I would like to loop through elements of a vector in C++.
I am very new at this so I don't understand the details very well.
For example:
for (elements in vector) {
if () {
check something
else {
//else add another element to the vector
vectorname.push_back(n)
}
}
Its the for (vector elements) that I am having trouble with.
You'd normally use what's called a range-based for loop for this:
for (auto element : your_vector)
if (condition(element))
// whatever
else
your_vector.push_back(something);
But note: modifying a vector in the middle of iteration is generally a poor idea. And if your basic notion is to add the element if it's not already present, you may want to look up std::set, std::map, std::unordered_set or std::unordered_map instead.
In order to do this properly (and safely), you need to understand how std::vector works.
vector capatity
You may know that a vector works much like an array with "infinite" size. Meaning, it can hold as many elements as you want, as long as you have enough memory to hold them. But how does it do that?
A vector has an internal buffer (think of it like an array allocated with new) that may be the same size as the elements you're storing, but generally it's larger. It uses the extra space in the buffer to insert any new elements that you want to insert when you use push_back().
The amount of elements the vector has is known as its size, and the amount of elements it can hold is known as its capacity. You can query those via the size() and capacity() member functions.
However, this extra space must end at some point. That's when the magic happens: When the vector notices it doesn't have enough memory to hold more elements, it allocates a new buffer, larger1 than the previous one, and copies all elements to it. The important thing to notice here is that the new buffer will have a different address. As we continue with this explanation, keep this in mind.
iterators
Now, we need to talk about iterators. I don't know how much of C++ you have studied yet, but think of an old plain array:
int my_array[5] = {1,2,3,4,5};
you can take the address of the first element by doing:
int* begin = my_array;
and you can take the address of the end of the array (more specifically, one past the last element) by doing:
int* end = begin + sizeof(my_array)/sizeof(int);
if you have these addresses, one way to iterate the array and print all elements would be:
for (int* it = begin; it < end; ++it) {
std::cout << *it;
}
An iterator works much like a pointer. If you increment it (like we do with the pointer using ++it above), it will point to the next element. If you dereference it (again, like we do with the pointer using *it above), it will return the element it is pointing to.
std::vector provides us with two member functions, begin() and end(), that return iterators analogous to our begin and end pointers above. This is what you need to keep in mind from this section: Internally, these iterators have pointers that point to the elements in the vector's internal buffer.
a simpler way to iterate
Theoretically, you can use std::vector::begin() and std::vector::end to iterate a vector like this:
std::vector<int> v{1,2,3,4,5};
for (std::vector<int>::iterator it = v.begin; it != v.end(); ++it) {
std::cout << *it;
}
Note that, apart from the ugly type of it, this is exactly the same as our pointer example. C++ introduced the keyword auto, that lets us get rid of these ugly types, when we don't really need to know them:
std::vector<int> v{1,2,3,4,5};
for (auto it = v.begin; it != v.end(); ++it) {
std::cout << *it;
}
This works exactly the same (in fact, it has the exact same type), but now we don't need to type (or read) that uglyness.
But, there's an even better way. C++ has also introduced range-based for:
std::vector<int> v{1,2,3,4,5};
for (auto it : v) {
std::cout << it;
}
the range-based for construct does several things for you:
It calls v.begin() and v.end()2 to get the upper and lower bounds of the range we're going to iterate;
Keeps an internal iterator (let's call it i), and calls ++i on every step of the loop;
Dereferences the iterator (by calling *i) and stores it in the it variable for us. This means we do not need to dereference it ourselves (note how the std::cout << it line looks different from the other examples)
putting it all together
Let's do a small exercise. We're going to iterate a vector of numbers, and, for each odd number, we are going to insert a new elements equal to 2*n.
This is the naive way that we could probably think at first:
std::vector<int> v{1,2,3,4,5};
for (int i : v) {
if (i%2==1) {
v.push_back(i*2);
}
}
Of course, this is wrong! Vector v will start with a capacity of 5. This means that, when we try using push_back for the first time, it will allocate a new buffer.
