Reaching the last element of our choice in the vector using find()

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what i want to do is commented in the code below.
as find gives an iterator pointing to the first value found.. i want to reach the last value in my given vector of my desired choice using find function.
Thanks.

C#

vector<int>a = {5,1,2,3,5,4,5,6 };
	//i want to make my iterator iter point to the second last "5" in the given vector  i.e a[6].
	auto iter = a.begin();
	do {
		auto ptr = find(iter, a.end(), 5);
		iter = ptr + 1;			
	} while (iter != a.end());
	// error shown is vector iterator + offset out of range and the program crashes

What I have tried:

i havent tried anything special. but maybe there must be some problems with the iterators addition.

one possible solution is instead or iter=ptr+1; use iter=iter+1;
but this may increase the time complexity and this is not the perfect solution to my question.

Posted 7-Jun-16 12:28pm

shivam gohel

Updated 8-Jun-16 11:33am

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Comments

barneyman 7-Jun-16 19:43pm

you don't have an exit condition ... it'll find 3 fives, then it won't find any - so it'll return .end() - which you then try to increment

3 solutions

Solution 1

you are using the iterator wrong. You add 1 (bit) to it and NOT the full size.

It should be:

C++

iter++;//incrementing the iterator

Take a look how cplpusplus is doing it and sue a debugger.

Tip: Read some documentation and search for example code ;-)

Posted 8-Jun-16 7:13am

KarstenK

Solution 3

The error you are getting is quite easy to explain: In the fourth iteration of your loop iter points to the last element in the sequence (the 6) and find will fail, so it returns a.end(). So ptr now has the value of a.end(). In the next statement you try to increment ptr and that fails, because ptr + 1 would be out of range. That's exactly what the error message is trying to tell you.

You may either use a reverse find as CPallini suggests or, if you want to salvage your approach, write it like this:

C++

auto iter = a.begin();
while (true) {
    auto ptr = find(iter, a.end(), 5);
    if (ptr == a.end())
        break; // not found, so break out and leave iter on the last found value
    iter = ptr + 1;
}

Posted 8-Jun-16 11:33am

nv3

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CPallini · Accepted Answer · 2016-06-08T10:07:00

Solution 2

What about

C++

auto iter = find( a.rbegin(), a.rend(), 5);

?

Try

C++

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

int main()
{
  vector<int>a = {5,1,2,3,5,4,5,6 };

  auto iter = find( a.rbegin(), a.rend(), 5);

  if ( (iter != a.rend())) std::cout << "found at position " << (a.rend() - iter- 1 ) << endl;

}

Posted 8-Jun-16 10:07am

CPallini

Comments

shivam gohel 9-Jun-16 3:19am

perfect!
thank you sir!
you have always been helpful to me...

CPallini 9-Jun-16 3:36am

You are welcome.