Can't change a vector's element value at index i - c++

Question

In Stroustrup's Programming: Principles and Practice Using C++, chapter 4, exercise 13, I have to write a program to find all the prime numbers in a given range using the Sieve of Erathostenes.
So far, I have come up with this:
vector<int> values;
void initialize_values()
{
for (int i{0}; i < 100; ++i)
values.push_back(1);
}
void remove_composites(vector<int> values)
{
for(int i{2}; i * i < values.size(); ++i)
{
if (values[i] == 1)
{
for (int p{i + i}; p < values.size(); p += i)
values[p] = 0; //not working
}
}
}
int main()
{
initialize_values();
remove_composites(values);
for (int i{2}; i < values.size(); ++i)
{
if (values[i] == 1)
cout << i << " ";
}
cout << '\n';
return 0;
}
I create a vector with size 100 (in this first case) and initialize elements to 1. Then I check all indexes multiples of i while i * i < 100 and change the value at those indexes to 0 and finally print the indexes whose value is 1.
The problem is that values[p] = 0 is not assigning the value 0 to the element at index p and I don't really understad why.
EDIT So after reading comments and answers I decided to move all code under main(), because I don't know how to use references.
The goal of this exercises, according to the book, is to get used to processing user input, working with loops, iterations, etc. It's not intended to be professional nor expert at this point.
To all people with many years coding and answering questions here, remember you were once learners and beginners. Have some respect and don't turn this community into a toxic one.
Updated code:
int main()
{
vector<int> values;
for (int i{0}; i < 100; ++i)
values.push_back(1);
for(int i{2}; i * i < values.size(); ++i)
{
if (values[i] == 1)
{
for (int p{i + i}; p < values.size(); p += i)
{
//cout << p << " ";
values[p] = 0;
}
//cout << '\n';
}
}
for (int i{2}; i < values.size(); ++i)
{
if (values[i] == 1)
cout << i << " ";
}
cout << '\n';
return 0;
}
And works as expected:
2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97
Thanks everyone.

Answer 1

The problem is that values[p] = 0 is not assigning the value 0 to the element at index p
It is assigning it.
The problem is that you are operating on a copy of the original vector, and that copy is local to remove_composites, which is why the calling code doesn’t see the changes.
Change your function signature to use a reference type parameter: std::vector<int>&.
There are also some other issues with your code (using the wrong type, i.e. int instead of bool; inconsistent signatures; mutable global variables).