使用基于范围的for循环将值分配给矢量

Question

I am trying to assign values to a vector using the for each loop. If I print the values after assigning them to x in loop 2, the order is correct.

But when I print the vector which was modified in loop 2, the vector remains unmodified. Can someone explain?

I tried using the normal for loop and then there is no problem.

The code that doesn't work:

#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

int main()
{
    vector<int> a = { -1, 150, 190, 170, -1, -1, 160, 180 };
    vector<int> v;

    // loop 1
    for (int x : a)
    {
        if (x != -1)
            v.push_back(x);
    }
    sort(v.begin(), v.end(), greater<int>());

    // loop2
    for (int x : a)
    {
        if (x != -1)
        {
            x = v.back();
            v.pop_back();
            cout << x << " ";
        }
        else
            cout << x << " ";
    }

    cout << endl << endl;

    // loop3
    for (int x : a)
        cout << x << " ";
}

The code works when loop 2 is replaced by:

for (int x = 0; x < a.size(); x++)
{
    if (a[x] != -1)
    {
        a[x] = v.back();
        v.pop_back();
    }
}

Actual result:

-1 150 160 170 -1 -1 180 190

-1 150 190 170 -1 -1 160 180

Desired result:

-1 150 160 170 -1 -1 180 190

-1 150 160 170 -1 -1 180 190

Answer 1

The problem is in the for loop:

for(int x:a) { // (1)
    if(x!=-1)
    {   
        x=v.back(); // (2)
        v.pop_back();
        cout<<x<<" ";}
    else
        cout<<x<<" ";
}

x is a copy of the element in a, and not the element directly. So when you change x (2), you change the copy of the element, and not the element in the vector.

If you want to change the elements in the vector, do

for(int& x : a)

then x will be a reference to the element in a, and when x if changed, the corresponding element in a is changed as well.