Doubt with vectors

help
1

I am having doubt with the following code. Here I have a vector of vectors wall . I have made some changes into it v[i]=v[i]+v[i-1] . But when I print wall, i get unchanged output…HELP ME.

#include <bits/stdc++.h>
using namespace std;

int main() {

vector<vector<int>> wall={{7, 1, 2}, {3, 5, 1, 1}, {10}};

 for (auto v : wall)
    {
        for (int i=1;i<v.size();i++)
        {
            v[i]=v[i]+v[i-1];
            
            //cout<<v[i]<<" ";
                
        }
       // cout<<endl;
    }
    
    
    for (auto v : wall)
    {
        for (int i=0;i<v.size();i++)
        {
            
            cout<<v[i]<<" ";     
        }
        cout<<endl;
    }


return 0;

}

Output:
7 1 2
3 5 1 1
10

Output I expect:
7 8 10
3 8 9 10
10

2

I am not sure, but aren’t you supposed to use for(auto &v: wall) instead of for(auto v: wall)?

Working Code 
#include <bits/stdc++.h>
using namespace std;

int main() {

vector<vector<int>> wall={{7, 1, 2}, {3, 5, 1, 1}, {10}};

 for (auto &v : wall)
    {
        for (int i=1;i<v.size();i++)
        {
            v[i]=v[i]+v[i-1];
            
            //cout<<v[i]<<" ";
                
        }
       // cout<<endl;
    }
    
    
    for (auto v : wall)
    {
        for (int i=0;i<v.size();i++)
        {
            
            cout<<v[i]<<" ";     
        }
        cout<<endl;
    }


return 0;
}
2 Likes
3

Thanks for your reply suman,
It is running fine…But can you please tell me the reason why we should put ‘&’ here…as we are not making a copy here , we are just iterating…

4

What you are asking is out of my knowledge. But the same question was asked here, it has answers.

5

Here is some explanation:

7

Thank you suman, this was helpful

8

Thank you kanchan, this was helpful

9

This part will help!

Modifying the elements in the container

If we want to modify the elements in a container using range-based for, the above for (auto elem : container) and for (const auto& elem : container) syntaxes are wrong.

In fact, in the former case, elem stores a copy of the original element, so modifications done to it are just lost and not stored persistently in the container, e.g.:

vector<int> v = {1, 3, 5, 7, 9};
for (auto x : v)  // <-- capture by value (copy)
    x *= 10;      // <-- a local temporary copy ("x") is modified,
                  //     *not* the original vector element.

for (auto x : v)
    cout << x << ' ';

The output is just the initial sequence:

1 3 5 7 9

Instead, an attempt of using for (const auto& x : v) just fails to compile.

g++ outputs an error message something like this:

TestRangeFor.cpp:138:11: error: assignment of read-only reference 'x'

      x *= 10;
        ^

The correct approach in this case is capturing by non-const reference:

vector<int> v = {1, 3, 5, 7, 9};
for (auto& x : v)
    x *= 10;

for (auto x : v)
    cout << x << ' ';

The output is (as expected):

10 30 50 70 90