifying the container itself. The range-based loop should be strictly reserved for situations where we only care about the elements. The syntax for the RBFL makes this clear by not even exposing the container inside the loop body.
Edit. Pre-C++11, you could not erase const-iterators. There you would have to say:
for (std::map<K,V>::iterator it = m.begin(); it != m.end(); ) { /* ... */ }
Erasing an element from a container is not at odds with constness of the element. By analogy, it has always been perfectly legitimate to delete p where p is a pointer-to-constant. Constness does not constrain lifetime; const values in C++ can still stop existing.
I personally prefer this pattern which is slightly clearer and simpler, at the expense of an extra variable:
for (auto it = m.cbegin(), next_it = it; it != m.cend(); it = next_it)
{
++next_it;
if (must_delete)
{
m.erase(it);
}
}
Advantages of this approach:
the for loop incrementor makes sense as an incrementor;
the erase operation is a simple erase, rather than being mixed in with increment logic;
after the first line of the loop body, the meaning of it and next_it remain fixed throughout the iteration, allowing you to easily add additional statements referring to them without headscratching over whether they will work as intended (except of course that you cannot use it after erasing it).
Assuming C++11, here is a one-liner loop body, if this is consistent with your programming style:
using Map = std::map<K,V>;
Map map;
// Erase members that satisfy needs_removing(itr)
for (Map::const_iterator itr = map.cbegin() ; itr != map.cend() ; )
itr = needs_removing(itr) ? map.erase(itr) : std::next(itr);
A couple of other minor style changes:
Show declared type (Map::const_iterator) when possible/convenient, over using auto.
Use using for template types, to make ancillary types (Map::const_iterator) easier to read/maintain.
The C++20 draft contains the convenience function std::erase_if.
So you can use that function to do it as a one-liner.
std::map<K, V> map_obj;
//calls needs_removing for each element and erases it, if true was reuturned
std::erase_if(map_obj,needs_removing);
//if you need to pass only part of the key/value pair
std::erase_if(map_obj,[](auto& kv){return needs_removing(kv.first);});
In short "How do I remove from a map while iterating it?"
With old map impl: You can't
With new map impl: almost as #KerrekSB suggested. But there are some syntax issues in what he posted.
From GCC map impl (note GXX_EXPERIMENTAL_CXX0X):
#ifdef __GXX_EXPERIMENTAL_CXX0X__
// _GLIBCXX_RESOLVE_LIB_DEFECTS
// DR 130. Associative erase should return an iterator.
/**
* #brief Erases an element from a %map.
* #param position An iterator pointing to the element to be erased.
* #return An iterator pointing to the element immediately following
* #a position prior to the element being erased. If no such
* element exists, end() is returned.
*
* This function erases an element, pointed to by the given
* iterator, from a %map. Note that this function only erases
* the element, and that if the element is itself a pointer,
* the pointed-to memory is not touched in any way. Managing
* the pointer is the user's responsibility.
*/
iterator
erase(iterator __position)
{ return _M_t.erase(__position); }
#else
/**
* #brief Erases an element from a %map.
* #param position An iterator pointing to the element to be erased.
*
* This function erases an element, pointed to by the given
* iterator, from a %map. Note that this function only erases
* the element, and that if the element is itself a pointer,
* the pointed-to memory is not touched in any way. Managing
* the pointer is the user's responsibility.
*/
void
erase(iterator __position)
{ _M_t.erase(__position); }
#endif
Example with old and new style:
#include <iostream>
#include <map>
#include <vector>
#include <algorithm>
using namespace std;
typedef map<int, int> t_myMap;
typedef vector<t_myMap::key_type> t_myVec;
int main() {
cout << "main() ENTRY" << endl;
t_myMap mi;
mi.insert(t_myMap::value_type(1,1));
mi.insert(t_myMap::value_type(2,1));
mi.insert(t_myMap::value_type(3,1));
mi.insert(t_myMap::value_type(4,1));
mi.insert(t_myMap::value_type(5,1));
mi.insert(t_myMap::value_type(6,1));
cout << "Init" << endl;
for(t_myMap::const_iterator i = mi.begin(); i != mi.end(); i++)
cout << '\t' << i->first << '-' << i->second << endl;
t_myVec markedForDeath;
for (t_myMap::const_iterator it = mi.begin(); it != mi.end() ; it++)
if (it->first > 2 && it->first < 5)
markedForDeath.push_back(it->first);
for(size_t i = 0; i < markedForDeath.size(); i++)
// old erase, returns void...
mi.erase(markedForDeath[i]);
cout << "after old style erase of 3 & 4.." << endl;
for(t_myMap::const_iterator i = mi.begin(); i != mi.end(); i++)
cout << '\t' << i->first << '-' << i->second << endl;
for (auto it = mi.begin(); it != mi.end(); ) {
if (it->first == 5)
// new erase() that returns iter..
it = mi.erase(it);
else
++it;
}
cout << "after new style erase of 5" << endl;
// new cend/cbegin and lambda..
for_each(mi.cbegin(), mi.cend(), [](t_myMap::const_reference it){cout << '\t' << it.first << '-' << it.second << endl;});
return 0;
}
prints:
main() ENTRY
Init
1-1
2-1
3-1
4-1
5-1
6-1
after old style erase of 3 & 4..
