I'm trying to use a range based for loop to iterate from the beginning of a vector to a variable index x as shown.
void algorithm(vector<int> arr, int n)
{
int count=0;
for (int i:arr[n])
count++;
}
I understand the above function is rather useless but I'm trying to conceptually understand if there is a way to do so. arr[n] gives me an error as does arr.begin()+n
You can increment a counter and break the loop when the desired count is reached.
Something like:
void func(const std::vector<int> & v, std::size_t n)
{
std::size_t count(0);
for(int e : v)
{
if(count < n)
{
std::cout << e << " "; // Do what you want with e.
++count;
}
else
break; // Exit the loop
}
}
But I would not recommend doing it. Range-based for loops are meant to iterate over the whole container.
The above solution is thus a less readable way of simply doing:
void func(const std::vector<int> & v, std::size_t n)
{
for(std::size_t i = 0; (i < v.size()) && (i < n); ++i)
{
std::cout << v[i] << " "; // Do what you want with v[i].
}
}
Or also:
void func(const std::vector<int> & v, std::size_t n)
{
std::size_t count(0);
for(std::vector<int>::const_iterator cit = v.cbegin(); (cit != v.cend()) && (count < n); ++cit, ++count)
{
std::cout << *cit << " "; // Do what you want with *cit
}
}
Range-based for loop is pure sugar - it can only iterate over the entire range.
If you need to play with the iterators you have to fall back on a for loop.
According to the documetion the for range loop is translate to :
{
auto && __range = range_expression ;
for (auto __begin = begin_expr, __end = end_expr; __begin != __end; ++__begin) {
range_declaration = *__begin;
loop_statement
}
}
so you can't set an ending point like you trying because its it a iterator to a place and its not arr.end(). but if you know the ending index you can try doing this:
vector<int> new_vec(arr.begin(), arr,(begin()+index));
for(auto& e:new_vec)
{
//some code
}
Just to add to the previous answers:
While at present this is not possible, C++20 will ship with much more support for ranges. I found this article helpful in understanding some of the proposed features. As far as I understand you will be able to write
void algorithm(vector<int> arr, int n){
int count=0;
for (int i: arr | take(n))
count++;
}
There is no implementation of this proposal yet, so I am not aware of a way to test my code.
arr) is passed by value and changes to it won't be visible to the calluer, simply resize based onn, and then use a range-based loopfor (int i: arr)...as usual. Otherwise, it is not possible to use a range-based for as you wish.for (int i=0; i<n; i++) std::cout << arr[i] << "\n";?