I do not know, why does it output 1024?
vector<int> default_container = { 1,2,3,4,5,6,7,78,8,1024 };
cout << *default_container.end() << endl; // 0
default_container.pop_back();
for (auto it : default_container)
{
cout << it << ",";
}
cout << endl;
cout << *default_container.end() << endl; // 1024 why？why？why？why？
cout << *--default_container.end() << endl; // 8
Your program has Undefined behavior!
You are de-referencing the end iterator, at the lines
cout << *default_container.end() << endl;
...
cout << *default_container.end() << endl;
which gives you undefined behavior. Form cppreference.com the std::vector::end, std::vector::cend
Returns an iterator to the element following the last element of the vector.
This element acts as a placeholder; attempting to access it results in undefined behavior.
Means, anything can be happened; therefore you shouldn't be relaying on its result and should not be doing it!
That being said, it looks like that you want to access the last element in the vector. If that the case, for a non-empty vector you have multiple other (safe) choices:
Using std::vector::back
// prints first 8 from last; due to "default_container.pop_back()"
std::cout << default_container.back();
Using std::prev
#include <iterator>
// prints first 8 from last; due to "default_container.pop_back()"
std::cout << *std::prev(default_container.end());
Using reverse iterator std::rbegin
// prints first 8 from last; due to "default_container.pop_back()"
std::cout << *std::rbegin(default_container);
As a side note, see: Why is "using namespace std;" considered bad practice?
The previous answer says it all.
In other words you're not supposed to use end() for anything else then comparing an iterator to.
Eg.
for (auto it = container.begin(); it < container.end(); ++it)
Also note that your line for (auto it : default_container)
isn't creating an iterator but literally an int.
// foreach value in container
for(int value : default_container)
{
cout << value;
}
note that if you're not planning to change the values you're iterating over you can use this :
for(const auto value : default_container) {}
or if your containter contains objects (instance of classes)
for(const auto& object : container) {}
Related
I'm trying to print elements of a vector of list pair in a hash-table program in C++.
If I use the C++11 auto it's working but if i use a iterator
for (vector<int>::iterator i = arr_Hash[i].begin(); i != arr_Hash[i].end(); ++i)
//for (auto index = arr_Hash[i].begin(); index != arr_Hash[i].end(); index++)
{
cout << i->second;
cout << " ";
}
Error list: https://i.imgur.com/rDejBGG.png
How can I use the iterator here?
vector<int>::iterator i = arr_Hash[i].begin()
You're reusing the variable i here. Call it something else.
std::cout << i->second;
i is a std::vector<int>::iterator. Dereferencing it gives you an int&, which has no second member. You probably just want std::cout << *i;
The iterator for arr_Hash[i] needs to be on the same type as the vector.
Namely, if the type of arr_Hash[i] is vector<pair<int,int>> then it's iterator needs to be vector<pair<int,int>>::iterator.
Howerver, you should Prefer a range-for-statement to a for-statement when there is a choice.
for (auto& e : arr_Hash[i])
cout << i->second << " ";
I have the following for each C++ code:
for (auto item : myVector)
{
std::cout << item;
if (item == orderBy.IsLast()) // <--- Check if this is the last element
std::cout << "(Is last element) " << std::endl;
else if (item == orderBy.IsFirst()) // <-- Check if this is the first element
std::cout << "(Is first element)" << std::endl;
}
Of course IfLast() and IfFirst() do not exist on std::vector. Is there a native std:: way to check for first and last element ?
You shouldn't use the range-based for in this case, as this kind of for "hides" the iterator, and you'd need an additional counter to keep track of the position in vector. You can simply do
for(auto it = myVector.begin(); it != myVector.end(); ++it)
{
if(it == myVector.begin()) // first element
{
// do something
}
else if(std::next(it) == myVector.end()) // last element
{
// do something else
}
}
Note that simply comparing my.Vector.back() with your element from a range-based for is OK only if you're sure that you don't have duplicates in the vector. But if e.g. the value of the last element appears multiple times in the vector, you're going to find only its first position. So that's why there's really no good way of using a range-based for without an additional index that keeps track of where exactly in the vector you are.
EDIT See also #thelink2012's answer for how to "trick" your range-based for so you can get the position of the element implicitly.
Use the std::vector::front and std::vector::back to get a reference to the data in the first and last positions.
Reference is a keyword here because you could efficiently check the address of your iterating item and the address of the respective front/back references. In your example you take the item by value not reference so this prehaps wouldn't work, take in consideration this example that'd work with this method:
for(auto& item : myVector) // take item by reference
{
std::cout << item;
if (&item == &myVector.back())
std::cout << "(last element) " << std::endl;
else if (&item == &myVector.front())
std::cout << "(first element)" << std::endl;
}
If the object overloads the address of operator & (though it's considered a bad practice) you might want to use std::addressof instead.
