How can I get the pre-last item of a map?

[Necip]

Hello again,

my code runs each item of a map and I would like to stop before the last item is reached. I would be thrilled if anybody can explain...

typedef map<int, int>   M;
typedef M::iterator     It;

M m;

m[1] = 1;
m[2] = 2;
m[3] = 3;


for (It it=m.begin(); it != m.end(); ++it) {
  
  if (it == m.end() -1)   // ?
    cout << "pre-last item found: " << it->second;   // result 2
}

sincerly,
Necip


a bit more about me on processing8.wordpress.com

[2kaud]

.end() points beyond the last element so the previous element to .end() is actually the last element. So if you want the pre-last element you need the previous to the previous of the .end(). Consider

#include <map>
#include <iterator>
#include <iostream>
using namespace std;

typedef map<int, int> M;

int main()
{
	M m { {1, 1}, {2, 2}, {3, 3} };

	const auto prelast = prev(prev(m.end()));

	for (auto it = m.begin(); it != m.end(); ++it)
		if (it == prelast)
			cout << "pre-last item found: " << it->second << endl;
}

[monarch_dodra]

It depends on what you are trying to achieve exactly. Anyways, the last item can be found at the index before end, which can be found with either "prev(m.end())" with C++11, or with "--m.end()" (less recommended, as it returns a reference to a temporary).

From there, you can do something like this:

It beforeLast = prev(m.end());
for (It it=m.begin(); it != beforeLast; ++it) {
  // do things
}
// do thing to beforeLast.

If you want to explicitly handle it in your loop, then this works:

It beforeLast = prev(m.end());
for (It it=m.begin(); it != m.end(); ++it) {
  if (it == beforeLast) {
    // special case
  }
}
// do thing to beforeLast.

[2kaud]

Also consider which displays all items of the map up to but not including the last element

#include <map>
#include <iterator>
#include <iostream>
using namespace std;

typedef map<int, int> M;

int main()
{
	M m { {1, 1}, {2, 2}, {3, 3} };

	const auto last = prev(m.end());

	for (auto it = m.begin(); it != last; ++it)
			cout << it->second << endl;
}

[Necip]

Thank you friends for the hint with "prev". unfortunately it does not work on C++ Builder XE7!?!
But I could now find the solution for me seeking "prev" in google :

boost
via stackoverflow

[2kaud]

If the compiler you are using does not support prev, then it is not c++11 conformant. IMO I would suggest you consider changing to a compiler that is at least c++11 compliant and preferably c++14 compliant.

[2kaud]

As monarch_dodra indicated in post #3, prev() can be replaced by pre-decrement.

const auto prelast = prev(prev(m.end()));

can be replaced by

const auto last = --(--m.end());

and

const auto last = prev(m.end());

by

const auto last = --m.end();

[Necip]

If the compiler you are using does not support prev, then it is not c++11 conformant. IMO I would suggest you consider changing to a compiler that is at least c++11 compliant and preferably c++14 compliant.

I figured out that the C++ Builder XE7 IS c++11 conformant, but NOT in the 32-bit Version which our application is compiled with.
Belief me I would love to work with the last releases of C++ but unfortunatelly I am restricted by enterprise policies...