If the buffer was reallocated, its address has changed. Then, what happens to the internal pointer that the range-based for is using to iterate the vector? It no longer points to the buffer!
This it what we call a reference invalidation. Look at the reference for std::vector::push_back. At the very beginning, it says:
If the new size() is greater than capacity() then all iterators and references (including the past-the-end iterator) are invalidated. Otherwise only the past-the-end iterator is invalidated.
Once the range-based for tries to increment and dereference the now invalid pointer, bad things will happen.
There are several ways to avoid this. For instance, in this particular algorithm, I know that we can never insert more than n new elements. This means that the size of the vector can never go past 2n after the loop has ended. With this knowledge in hand, I can increase the vector's capacity beforehand:
std::vector<int> v{1,2,3,4,5};
v.reserve(v.size()*2); // Increases the capacity of the vector to at least size*2.
// The code bellow now works properly!
for (int i : v) {
if (i%2==1) {
v.push_back(i*2);
}
}
If for some reason I don't know this information for a particular algorithm, I can use a separate vector to store the new elements, and then add them to our vector at the end:
std::vector<int> v{1,2,3,4,5};
std::vector<int> doubles;
for (int i : v) {
if (i%2==1) {
doubles.push_back(i*2);
}
}
// Reserving space is not necessary because the vector will allocate
// memory if it needs to anyway, but this does makes things faster
v.reserve(v.size() + doubles.size());
// There's a standard algorithm (std::copy), that, when used in conjunction with
// std::back_inserter, does this for us, but I find that the code bellow is more
// readable.
for (int i : doubles) {
v.push_back(i);
}
Finally, there's the old plain for, using an int to iterate. The iterator cannot be invalidated because it holds an index, instead of a pointer to the internal buffer:
std::vector<int> v{1,2,3,4,5};
for (int i = 0; i < v.size(); ++i) {
if (v[i]%2==1) {
doubles.push_back(v[i]*2);
}
}
Hopefully by now, you understand the advantages and drawbacks of each method. Happy studies!
1 How much larger depends on the implementation. Generally, implementations choose to allocate a new buffer of twice the size of the current buffer.
2 This is a small lie. The whole story is a bit more complicated: It actually tries to call begin(v) and end(v). Because vector is in the std namespace, it ends up calling std::begin and std::end, which, in turn, call v.begin() and v.end(). All of this machinery is there to ensure that the range-based for works not only with standard containers, but also with anything with a proper implementation for begin and end. That includes, for instance, regular plain arrays.
Here is the quick code snippet using iterators to iterate the vector-
#include<iostream>
#include<iterator> // for iterators to include
#include<vector> // for vectors to include
using namespace std;
int main()
{
vector<int> ar = { 1, 2, 3, 4, 5 };
// Declaring iterator to a vector
vector<int>::iterator ptr;
// Displaying vector elements using begin() and end()
cout << "The vector elements are : ";
for (ptr = ar.begin(); ptr < ar.end(); ptr++)
cout << *ptr << " ";
return 0;
}
Article to read more - Iterate through a C++ Vector using a 'for' loop
.
Hope it will help.
Try this,
#include<iostream>
#include<vector>
int main()
{
std::vector<int> vec(5);
for(int i=0;i<10;i++)
{
if(i<vec.size())
vec[i]=i;
else
vec.push_back(i);
}
for(int i=0;i<vec.size();i++)
std::cout<<vec[i];
return 0;
}
Output:
0123456789
Process returned 0 (0x0) execution time : 0.328 s
Press any key to continue.
I have to copy the first size element from a set of Solution (a class) named population to an array of solution named parents. I have some problems with iterators because i should do an hybrid solution between a normal for loop
and a for with iterators. The idea is this: when I'm at the ith iteration of the for I declare a new iterator that's pointing the beginning
of population, then I advance this iterator to the ith position, I take this solution element and I copy into parents[i]
Solution* parents; //it is filled somewhere else
std::set<Solution> population; //it is filled somewhere else
for (int i = 0; i < size; i++) {
auto it = population.begin();
advance(it, i);
parents[i] = *it;
}
Two error messages popup with this sentence: 'Expression: cannot dereference end map/set iterator'
and 'Expression: cannot advance end map/set iterator'
Any idea on how to this trick? I know it's kinda bad mixing array and set, i should use vector instead of array?
