cannot assign to return value because function 'operator[]' returns a const value [closed]

Question

I am writing a function in C++ that doubles all the numbers in a vector. My code is:

std::vector<int> maps(const std::vector<int> & values) {
    for (unsigned i = 0; i < values.size(); i++) 
    {
        values[i] = values[i] * 2;
    }
    return values;
}

However this gives the error:

error: cannot assign to return value because function 'operator[]' returns a const value

values[i] = values[i] * 2;

I don't understand this error or what is wrong with my code.

Answer 1

You are passing the vector as const and you are trying to modifying it. Remove const qualifier if you want to modify it.

Answer 2

I don't understand this error...

The error says that the assignment values[i] = ... is illegal

or what is wrong with my code.

You pass values as const vector. This lets the code inside maps to read values at index i, but not write back a new value.

You can fix it by copying the array first, and doing modifications to the copy. It can be done implicitly by removing const and & from the signature of maps:

std::vector<int> maps(std::vector<int> values)

Now that values is passed by value, C++ makes a copy for you before entering maps.