I am new to C programming and I am having trouble with the following code:

#include <iostream>
#include <vector>

using namespace std;

class Absolute {

    public: 
        vector<int> nums;

        Absolute(vector<int> nums) { 
            nums = nums; //<--- this size is not 0
        }

        vector<int> getNums() { return nums; }

        int addr() { 
            int total = 0;
            for(int i=0; i<nums.size(); i  ) { //<--- nums.size() here is 0 even though it should not be 
                total  = nums[i];
            }

            return total;
        }

};
    
int main() {

    vector<int> numbers = {1, 3, 3, 4, 5, 6, 7, 8}; //<--- passing this to addNums but still returns size of 0
    Absolute addNums = Absolute(numbers);

    cout << addNums.addr() << endl;

    return 0;
}

My goal is to add up all the numbers in the vector together using the addr() method, but for some reason the size of the vector is 0 inside of the addr() method, so I end up printing these very large numbers:

15152333321

What is going on here? Could it be that it's not being initialized with the vector in the first place?

CodePudding user response：

In this code:

Absolute(vector<int> nums)
{
    nums = nums; 
}

You encounter what I call "Highlander's Law" (There can be only one). In this function, nums refers to the parameter nums, not the member nums. The member nums is shadowed, hidden, by the parameter. So nums = nums; means assign the parameter to itself. The member nums is unchanged and stays at size 0.

Fix: You can change the name of one of the variables so that they do not collide. You can be extra explicit with this->nums = nums, but there's a better and faster option: Use the member initializer list and initialize the member nums with the correct value rather than initialize it to a default and then assign it.

Absolute(vector<int> nums): nums(nums)
{
}

You can use the same name here because there is no ambiguity. The member nums is being initialized, so it must be the left-most nums. It is initialized to the parameter nums because the parameter is the current holder of the identifier nums.

Side note: Consider passing by reference and save yourself a copy:

Absolute(const vector<int> & nums): nums(nums)
{
}

CodePudding user response：

As stated by @Eljay, you have the same name for your parameter and your member variable in Absolute constructor.

This change

        Absolute(vector<int> nums) { 
            this->nums = nums; //<--- this size is not 0
        }

should be enough for your example to work if you really want to keep the same name. I strongly suggest you to have a different name for your parameter, _nums for example.

CodePudding user response：

Here is your code btw

/*
 *
 *
 * 
 * 
 */


#include <iostream>
#include <vector>

using namespace std;


class Absolute {

    public: 
        vector<int> nums;

        

        vector<int> getNums() { return nums; }
        int total;    
        int addr() { 
            for(int i=0; i<nums.size();i  ) { //<--- nums.size() here is 0 even though it should not be 
                total  = nums[i];
            }

            return total;
        }
Absolute(vector<int> num) { 
            nums = num;
            total = 0; //<--- this size is not 0
        }
};



int main() {

    vector<int> numbers;
    numbers.push_back(1);
    numbers.push_back(3);
    numbers.push_back(3);
    numbers.push_back(4);
    numbers.push_back(5);
    numbers.push_back(6);
    numbers.push_back(7);
    numbers.push_back(8);

    Absolute addNums = Absolute(numbers);
    

    cout << addNums.addr() << endl;
    

    return 0;
}

now that the thing is there, producing right result... Please look at the fact that:

1. Parameter and attributes were named the same, I don't know why you would want to do that but thats not appropriate.
2. I moved 'Total' variable to the other section where is was more appropriate to initialize variables coming up for that class object.
3. for some reason, My version of Sublime Text was not comfortable with your style of vector initialization, (Which was valid though), SO I used the other valid variant.