This is a continuation of my previous question: Nested vector<float> and reference manipulation.
I got the loops and all working, but I'm trying to add new instances of arrays to a total vector.
Here's one example of what I mean:
array<float, 3> monster1 = { 10.5, 8.5, 1.0 };
// ...
vector<array<float, 3>*> pinkys = { &monster1};
// ...
void duplicateGhosts() {
int count = 0;
int i = pinkys.size(); // this line and previous avoid overflow
array<float, 3>& temp = monster1; // this gets the same data, but right now it's just a reference
for (auto monster : pinkys) { // for each array of floats in the pinkys vector,
if (count >= i) // if in this instance of duplicateGhosts they've all been pushed back,
break;
pinkys.push_back(&temp); // this is where I want to push_back a new instance of an array
count++;
}
}
With the current code, instead of creating a new monster, it is adding a reference to the original monster1 and therefore affecting its behavior.
As mentioned in a comment you cannot insert elements to a container you are iterating with a range based for loop. That is because the range based for loop stops when it reaches pinkys.end() but that iterator gets invalidated once you call pinkys.push_back(). It is not clear why you are iterating pinkys in the first place. You aren't using monster (a copy of the elements in the vector) in the loop body.
The whole purpose of the loop seems to be to have as many iterations as there are already elements in the container. For that you need not iterate elements of pinkys but you can do:
auto old_size = pinkys.size();
for (size_t i=0; i < old_size; ++i) {
// add elements
}
Further, it is not clear why you are using a vector of pointers. Somebody has to own the monsters in the vector. If it isnt anybody else, it is the vector. And in that case you should use a std::vector<monster>. For shared ownership you should use std::shared_ptr. Never use owning raw pointers!
Don't use a plain array for something that you can give a better name:
struct monster {
float hitpoints; // or whatever it actually is.
float attack; // notice how this is much clearer
float defense; // than using an array?
};
With those modifications the method could look like this:
void duplicateGhosts() {
auto old_size = pinkys.size();
for (size_t i=0; i < old_size; ++i) {
pinkys.push_back( pinkys[i] );
}
}
From the name of the method I assumed you want to duplciate the vectors elements. If you want to just add the same monster as many times as there were elements before, that is
void duplicateGhosts() {
auto old_size = pinkys.size();
for (size_t i=0; i < old_size; ++i) {
pinkys.push_back( monster{} );
}
}
Related
I have a game where I check collision between bullets and enemies which I store as 2 vector containers. People say if you're gonna erase an element in the for loop you better use iterators and so I did. But I have a problem now with passing the iterator to a function. The thing is I don't necessarily need to erase the element so it has to be a bit more complex.
This is the way I check collision. "CircularCollision" works fine, no mistakes there.
void ResolveColision(Weapon &weap, Map &map)
{
std::vector<Bullet> bullets = weap.GetBullets();
if (!bullets.empty())
{
for (std::vector<Bullet>::iterator i = bullets.begin(); i != bullets.end(); ++i)
{
std::vector<Enemy> enemies = map.GetEnemies();
if (!enemies.empty())
{
for (std::vector<Enemy>::iterator j = enemies.begin(); j != enemies.end(); ++j)
{
if (CircularCollision((*i), (*j)))
{
weap.DeleteByIndex(i);
map.TakeDamageByIndex(j, weap.GetDamage());
std::cout << "HIT!\n";
}
}
}
}
}
}
Here's the method which is supposed to decrease the health of an enemy:
void Map::TakeDamageByIndex(std::vector<Enemy>::iterator &itr, int damage)
{
(*itr).SetHealth((*itr).GetHealth() - damage);
}
Here's the method which deletes the bullet:
void Weapon::DeleteByIndex(std::vector<Bullet>::iterator &itr)
{
destroySprite((*itr).GetSprite());
bullets.erase(itr);
}
I'm sure it looks horrible and it shouldn't work but I have no idea how to do it properly. Please help!
Also, both methods work properly when the for loops operate with indexes (e.g. bullets[i]), in that case the problem is with "Vector subscript out of range" error.
In DeleteByIndex(), change this:
bullets.erase(itr);
To this:
itr = bullets.erase(itr);
std::vector::erase() returns an iterator to the next remaining element after the element that was erased. That next element is where your outer loop needs to continue from on its next iteration.