This method won't work however for the std::vector<bool> specialization since it optimizes the vector to store booleans efficiently with bits, and since we cannot have references to bits, all references taken out this data structure is a proxy object not exactly tied to the address of the internal data.
Use std::vector::front() for the first element.
Use std::vector::back() for the last element.
Before you call those functions, make sure that the vector is not empty.
if (!orderBy.empty() && item == orderBy.back()) <--- Check if this is the last element
else if (!orderBy.empty() && item == orderBy.front()) <-- Check if this is the first element
For value based comparison, you may use myVector.front()/myVector[0] as the first and myVector.back()/myVector[myVector.size()-1] as the last element.
Suggestion
Capture the reference by default to avoid unwanted copies. e.g.
for(const auto& I : myVector)
You could search for it again, but that would be rather inefficient. If you need information on the position of the current item, you probably want to use an iterator or an index:
for (std::size_t i=0; i<myVector.size(); ++i)
{
auto& item = myVector[i];
std::cout << item;
if (i == (myVector.size() - 1))
std::cout << "(Is last element) " << std::endl;
else if (i == 0)
std::cout << "(Is first element)" << std::endl;
}
If you have special cases for the boundaries you should use the ol' iterator version, but separating the first and last cases away from the loop.
If the cases share the code after that if you should encapsulate it on a function.
I can't write code from my phone :c
This works for me
vector<int> vi {1,2,3,4};
cout << "vi = {";
for (const auto &e : vi) {
cout << e;
if (&e != &vi.back())
cout << ',';
}
cout << '}' << endl;
I do not understand why iterating through a container with a for loop produces different results than iterating through it with a while loop. The following MWE illustrates this with a vector and a set of 5 integers.
#include <iostream>
#include <vector>
#include <set>
using namespace std;
int main()
{
vector<int> v;
set<int> s;
// add integers 0..5 to vector v and set s
for (int i = 0; i < 5; i++) {
v.push_back(i);
s.insert(i);
}
cout << "Iterating through vector with for loop.\n";
vector<int>::const_iterator itv;
for (itv = v.begin(); itv != v.end(); itv++) cout << *itv << ' ';
cout << '\n';
cout << "Iterating through set with for loop.\n";
set<int>::const_iterator its;
for (its = s.begin(); its != s.end(); its++) cout << *its << ' ';
cout << '\n';
cout << "Iterating through vector with while loop.\n";
itv = v.begin();
while (itv++ != v.end()) cout << *itv << ' ';
cout << '\n';
cout << "Iterating through set with while loop.\n";
its = s.begin();
while (its++ != s.end()) cout << *its << ' ';
cout << '\n';
}
The above produces:
Iterating through vector with for loop.
0 1 2 3 4
Iterating through set with for loop.
0 1 2 3 4
Iterating through vector with while loop.
1 2 3 4 0
Iterating through set with while loop.
1 2 3 4 5
The for loops work as expected but not the while loops. Since I'm using ++ as a postfix, I don't understand why the while loops behave as they do. Another mystery is why the while loop prints a 5 for set s, since this number was not inserted in s.
Your while loop is not equivalent to the for loop.
The for loop is equivalent to
itv = v.begin();
while(itv != v.end()) {
cout << *itv << ' ';
itv++;
}
Note that the increment happens after the cout. In your while loops, you do the increment in the test, before cout. Even though you use postincrement, the increment takes effect before your loop body is executed.
Write your while loops like I did there and the discrepancy should disappear.
When you iterate using the for loop you increment the iterator only after the body is evaluated. When you iterate using the while loop you increment the iterator after the check but before the body of the loop. Dereferencing the iterator in the last iteration of your while loops causes undefined behavior.
It could be because the compiler evaluates the its++ in the while expression first before evaluating the rest of the expression.
Since I'm using ++ as a postfix, I don't understand why the while loops behave as they do.
That's because first the while predicate is evaluated, and then (if the predicate was true) the body of the while loop. By the time you try to access the value in the body, the iterator was already incremented.
Just a few 'random' style hints, mainly showing algorithm use and modern C++11 features.
#include <iostream>
#include <vector>
#include <set>
#include <algorithm>
#include <iterator>
int main()
{
const std::vector<int> v { 0,1,2,3,4 };
const std::set<int> s { 0,1,2,3,4 };
for (auto element : v)
std::cout << element << ' ';
std::cout << '\n';
for (auto element : s)
std::cout << element << ' ';
std::cout << '\n';
std::copy(v.begin(), v.end(), std::ostream_iterator<int>(std::cout, " "));
std::cout << '\n';
std::copy(s.begin(), s.end(), std::ostream_iterator<int>(std::cout, " "));
std::cout << '\n';
}
I have a Playlist class that has a vector with Tracks and each Track has a multimap<long, Note> as datamember.
