Why is my simple merge algorithm c++ not working?

January 9, 2022

I’m trying to merge two sorted vectors into a third one. However, the compiler gives me 0 in the terminal!!

Can someone please tell me what I’m doing wrong?

Thanks!



// merging two sorted vectors

std::vector<int> vec1{5};
std::vector<int> vec2{5};
std::vector<int> vec3{10};

for(int i = 0; i < 5; i++){
 vec1[i] =  2 * i;
}

for(int i = 0; i < 5 ; i++){
 vec2[i] = 2 + 2 * i;
}

std::sort(vec1.begin(), vec1.end());
std::sort(vec2.begin(), vec2.end());

std::merge(vec1.begin(), vec1.end(), vec2.begin(), vec2.end(), vec3.begin());

for(auto itr = vec3.begin(); itr != vec3.end(); ++itr){
    std::cout << " " << *itr; 
}

>Solution :

All your vectors have .size() == 1, so you’re going out of bounds, and the behavior is undefined.

Use () instead of {}:

std::vector<int> vec1(5);
std::vector<int> vec2(5);
std::vector<int> vec3(10);

You should never use the {...} syntax with containers (as opposed to = {...} or {}), because it has a rather peculiar behavior.

If the elements were not ints (and not constructible from ints), it would’ve worked as you expect:

std::vector<int> x(3); // size=3, [0,0,0]
std::vector<int> x{3}; // size=1, [1]
std::vector<int> x = {3}; // size=1, [1]

struct A {};
std::vector<A> x(3); // size=3, [A{}, A{}, A{}]
std::vector<A> x{3}; // size=3, [A{}, A{}, A{}]
std::vector<A> x = {3}; // compilation error
std::vector<A> x = {A{}}; // size=1, [A{}]

Notice that the behavior of {...} changes depending on the element type, but that doesn’t happen with (...) or = {...}.