As such, you need to change your outer loop from a for to a while instead, or else you will skip elements (in fact, your original code suffers from that problem when you were still using indexes):
void ResolveColision(Weapon &weap, Map &map)
{
std::vector<Bullet> bullets = weap.GetBullets();
std::vector<Bullet>::iterator bullerItr = bullets.begin();
while (bullerItr != bullets.end())
{
std::vector<Enemy> enemies = map.GetEnemies();
bool wasAnyHit = false;
for (std::vector<Enemy>::iterator enemyItr = enemies.begin(); enemyItr != enemies.end(); ++enemyItr)
{
if (CircularCollision(*bulletItr, *enemyItr))
{
wasAnyHit = true;
weap.DeleteByIndex(bulletItr);
map.TakeDamageByIndex(enemyItr, weap.GetDamage());
std::cout << "HIT!\n";
break;
}
}
if (!wasAnyHit)
++bulletItr;
}
}
That being said, I would suggest replacing the inner loop with std::find_if() instead. And renaming DeleteByIndex() and TakeDamageByIndex() since they don't take an index anymore. In fact, I would not pass an iterator to TakeDamage...() at all, pass the actual Enemy object instead. Or better, move TakeDamage() into Enemy itself.
Try something more like this:
void ResolveColision(Weapon &weap, Map &map)
{
auto bullets = weap.GetBullets();
auto bulletItr = bullets.begin();
while (bulletItr != bullets.end())
{
auto enemies = map.GetEnemies();
auto &bullet = *bulletItr;
auto enemyHit = std::find_if(enemies.begin(), enemies.end(),
[&](Enemy &enemy){ return CircularCollision(bullet, enemy); }
);
if (enemyHit != enemies.end())
{
weap.DeleteBulletByIterator(bulletItr);
enemyHit->TakeDamage(weap.GetDamage());
std::cout << "HIT!\n";
}
else
++bulletItr;
}
}
void Enemy::TakeDamage(int damage)
{
SetHealth(GetHealth() - damage);
}
void Weapon::DeleteBulletByIterator(std::vector<Bullet>::iterator &itr)
{
destroySprite(itr->GetSprite());
itr = bullets.erase(itr);
}
A few other comments in addition to Remy Lebeau’s answer.
It’s as efficient to pass a STL iterator by value as by reference, so the only reason you would need to pass one by reference is: when you intend to change the index and you want that change to be visible in the caller’s scope. (For example, a UTF-8 parser needs to consume anywhere from one to four bytes.) Since this code doesn’t need to do that, you’re better off just passing the iterator by value.
In general, if you aren’t modifying the variable you pass by reference, you should pass by const reference instead. In the case of Enemy::TakeDamage(), the only thing you do with the iterator is dereference it, so you might as well just pass in an Enemy& and call it with *i as the parameter.
The algorithm is not very efficient: if you delete a lot of items near the start of the list, you would need to move all remaining items of the array multiple times. This runs in O(N²) time. A std::list, although it has a high overhead compared to std::vector, can delete elements in constant time, and might be more efficient if you have a lot of insertions and deletions that are not at the end. You might also consider moving only the objects that survive to a new list and then destroying the old one. At least this way, you only need to copy once, and your pass runs in O(N) time.
If your containers store smart pointers to the objects, you only have to move the pointers to a new location, not the entire object. This will not make up for the overhead of lots of heap allocations if your objects are small, but could save you a lot of bandwidth if they are large. The objects will still be automatically deleted when the last reference to them is cleared.
You could do something like this:
void delByIndex(vector<int>::iterator &i, vector<int>& a)
{
a.erase(i);
}
int main()
{
vector<int> a {1,5,6,2,7,8,3};
vector<int> b {1,2,3,1};
for(auto i=a.begin();i!=a.end();)
{
bool flag = false;
for(auto j=b.begin();j!=b.end();j++)
{
if(*i==*j)
{
flag = true;
}
}
if(flag)
{
delByIndex(i, a);
}
else
i++;
}
for(auto i:a)
cout << i << " ";
return 0;
}
Be careful when using erase as it will change the size of the vector and also invalidates the vector iterator.
