How do you use a vector in c++? - c++

Question

I am a beginner in c++ and I am trying to understand vectors.
I know the basic format which is:
vector <dataType> vectorName;
People are telling me that vectors are like arrays. But, what I don't
understand is that for arrays you can do this:
array[] = {1, 2, 3}
But for vectors you don't seem to get to set it to a list. Or do you have
to keep using .push_back().
Also, can you use something like vectorName[1] or not?
Can anyone explain this to me?
Thanks.

Answer 1

You can use the style if you use C++11 or later.
#include <iostream>
#include <vector>
int main(void) {
std::vector<int> vec = {1, 2, 3};
std::cout << vec[1] << std::endl;
return 0;
}

Answer 2

The whole purpose of vectors is to be "infinite", so you don't have to redefine it everytime you need to expand it.
push_back is made so you can add/expand to the array without redefining it; you still access and modify like a normal array:
std::vector<int> a;
a.push_back(2);
a.push_back(6);
std::cout << a[0] << std::end; //2
std::cout << a[1] << std::end; //6
a[0] = 5;
a[1] = 7;
std::cout << a[0] << std::end; //5
std::cout << a[1] << std::end; //7
You can also initialize it old-school style (the = is optional):
std::vector<int> a {2, 6};
std::cout << a[0] << std::end; //2
std::cout << a[1] << std::end; //6

Answer 3

Try this for C++:
#include <iostream>
#include <vector>
int main(void) {
std::vector<int> vec { 34,23 };
return 0;
}
Or even:
#include <iostream>
#include <vector>
int main(void) {
std::vector<int> v(2);
v = { 34,23 };
return 0;
}

Answer 4

seems non of the above gave you any hint on dealing with the vector after you created it, so, let's say you created it with few initial values.
#include <iostream>
#include <vector>
int main(void) {
std::vector<int> vec { 34,23 };
// use push_back(some_value) if you have multiple values to put inside, say 1000.
// instead of using the index [] brackets, try .at() method
std::cout << vec.at(1) << std::endl;
// use both the .size() or the new syntax to loop over it
for (unsigned i = 0 ; i < vec.size() ; i++){
std::cout << vec.at(i) << std::endl;
}
// or the new syntax
for (auto & i : vec){
std::cout << i << std::endl;
}
return 0;
}
Have fun :)

Answer 5

Vectors are expandable arrays. Unlike an array, you are not restricted to the size you initialized it with.
Growth of vector: the vector doubles it size whenever you overflow it. Underneath the hood its still an array but the "expandable" property of it comes by copying the contents of the previous array into a new larger array.
Few things that you can do with vectors in C++
Initialization of vector
vector<Type> one (size, defaultValue);
vector<Type> two (one); // makes a new vector *two* with the contents of *one*
vector<int> three {1,2,3}; // using initializer list
Accessing an element
int temp = three[2]; //array like syntax
int temp = three.at(2);
You cannot increase the size of a vector using this syntax i.e. you cannot dothree[3]=4;
Expanding
three.pusk_back(4);
Shrinking
three.pop_back();