﻿ CPP - Concatenate 3 copies of each string in a list of strings

# C++ Exercises: Concatenate three copies of each string in a given list of strings

## C++ Basic Algorithm: Exercise-127 with Solution

Write a C++ program to create a new list from a given list of strings where each element is replaced by 3 copies of the string concatenated together.

Test Data:
Sample Input:
{ "1", "2", "3" , "4" }
Expected Output :
111 222 333 444

Sample Solution:

C++ Code :

``````#include <bits/stdc++.h>
#include <list>

using namespace std;

list<string> test(list<string> nums) {
list<string> new_list;
list<string>::iterator it;
for (it = nums.begin(); it != nums.end(); ++it)
{
new_list.push_back(*it + *it + *it);
}

return new_list;
}

display_list(list<string> g)
{
list<string>::iterator it;
for (it = g.begin(); it != g.end(); ++it)
cout << *it << ' ';
cout << '\n';
}

int main() {

list<string> text = { "1", "2", "3" , "4", "5", "6" };
cout << "Original list of elements:\n";
display_list(text);
list<string> result_list;
result_list = test(text);
cout << "\nNew list from the said list where each element is replaced\nby 3 copies of the string concatenated together:\n";
display_list(result_list);
return 0;
}

``````

Sample Output:

```Original list of elements:
1 2 3 4 5 6

New list from the said list where each element is replaced
by 3 copies of the string concatenated together:
111 222 333 444 555 666
```

Flowchart: C++ Code Editor:

What is the difficulty level of this exercise?

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## C++ Programming: Tips of the Day

What is the usefulness of `enable_shared_from_this?

It enables you to get a valid shared_ptr instance to this, when all you have is this. Without it, you would have no way of getting a shared_ptr to this, unless you already had one as a member.

```class Y: public enable_shared_from_this
{
public:

shared_ptr f()
{
return shared_from_this();
}
}

int main()
{
shared_ptr p(new Y);
shared_ptr q = p->f();
assert(p == q);
assert(!(p < q || q < p)); // p and q must share ownership
}
```

The method f() returns a valid shared_ptr, even though it had no member instance. Note that you cannot simply do this:

```class Y: public enable_shared_from_this
{
public:

shared_ptr f()
{
return shared_ptr(this);
}
}
```

The shared pointer that this returned will have a different reference count from the "proper" one, and one of them will end up losing and holding a dangling reference when the object is deleted.

Ref : https://bit.ly/3pwVzzz