Declaring the vector of objects in this way...
std::vector< std::shared_ptr<Boid> >m_flock;
std::vector< std::shared_ptr<Boid> >m_flock2;
And this is how I created my objects in a loop
//Create objects in first container
m_flock.resize(2);
for(auto &boid : m_flock)
{
boid = std::shared_ptr<Boid>(new Boid);
boid->setPos(20);
std::cout<< boid->getPos() <<std::endl;
}
//Create objects in second container
m_flock2.resize(3);
int num = 0;
for(auto &boid2 : m_flock2)
{
num +=10;
boid2 = std::shared_ptr<Boid>(new Boid);
boid2->setPos(num);
std::cout<< boid2->getPos() <<std::endl;
}
This is the main part i need help with.. how do I properly compare the values of objects in first container and object in second container?
// Compare 2 objects function from one container to another
// If it's 2nd object container's value is equals to the value of the first container,
// Then add this to the first container.
// And erase that specific element in the 2nd container
int i=0;
for(auto &boid : m_flock) //Container 1
{
for(auto &boid2 : m_flock2) //Container 2
{
//If they are the same position, then push_back to container 1.
if( boid2->getPos() == boid->getPos() )
{
//Push object of 2nd container to 1st container
m_flock.push_back(boid2);
//Delete the object that was being pushed into the 1st container.
m_flock2.erase(m_flock2.begin()+i); //Also is this the best way to delete that specic object?
++i;
}
}
}
So now when I run into problems get a segmentation fault. Can someone please advise me? Thank you.
You likely want std::partition or, if the order of elements matters, std::stable_partition. Something like this (untested):
auto middle = std::partition(m_flock2.begin(), m_flock2.end(),
[&](const std::shared_ptr<Boid>& boid2) {
return m_flock.end() == std::find_if(m_flock.begin(), m_flock.end(),
[&](const std::shared_ptr<Boid>& boid) {
return boid->getPos() == boid2->getPos();
});
});
// Now in m_flock2, elements between begin() and middle don't match any
// in m_flock, while elements between middle and end() all do.
// Move the second half over to the end of m_flock, then erase from m_flock2
m_flock.insert(m_flock.end(),
std::make_move_iterator(middle),
std::make_move_iterator(m_flock2.end()));
m_flock2.erase(middle, m_flock2.end());
I have a list of line segments (a std::vector<std::pair<int, int> > that I'd like to iterate through and subdivide. The algorithm would be, in psuedocode:
for segment in vectorOfSegments:
firstPoint = segment.first;
secondPoint = segment.second;
newMidPoint = (firstPoint + secondPoint) / 2.0
vectorOfSegments.remove(segment);
vectorOfSegments.push_back(std::make_pair(firstPoint, newMidPoint));
vectorOfSegments.push_back(std::make_pair(newMidPoint, secondPoint));
The issue that I'm running into is how I can push_back new elements (and remove the old elements) without iterating over this list forever.
It seems like the best approach may be to make a copy of this vector first, and use the copy as a reference, clear() the original vector, and then push_back the new elements to the recently emptied vector.
Is there a better approach to this?
It seems like the best approach may be to make a copy of this vector first, and use the copy as a reference, clear() the original vector, and then push_back the new elements to the recently emptied vector.
Almost. You don't need to copy-and-clear; move instead!
// Move data from `vectorOfSegments` into new vector `original`.
// This is an O(1) operation that more than likely just swaps
// two pointers.
std::vector<std::pair<int, int>> original{std::move(vectorOfSegments)};
// Original vector is now in "a valid but unspecified state".
// Let's run `clear()` to get it into a specified state, BUT
// all its elements have already been moved! So this should be
// extremely cheap if not a no-op.
vectorOfSegments.clear();
// We expect twice as many elements to be added to `vectorOfSegments`
// as it had before. Let's reserve some space for them to get
// optimal behaviour.
vectorOfSegments.reserve(original.size() * 2);
// Now iterate over `original`, adding to `vectorOfSegments`...