You use std::copy_n.
#include <algorithm>
extern Solution* parents; //it is filled somewhere else
extern std::set<Solution> population; //it is filled somewhere else
std::copy_n(population.begin(), size, parents);
It seems like size may be incorrectly set. To ensure that your code behaves as expected, you should just use the collection's size directly:
auto it = population.begin();
for (int i = 0; i < population.size(); i++) {
parents[i] = *it;
++it;
}
This can also be solved with a much simpler expression:
std::copy(population.begin(), population.end(), parents);
I have to copy the first size element from a set [..] to an array
You can use std::copy_n.
for (int i = 0; i < size; i++) {
auto it = population.begin();
advance(it, i);
The problem with this is that you're iterating over the linked list in every iteration. This turns the copy operation from normally linear complexity to quadratic.
Expression: cannot dereference end map/set iterator'
The problem here appears to be that your set doesn't contain at least size number of elements. You cannot copy size number of elements if there aren't that many. I suggest that you would copy less elements when the set is smaller.
i should use vector instead of array?
Probably. Is the array very large? Is the size of the vector not known at compile time? If so, use a vector.
I've been trying to use an array of pointers to point to vectors, which I have so far been able to implement, however, in trying to add an element to one of the sub-vectors, I repeatedly get an unknown error on run-time.
I have previously defined my array as so:
std::vector<std::string> *frequency_table[10000];
I then try to add an element to a specific one of the vectors. This is the line that causes the run-time error.
frequency_table[index]->push_back(value);
Any ideas?
At first glance the problem looks like you haven't allocated any memory for the pointer so it has nowhere to push the value to. Though i can't be sure without the error message.
If that is the case however you would need to use new to allocate the memory
Your approach involves mixing vectors (and vectors are a good thing) and C-style arrays of pointers to objects (which betrays a mix-up since there are already vectors in your code).
If you want 10000 vectors of vectors of string, then just write
std::vector<std::vector<std::string> > frequency_table(10000);
. . .
frequency_table[index].push_back(value);
The first line declares a vector, each element of which is a vector<string>, allocates 10000 elements to it and initializes each element.
Using the input from you guys, I realized that my problem was the vectors were not being initialized, so I added this loop before the loop in which I populate the vectors, and it works now:
for(int i = 0; i < 5000; ++i)
{
frequency_table[i] = new std::vector<std::string>;
}
The final code looks like this:
for(int i = 0; i < 5000; ++i)
{
frequency_table[i] = new std::vector<std::string>;
}
for(auto itr = frequency_map.begin(); itr != frequency_map.end(); ++itr)
{
std::string key = itr->first;
double value = itr->second;
frequency_table[(int)value]->push_back(key);
}
Thanks all!
ps I halved the 10000 for testing purposes
I'm creating a custom vector class as part of a homework assignment. What I am currently trying to do is implement a function called erase, which will take an integer as an argument, decrease my array length by 1, remove the element at the position specified by the argument, and finally shift all the elements down to fill in the gap left by "erased" element.
What I am not completely understanding, due to my lack of experience with this language, is how you can delete a single element from an array of pointers.
Currently, I have the following implemented:
void myvector::erase(int i)
{
if(i != max_size)
{
for(int x = i; x < max_size; x++)
{
vec_array[x] = vec_array[x+1];
}
vec_size --;
//delete element from vector;
}
else
//delete element from vector
}
The class declaration and constructors look like this:
template <typename T>
class myvector
{
private:
T *vec_array;
int vec_size;
int max_size;
bool is_empty;
public:
myvector::myvector(int max_size_input)
{
max_size = max_size_input;
vec_array = new T[max_size];
vec_size = 0;
}
I have tried the following:
Using delete to try and delete an element
delete vec_size[max_size];
vec_size[max_size] = NULL;
Setting the value of the element to NULL or 0
vec_size[max_size] = NULL
or
vec_size[max_size] = 0
None of which are working for me due to either operator "=" being ambiguous or specified type not being able to be cast to void *.