class Track {
private:
multimap<long, Note> noteList;
}
Using an iterator to acces the tracks is no problem, so this part here is working fine:
vector<Track>::iterator trackIT;
try{
for(noteIT = trackIT->getNoteList().begin(); noteIT != trackIT->getNoteList().end(); noteIT++){
cout << "---" << noteIT->second.getName() << endl;
}
}catch (int e){
cout << "exception #" << e << endl;
}
What I want to do next is iterate the Notes of each Track. But starting from this part all output is stopped. So I only get to see the first tracks name. Any cout's after that are not shown and the compiler isn't giving me any errors. Even the cout inside the try catch block isn't working..
vector<Track>::iterator trackIT;
multimap<long, Note>::iterator noteIT;
for(trackIT = this->playlist.getTracklist().begin(); trackIT < this->playlist.getTracklist().end(); trackIT++){
cout << trackIT->getTrackName() << endl;
for(noteIT = trackIT->getNoteList().begin(); noteIT != trackIT->getNoteList().end(); noteIT++){
cout << "---" << noteIT->second.getName() << endl;
}
}
cout << "random cout that is NOT shown" << endl; // this part doesn't show up in console either
Also, the method in my Track class that I'm using to add the Note objects looks like this:
void Track::addNote(Note ¬e) {
long key = 1000009;
this->noteList.insert(make_pair(key, note));
}
// I'm adding the notes to the track like this:
Note note1(440, 100, 8, 1, 1);
note1.setName("note1");
synthTrack.addNote(note1);
Any ideas why the iterator won't work?
Change
noteIT < trackIT->getNoteList().end()
To
noteIT != trackIT->getNoteList().end()
Not all iterators support less than / greater than comparisons.
If you have c++11 you can use a range-based for loop:
for (Note& note : trackIT->getNoteList())
Or you can use BOOST_FOREACH
BOOST_FOREACH (Note& note, trackIT->getNoteList())
You haven't shown the definitions of getTrackList or getNoteList, but there's a common mistake people make - if you return a copy of the container instead of a reference to it, the iterators will be pointing to different containers making comparisons impossible. Not only that but since the containers are temporary any use of the iterators results in undefined behavior.
If you are really hardcoding the track key, then there will only ever be one track in the map because std::map stores unique keys...
long key = 1000009; //If yo are really doing this, this key is already inserted so it will fail to insert more.
Also, if you would like a more elegant approach you could use function object.
struct print_track
{
void operator()(const Track& track)
{
cout << track.getTrackName() << endl;
std::for_each(track.getNoteList().begin(), track.getNoteList().end(), print_track_name());
}
};
struct print_note_name
{
void operator()(const std::pair<long,Note>& note_pair)
{
cout << "---" << note_pair.second.getName() << endl;
}
};
//In use...
std::for_each(playlist.getTracklist().begin(), playlist.getTracklist.end(), print_track());
I was trying to use the vector STL, where iam facing a weird response from the following sample program :
#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;
vector<int>::iterator fun();
vector<int> myvector;
bool IsOdd (int i) {
return ((i%2)==1);
}
int main()
{
vector<int>::iterator it;
it = fun();
if (it == myvector.end() )
cout << "Reached end, Global" << endl;
else
cout << "Not end" << endl;
}
vector<int>::iterator fun() {
vector<int>::iterator it;
myvector.push_back(10);
myvector.push_back(26);
myvector.push_back(40);
myvector.push_back(56);
it = find_if (myvector.begin(), myvector.end(), IsOdd);
cout << "The first odd value is " << *it << endl;
if (it == myvector.end() )
cout << "Reached end, inside the function" << endl;
else
cout << "Not end" << endl;
return it;
}
Iam getting "Reached End" inside the function fun(), whereas, in the main program, it is showing up as "Not End".
Not sure, what might be the reason. Also, found that, the Address of myvector.end() is showing up as Zero in the main program [ after the fun() call ], where-as, showing a non-zero value inside the function fun().
The function is using the local myvector, main is using the global one.
Your modified code produces:
Reached end, inside the function
Reached end, Global
as expected.
Edit: Well, not as expected - as others have pointed out:
it = find_if (myvector.begin(), myvector.end(), IsOdd);
cout << "The first odd value is " << *it << endl;
will cause undefined behaviour with your dataset, because you don't have any odd values. You want:
it = find_if (myvector.begin(), myvector.end(), IsOdd);
if ( it != myvector.end() ) {
cout << "The first odd value is " << *it << endl;
}
There are two different myvectors, one is global, another is in fun. Therefore, you're comparing an iterator to the global vector with an iterator to a local vector which moreover doesn't exist anymore.
You have two instances of vector with name myvector. One global and one local to function fun. Inside fun this local vector hides the global vector. Since you are operating on two totally different vector objects you are seeing these results.
You dereferenced *it without checking if it was before the end. None of your vector values are odd, so it will be end and you will cause undefined behaviour by dereferencing it.