1-1
2-1
5-1
6-1
after new style erase of 5
1-1
2-1
6-1
Process returned 0 (0x0) execution time : 0.021 s
Press any key to continue.
Pretty sad, eh? The way I usually do it is build up a container of iterators instead of deleting during traversal. Then loop through the container and use map.erase()
std::map<K,V> map;
std::list< std::map<K,V>::iterator > iteratorList;
for(auto i : map ){
if ( needs_removing(i)){
iteratorList.push_back(i);
}
}
for(auto i : iteratorList){
map.erase(*i)
}
I'am having problems while trying to iterate some maps.
Basically i have a Deposit class. Each deposit class has a multimap containing a destination Deposit and a distance. (This will be used to create a graph).
When i try to iterate all the maps i'm getting a segmentation fault error.
Here's the code:
for (int j = 0; j < deposit.size(); j++) {
for (typename multimap< Deposit<Product>*, int>::iterator it = deposit.at(j)->getConnections().begin(); it != deposit.at(j)->getConnections().end(); it++) {
cout << "From the depo. " << deposit.at(j)->getKey() << " to " << it->first->getKey() << " with the distance " << it->second << endl;
}
}
EDIT:
Deposit Class:
template<class Product>
class Deposit {
private:
multimap <Deposit<Product>*, int> connections;
public:
void addConnection(Deposit<Product>* dep, int dist);
multimap <Deposit<Product>*, int> getConnections() const;
};
(...)
template<class Product>
void Deposit<Product> ::addConnection(Deposit<Product>* depKey, int dist) {
this->connections.insert(pair<Deposit<Product>*, int>(depKey, dist));
}
template<class Product>
multimap < Deposit<Product>*, int> Deposit<Product> ::getConnections() const {
return this->connections;
}
Storage Class - This is where I populate the multimaps.
(...)
ligs = rand() % 10;
do{
ligIdx = rand() % deposit.size();
dist = rand() % 100;
deposit.at(i)->addConnection(deposit.at(ligIdx), dist);
ligs--;
}while(ligs>0);
(...)
My deposit class has 2 subclasses. I dont know why the error occurs. Is there any problem with the iterator?
Thank you very much!!!
The problem you have is pretty nasty: getConnections() returns a multimap by value.
This means that successive calls to deposit.at(j)->getConnections() refer to different temporary copies of the original multimap. Thus the the iterator created on the begin of the first temporary copy, will never match the end of the second copy, without first accessing illegally some invalid places.
Two alternatives:
if you want to iterate on a copy, make one local copy auto cnx = deposit.at(j)->getConnections(); and change your inner loop to iterate on cnx.
if you intended to iterate on the original multimap, change the signature of getConnections() to return a reference.
By the way, if you use c++11 or higher, you could consider defining the iterator in a more readable way: for (auto it = ....) or even better, using the range-for syntax as proposed by Norah Attkins in her answer.
If you have a c++11 (or 14) compiler (and you should - unless it's a work/company barrier involved) consider using range based for loops to make your code clearer
for (auto const& elem : deposit)
{
for (auto const& product : elem)
{
}
}
Apart from the stylist guidance, lacking info on what the containers actrually hold, we'd just be guessing what's wrong when answering this question. My guess is that invalid reads happen and the pointers you're accessing are not allocated (but that's a guess)
Is there a way to access the iterator (I suppose there's no loop index?) in a C++11 range-based for loop?
Often we need to do something special with the first element of a container and iterate over the remaining elements. So I'm looking for something like the c++11_get_index_of statement in this pseudo-code:
for (auto& elem: container)
{
if (c++11_get_index_of(elem) == 0)
continue;
// do something with remaining elements
}
I'd really like to avoid going back to old-style manual iterator handling code in that scenario.
Often we need to do something special with the first element of a
container and iterate over the remaining elements.
I am surprised to see that nobody has proposed this solution so far:
auto it = std::begin(container);
// do your special stuff here with the first element
++it;
for (auto end=std::end(container); it!=end; ++it) {
// Note that there is no branch inside the loop!