I'm probably missing something simple, but I just can't seem to get passed this. Any help would be much appreciated. Again, sorry for the lack of experience if this is something silly.
If your custom vector class is supposed to work like std::vector, then don't concern yourself with object destruction. If you need to erase an element, you simply copy all elements following it by one position to the left:
void myvector::erase(int i)
{
for (int x = i + 1; x < vec_size; x++) {
vec_array[x - 1] = vec_array[x];
}
vec_size--;
}
That's all the basic work your erase() function has to do.
If the elements happen to be pointers, you shouldn't care; the user of your vector class is responsible for deleting those pointers if that's needed. You cannot determine if they can actually be deleted (the pointers might point to automatic stack variables, which are not deletable.)
So, do not ever call delete on an element of your vector.
If your vector class has a clear() function, and you want to make sure the elements are destructed, simply:
delete[] vec_array;
vec_array = new T[max_size];
vec_size = 0;
And this is how std::vector works, actually. (Well, the basic logic of it; of course you can optimize a hell of a lot of stuff in a vector implementation.)
Since this is homework i wont give you a definitive solution, but here is one method of erasing a value:
loop through and find value specified in erase function
mark values position in the array
starting from that position, move all elements values to the previous element(overlapping 'erased' value)
for i starting at position, i less than size minus one, i plus plus
element equals next element
reduce size of vector by 1
see if this is a big enough hint.
I ran into this problem when I tried to write out an new algorithm to reorder elements in std::vector. The basic idea is that I have std::list of pointters pointing into std::vector in such way that *list.begin() == vector[0], *(++list.begin()) == vector[1] and so on.
However, any modifications on list's element positions breaks the mapping. (Including appended pointers) When the mapping is broken the list's elements can be in random order but they point still into correct elements on vector. The task would be to reorder the elements in vector to correct the mapping.
Simplest method to do it (How I have done it now):
create new empty std::vector and resize it to equal size of the old vector.
iterate through the list and read elements from the old vector and write them into new vector. Set the pointer to point into new vector's element.
swap vectors and release the old vector.
Sadly the method is only useful when I need more capacity on the vector. It's inefficient when the current vector holding the elements has enough capacity to store all incoming elements. Appended pointers on the list will point into diffrent vector's storgate. The simple method works for this because it only reads from the pointers.
So I would want to reorder the vector "in place" using constant amount of memory. Any pointer that was not pointing into current vector's storgate are moved to point into current vector's storgate. Elements are simple structures. (PODs)
I'll try post an example code when I have time..
What should I do to achieve this? I have the basic idea done, but I'm not sure if it is even possible to do the reordering with constant amount of memory.
PS: I'm sorry for the (possibly) bad grammar and typos in the post. I hope it's still readable. :)
First off, why do you have a list of pointers? You might as well keep indices into the vector, which you can compute as std::distance(&v[0], *list_iter). So, let's build a vector of indices first, but you can easily adapt that to use your list directly:
std::vector<T> v; // your data
std::list<T*> perm_list; // your given permutation list
std::vector<size_t> perms;
perms.reserve(v.size());
for (std::list<T*>::const_iterator it = perm_list.begin(), end = perm_list.end(); it != end; ++it)
{
perms.push_back(std::distance(&v[0], *it));
}
(There's probably a way to use std::transform and std::bind, or lambdas, to do this in one line.)
Now to do the work. We simply use the cycle-decomposition of the permutation, and we modify the perms vector as we go along:
std::set<size_t> done;
for (size_t i = 0; i < perms.size(); while(done.count(++i)) {})
{
T tmp1 = v[i];
for (size_t j = perms[i]; j != i; j = perms[j])
{
T tmp2 = v[j];
v[j] = tmp1;
tmp1 = tmp2;
done.insert(j);
}
v[i] = tmp1;
}
I'm using the auxiliary set done to track which indices have already been permuted. In C++0x you would add std::move everywhere to make this work with movable containers.