在 C++ 程序中使用向量不会打印任何内容

Question

I have written a C++ program to find fibonacci numbers. It's running successfully but is not printing anything.

#include<bits/stdc++.h>

using namespace std;

int main(){
int n = 5;

vector<int> fib;

fib[0] = 0;
fib[1] = 1;


for (int i = 2; i <= n; i++){
    fib[i] = fib[i-1] + fib[i-2];
}

cout<<fib[n];
}

If I do the same thing using array instead of vector it prints successfully.

#include<bits/stdc++.h>

using namespace std;

int main(){
int n = 5;

int fib[10];

fib[0] = 0;
fib[1] = 1;


for (int i = 2; i <= n; i++){
    fib[i] = fib[i-1] + fib[i-2];
}

cout<<fib[n];
}

I have tested this on sublime text and onlinegdb.

Answer 1

int fib[10];

This creates an array of 10 integers.

vector<int> fib;

This creates a vector of size 0, with 0 integers.

For these two snippets to match, you need to initialize the vector with 10 integers like the array. So:

vector<int> fib(10);

---

Some notes on vectors

One of the big differences between std::vector and primitive arrays is that std::vector is resizable, So while initializing the vector with 10 ints like above will work, you could also add them on the fly using push_back() or by callingresize() .

Likewise, std::vector has a .at() function for access. It's marginally slower than the subscript operator ( [] ), so I would not suggest using it in production-level code. But while you're learning, I would strongly suggest using .at() , as it will do bounds checking for you. So this program would've told you you were trying to access locations in the vector that don't exist--instead of just running with Undefined Behavior.

Answer 2

Note that when you wrote:

vector<int> fib;//creates an empty vector that is a vector of size 0

fib[0] = 0; //UNDEFINED BEHAVIOR
fib[1] = 1; //UNDEFINED BEHAVIOR

In the above code snippet, you created an empty std::vector . That is a vector of size 0 .

Next when you wrote fib[0] = 0; you're trying access the first element of the vector. That is, the element at index 0 . But note that there is no element inside the vector and so you have undefined behavior in your program.

Similarly when you wrote fib[1] = 1; , you're trying to access the second element of the vector. But since there is no element inside the vector, this again leads to undefined behavior.

To solve this you can create a vector of some size(say 10) and then access its elements as shown below:

vector<int> fib(10);//create vector of size 10

fib[0] = 0;//OK NOW
fib[1] = 1;//OK NOW

Since you're using indices to access the elements of the vector, i have not suggested push_back . std::vector::push_back is a member function that can be used to add an element into the vector.