Don't remove elements while you insert new segments. Then, when finished with inserting you could remove the originals:
int len=vectorOfSegments.size();
for (int i=0; i<len;i++)
{
std::pair<int,int>& segment = vectorOfSegments[i];
int firstPoint = segment.first;
int secondPoint = segment.second;
int newMidPoint = (firstPoint + secondPoint) / 2;
vectorOfSegments.push_back(std::make_pair(firstPoint, newMidPoint));
vectorOfSegments.push_back(std::make_pair(newMidPoint, secondPoint));
}
vectorOfSegments.erase(vectorOfSegments.begin(),vectorOfSegments.begin()+len);
Or, if you want to replace one segment by two new segments in one pass, you could use iterators like here:
for (auto it=vectorOfSegments.begin(); it != vectorOfSegments.end(); ++it)
{
std::pair<int,int>& segment = *it;
int firstPoint = segment.first;
int secondPoint = segment.second;
int newMidPoint = (firstPoint + secondPoint) / 2;
it = vectorOfSegments.erase(it);
it = vectorOfSegments.insert(it, std::make_pair(firstPoint, newMidPoint));
it = vectorOfSegments.insert(it+1, std::make_pair(newMidPoint, secondPoint));
}
As Lightning Racis in Orbit pointed out, you should do a reserve before either of these approaches. In the first case do reserve(vectorOfSegmets.size()*3), in the latter reserve(vectorOfSegmets.size()*2+1)
This is easiest solved by using an explicit index variable like this:
for(size_t i = 0; i < segments.size(); i++) {
... //other code
if(/*condition when to split segments*/) {
Point midpoint = ...;
segments[i] = Segment(..., midpoint); //replace the segment by the first subsegment
segments.emplace_back(Segment(midpoint, ...)); //add the second subsegment to the end of the vector
i--; //reconsider the first subsegment
}
}
Notes:
segments.size() is called in each iteration of the loop, so we really reconsider all appended segments.
The explicit index means that the std::vector<> is free to reallocate in the emplace_back() call, there are no iterators/pointers/references that can become invalid.
I assumed that you don't care about the order of your vector because you add the new segments to the end of the vector. If you do care, you might want to use a linked list to avoid quadratic complexity of your algorithm as insertion/deletion to/from an std::vector<> has linear complexity. In my code I avoid insertion/deletion by replacing the old segment.
Another approach to retain order would be to ignore order at first and then reestablish order via sorting. Assuming a good sorting algorithm, that is O(n*log(n)) which is still better than the naive O(n^2) but worse than the O(n) of the linked list approach.
If you don't want to reconsider the new segments, just use a constant size and omit the counter decrement:
size_t count = segments.size();
for(size_t i = 0; i < count; i++) {
... //other code
if(/*condition when to split segments*/) {
Point midpoint = ...;
segments[i] = Segment(..., midpoint); //replace the segment by the first subsegment
segments.emplace_back(Segment(midpoint, ...)); //add the second subsegment to the end of the vector
}
}
I need some assistance with a C++ project. What I have to do is remove the given element from an array of pointers. The technique taught to me is to create a new array with one less element and copy everything from the old array into the new one except for the specified element. After that I have to point the old array towards the new one.
Here's some code of what I have already:
I'm working with custom structs by the way...
Data **values = null; // values is initialized in my insert function so it is
// populated
int count; // this keeps track of values' length
bool remove(Data * x) {
Data **newArray = new Data *[count - 1];
for (int i = 0; i < count; i++) {
while (x != values[i]) {
newArray[i] = values[i];
}
count -= 1;
return true;
}
values = newArray;
return false;
}
So far the insert function works and outputs the populated array, but when I run remove all it does is make the array smaller, but doesn't remove the desired element. I'm using the 0th element every time as a control.
This is the output I've been getting:
count=3 values=[5,6,7] // initial insertion of 5, 6, 7
five is a member of collection? 0
count=3 values=[5,6] // removal of 0th element aka 5, but doesn't work
five is a member of collection? 0
count=4 values=[5,6,5] // re-insertion of 0th element (which is stored in
five is a member of collection? 0 // my v0 variable)
Could anyone nudge me in the right direction towards completing this?
First of all, your code is leaking memory like no good! Next you only copy the first element and not even that if the first element happens to be the one you want to remove. Also, when you return from your function, you haven't changed your internal state at all. You definitely want to do something along the lines of
Data** it = std::find(values, values + count, x);
if (it != values + count) {
std::copy(it + 1, values + count, it);
--count;
return true;
}
return false;
That said, if anybody taught you to implement something like std::vector<T> involving reallocations on every operation, it is time to change schools! Memory allocations are relatively expensive and you want to avoid them. That is, when implementing something like a std::vector<T> you, indeed, want to implement it like a std::vector<T>! That is you keep an internal buffer of potentially more element than there are and remember how many elements you are using. When inserting a new element, you only allocate a new array if there is no space in the current array (not doing so would easily result in quadratic complexity even when always adding elements at the end). When removing an element, you just move all the trailing objects one up and remember that there is one less object in the array.
