How can I use function in C++ to create a sequence? - c++

Question

My goal is to create an immutable function (functional programming) using "accumulate" in C++. I created a dummy list that generates 1's based on the position I send, which will be 6. So the list at the start contains {1,1,1,1,1,1}. I tried using accumulate to somehow use the information on this list and make the fibonacci sequence into a new list. The result has to be {1,1,2,3,5,8}
Here is what I have.
list<int> immutableFibonacci(int position)
{
const size_t fixedListSize(position);
list<int> newList(position, int(1));
list<int> copyList;
list<int>::iterator it = newList.begin();
if (position <=2)
{
return newList; //returns {1,1} or {1}
}
while (position>0)
{
advance(it, 1);
sum = accumulate(newList.begin(),it, 0);
copyList.push_back(sum);
position--;
}
return copyList;
}
What I have so far will return copyList as {1,2,3,4,5,6}. Can someone please push me in the right direction on what to do? I tried researching quite a bit.

Answer 1

this method creates a 'container-like' object which exposes iterators via begin() and end()
#include <iterator>
#include <iostream>
struct fib_iterator : std::iterator<std::forward_iterator_tag, long long>
{
fib_iterator(std::size_t torun = 0) : to_run(torun) {}
value_type operator*() const {
return value();
}
fib_iterator& operator++()
{
--to_run;
switch(preamble)
{
case 2:
--preamble;
return *this;
case 1:
--preamble;
return *this;
}
auto next = value();
x = y;
y = next;
return *this;
}
value_type value() const
{
switch(preamble)
{
case 2:
return 0;
case 1:
return 1;
}
return x + y;
}
bool operator==(const fib_iterator& r) const {
return to_run == r.to_run;
}
bool operator!=(const fib_iterator& r) const {
return to_run != r.to_run;
}
long long x = 0;
long long y = 1;
std::size_t preamble = 2;
std::size_t to_run;
};
struct fibonacci_sequence
{
fibonacci_sequence(std::size_t length) : length_(length) {}
fib_iterator begin() const { return { length_ }; }
fib_iterator end() const { return { }; }
std::size_t length_;
};
int main()
{
for (auto i : fibonacci_sequence(50))
std::cout << i << ", ";
std::cout << '\n';
}
sample output:
0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987,
1597, 2584, 4181, 6765, 10946, 17711, 28657, 46368, 75025, 121393,
196418, 317811, 514229, 832040, 1346269, 2178309, 3524578, 5702887,
9227465, 14930352, 24157817, 39088169, 63245986, 102334155, 165580141,
267914296, 433494437, 701408733, 1134903170, 1836311903, 2971215073,
4807526976, 7778742049,

Answer 2

How about this:
#include <iostream>
#include <vector>
#include <numeric>
#include <string>
#include <functional>
int main()
{
std::vector<int> v{1, 1, 1, 1, 1, 1, 1, 1, 1, 1};
std::vector<int> s = std::accumulate(v.begin(), v.end(),std::vector<int>{},
[](const std::vector<int>& a, int b)
{
std::vector<int> d = a;
if(a.size()<2)
{
d.push_back(1);
}
else
{
auto start = d.rbegin();
auto first = *start;
start++;
auto second = *start;
d.push_back(first+second);
}
return d;
});
std::cout << "Fibo: " <<'\n';
for( auto c : s )
{
std::cout << c << "-";
}
std::cout << '\n';
}
But I also think it is a bit too much overhead, for something that simple.
EDIT: Remember to compile that with: g++ --std=c++14 fibo.cpp -o fibo.
EDIT: If you don't want to use the lambda function look here: How can I modify this Fibonacci code in C++ to use a function instead of lambda?