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Error main: malloc.c:2401: sysmalloc: Assertion Doing a simple C++ fibonacci assignment [duplicate]

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I am working on this assignment and i trying to figure this out. When i input any other numbers between 1 to 10 except 6. The fibonacci_fast(n) outputs an error main: malloc.c:2401: sysmalloc: Assertion. However, when i use an array to replace the vector or just uncomment the std::cout. It will work fine. Can anyone enlighten me what's the error that i have made.

#include <iostream> #include <cassert> #include <vector> #include <algorithm> int fibonacci_naive(int n) { if (n <= 1) return n; return fibonacci_naive(n - 1) + fibonacci_naive(n - 2); int fibonacci_fast(int n) { std::vector<int> numbers{0,n}; numbers[0] =0; numbers[1] =1; int ans=0; if (n <= 1) return n; //std::cout<<std::endl; //int numbers[n]; for(int i=2;i<=n;i++) numbers[i] = numbers[i-1] + numbers[i-2]; ans = numbers[n]; return ans; void test_solution() { assert(fibonacci_fast(3) == 2); assert(fibonacci_fast(10) == 55); for (int n = 0; n < 20; ++n) assert(fibonacci_fast(n) == fibonacci_naive(n)); int main() { int n = 0; std::cin >> n; //std::cout << fibonacci_naive(n) << '\n'; //test_solution(); std::cout << fibonacci_fast(n) << std::endl; return 0; std::vector<int> numbers{0,n}; creates a vector of size() 2, with values 0 and n . You need to use parantheses to call the fill-n constructor: std::vector<int> numbers(n,0); . Also, the length goes first. – Yksisarvinen Feb 7, 2022 at 15:15 @Yksisarvinen it's not actually a dupe (I can see that now) but they are using a VLA in their "success" case. it's the int numbers[n]; . They should be aware that's not standard behavior and may work funky. – Mgetz Feb 7, 2022 at 15:19 Also, you go out-of-bound in the loop. Max index for vector of size n is n-1 , numbers[n]; is already out of bounds. – Yksisarvinen Feb 7, 2022 at 15:20 Also, you go out-of-bounds if n <= 1 . Before checking if n is that small, you attempt to write to numbers[1] . Constructing your vector will also fail spectacularly if n < 0 – Drew Dormann Feb 7, 2022 at 15:23 @Mgetz Ah, I didn't notice the comment. Still, StackOverflow decided for me and added both, seems reasonable. – Yksisarvinen Feb 7, 2022 at 15:23