Try this:
bool remove(Data * x)
{
bool found = false;
// See if x is in the array.
for (int i = 0; i < count; i++) {
if (x != values[i]) {
found = true;
break;
}
}
if (!found)
{
return false;
}
// Only need to create the array if the item to be removed is present
Data **newArray = new Data *[count - 1];
// Copy the content to the new array
int newIndex = 0;
for (int i = 0; i < count; i++)
{
if (x != values[i])
newArray[newIndex++] = values[i];
}
// Now change the pointers.
delete[] values;
count--;
values = newArray;
return true;
}
Note that there's an underlying assumption that if x is present in the array then it's there only once! The code will not work for multiple occurrences, that's left to you, seeing as how this is a school exercise.
I have a Deque that contains this kind of stucts.
struct New_Array {
array<array<int,4>,4> mytable;
int h;
};
In this stuct 2 different arrays may have same value of h.
deque<New_Array> Mydeque;
I also know how many different h are in the deque(the value of steps). And how many stucts are in the deque(Mydeque.size()).
I need to print one array for each h. Starting from h=0 till h=steps (steps is a known int value). Each array that is going to be printed must be the closer to the end of the deque.
I tried something like this:
void foo(deque<New_Array> Mydeque, int steps)
for(int i=0; i<steps; i++)
{
deque<New_Array>::iterator it;
it = find(Mydeque.begin(),Mydeque.end(),i);
PrintBoard(*it); // This if a function where you enter the New_Array struct
// and it prints the array
}
}
The above gives me : error C2679: binary '==' : no operator found which takes a right-hand operand of type 'const bool' (or there is no acceptable conversion)
Or something like this:
void foo(deque<New_Array> Mydeque, int steps)
for(int i=0; i<steps; i++)
{
deque<New_Array>::iterator it;
for(unsigned int j=0;j<Mydeque.size();j++)
{
it = find_if(Mydeque.begin(),Mydeque.end(),Mydeque[j].h==i);
PrintBoard(*it);
break;
}
}
The above gives me: error C2064: term does not evaluate to a function taking 1 arguments
EDIT: The deque is not sorted. For each h an array should be printed. This array should be the one that is at this moment closer to the end of the deque.
Remember the last value and skip:
assert(!Mydeque.empty());
int old_h = Mydeque[0].h + 1; // make sure it's different!
for (std::size_t i = 0, end != Mydeque.size(); i != end; ++i)
{
if (Mydeque[i].h == old_h) continue;
print(Mydeque[i]);
old_h = Mydeque[i].h;
}
Firstly, note that you declare your std::array on the stack, so the storage will also be on the stack. This means that iterating over the structure involves loading a (4*4+1)*int for each comparison. If this is performance-sensitive, I would suggest using std::vector since the load will be only of the outer vector pointer and the h when only comparing h.
struct New_Array {
vector<vector<int,4>,4> mytable;
int h;
};
Secondly, if you need to access these tables through their h values, or access all the tables with a given h at once, make it easier for everyone and store them as vectors in a map, or a sorted vector of vectors:
std::map<int,std::vector<New_Array> > rolodex;
rolodex[someNewArray.h].push_back(someNewArray);
If you construct this in-order, then the first item in each vector will be the one to print:
for(auto it : rolodex) {
vector<New_Array> tablesForThisH = it->second;
if(tablesForThisH.begin() != tablesForThisH.end())
PrintBoard(it->second[0]);
}
Since std:map stores (and iterates) its keys in ascending (I think) order, this will run over the different h values in ascending order. Again it will only need to load the stack-stored struct, which is just the h int and the vector header (probably 12 bytes, as mentioned in this question).
Forgive me if the code is wrong, my stl is a little rusty.
Loop through the deque, and insert all elements into a map, using h as the key. Since your set of h values seems to be sequential, you can use a vector instead, but testing whether an element has already been found will be more difficult.
The solution is :
void Find_Solution_Path(deque<New_Array> Mydeque, int steps)
{
for(int i=0; i<steps+1; i++)
{
for(int j=Mydeque.size()-1;j>-1;j--)
{
if (Mydeque[j].h==i)
{
PrintBoard(Mydeque[j]);
cout<<endl;
break;
}
}
}
}