// iterate over the rest of the container
}
It has the big advantage that the branch is moved out of the loop. It makes the loop much simpler and perhaps the compiler can also optimize it better.
If you insist on the range-based for loop, maybe the simplest way to do it is this (there are other, uglier ways):
std::size_t index = 0;
for (auto& elem : container) {
// skip the first element
if (index++ == 0) {
continue;
}
// iterate over the rest of the container
}
However, I would seriously move the branch out of the loop if all you need is to skip the first element.
Boost provides a nice succinct way to do this:
std::vector<int> xs{ 1, 2, 3, 4, 5 };
for (const auto &x : boost::make_iterator_range(xs.begin() + 1, xs.end())) {
std::cout << x << " ";
}
// Prints: 2 3 4 5
You can find make_iterator_range in the boost/range/iterator_range.hpp header.
How about using a simple for loop with iteratos:
for(auto it = container.begin(); it != container.end(); it++)
{
if(it == container.begin())
{
//do stuff for first
}
else
{
//do default stuff
}
}
It's not range based, but it's functional.
In case you may still want to use the range loop:
int counter = 0;
for(auto &data: container)
{
if(counter == 0)
{
//do stuff for first
}
else
{
//do default stuff
}
counter++;
}
No, you can't get the iterator in a range-based for loop (without looking up the element in the container, of course). The iterator is defined by the standard as being named __begin but this is for exposition only. If you need the iterator, it is intended that you use the normal for loop. The reason range-based for loop exists is for those cases where you do not need to care about handling the iteration yourself.
With auto and std::begin and std::end, your for loop should still be very simple:
for (auto it = std::begin(container); it != std::end(container); it++)
When iterating over elements, always prefer to use an algorithm, and use a plain for loop only if none of the algorithms fit.
Picking the right algorithm depends on what you want to do with the elements... which you haven't told us.
If you want to skip the first element, dump example:
if (!container.empty()) {
for_each(++container.begin(), container.end(), [](int val) { cout << val; });
}
There is no way of knowing how far an element is within the container without having an iterator, pointer or an intrusive index. Here's a simple way of doing it:
int index= 0;
for (auto& elem: container)
{
if (index++ == something)
continue;
// do something with remaining elements
}
If you want to skip the first element, another way is to use a std::deque and pop_front the first element. Then you can do your ranged for loop with the container as usual.
When I need to do something like this on a random access container, my habit is to iterate over the indexes.
for( std::size_t i : indexes( container ) ) {
if (i==0) continue;
auto&& e = container[i];
// code
}
the only tricky part is writing indexes, which returns a range of what boost calls counting iterators. Creating a basic iterable range from iterators is easy: either use boost's range concept, or roll your own.
A basic range for an arbitrary iterator type is:
template<typename Iterator>
struct Range {
Iterator b; Iterator e;
Range( Iterator b_, Iterator e_ ):b(b_), e(e_) {};
Iterator begin() const { return b; }
Iterator end() const { return e; }
};
which you can gussy up a bunch, but that is the core.
I would try to avoid using iterators, because the idea of a range-based for loop is to get rid of them. As of C++20, to skip the first element in your container, I would take one of the following approaches. I also include, for the sake of completeness, how to handle the first element separately:
Handling the first element outside the loop
You can use container.front() which exists for all sequence containers to access the first element. However, you must make sure that the container is not empty to avoid a segmentation fault. Then, to skip the first element (or more) in the loop, you can use the range adapter std::views::drop from the Ranges library. All together it looks as follows:
std::vector<int> container { 1, 2, 3 };
if(!container.empty()) {
// do something with first element
std::cout << "First element: " << container.front() << std::endl;
}
for (auto& elem : container | std::views::drop(1)) {
// do something with remaining elements
std::cout << "Remaining element: " << elem << std::endl;
}
Instead of container.front() you can also use another range-based for loop together with the range adapter std::views::take(1). The advantage of take() and drop() is that they work safely even if their arguments exceed the count of elements in your container.
Handling the first element inside the loop
You can use an init-statement in a range-based for loop to define a Boolean flag (or even a counter). This way, the flag is visible only within the scope of the loop. You can use the flag inside the loop as follows:
std::vector<int> container { 1, 2, 3 };
for(bool isFirst(true); auto& elem : container) {
if(isFirst) {
// do something with first element
std::cout << "First element: " << elem << std::endl;
isFirst = false;
continue;
}
// do something with remaining elements
std::cout << "Remaining element: " << elem << std::endl;
}
Output for both approaches shown:
First element: 1
Remaining element: 2
Remaining element: 3
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