CODEWITHSUNDEEP
c++dictionaryiteratorstd
DJMcMayhem
DJMcMayhem

Reputation: 7669

Detect if iterator is last element of std::map

This is similar to this question but not a duplicate. I'm trying to iterate through a map and print the values of each element, but with slightly different output on the last element. In that question, they recommend using map.rbegin().base(), but it's not working for me.

Here is my code:

#include <iostream>
#include <map>

int main()
{
    std::map <char, int> charMap = { {'a', 1}, {'b', 2}, {'c', 3}, {'d', 4} };
    for (auto iter = charMap.begin(); iter != charMap.end(); iter++)
    {
        std::cout << iter->first << ":\t" << iter->second;
        if (iter == charMap.rbegin().base())
            std::cout << "\t//This is the last element.\n";
        else
            std::cout << "\n";
    }
}

I'd expect my output to look like this:

a:    1
b:    2
c:    3
d:    4    //This is the last element.

But instead, I am getting this output:

a:    1
b:    2
c:    3
d:    4

Now I realize that there is a better way to do this, but I would expect this to work also. Why can't I compare iter and charMap.rbegin().base()?

Upvotes: 10

Views: 11796

Answers (7)

Ikbel
Ikbel

Reputation: 7851

This is working:

#include <iostream>
#include <map>

int main()
{
    std::map <char, int> charMap = { {'a', 1}, {'b', 2}, {'c', 3}, {'d', 4} };
    for (auto iter = charMap.begin(); ;)
    {
        std::cout << iter->first << ":\t" << iter->second;          
        if (++iter== charMap.end()){
            std::cout << "\t//This is the last element.\n"; break;
        }
        else
            std::cout << "\n";
    }

}

Upvotes: 0

Camiel Coppelmans
Camiel Coppelmans

Reputation: 27

Actually, this is not expect to work as iter is indeed different from charMap.rbegin().base().

Your logic is correct, the only problem is that charMap.rbegin().base() iterator is equal to charMap.end(), but you leave your for statement at the moment iter reaches the charMap.end() value, so it never got the chance to test it in the if statement inside the for loop.

To make it work you could rewrite the if statement in the following way:

if (iter->first == charMap.rbegin()->first)
    std::cout << "\t//This is the last element.\n";
else
    std::cout << "\n";

So you will be comparing the map key instead of the iterator itself.

Upvotes: 1

yuri kilochek
yuri kilochek

Reputation: 13486

Because std::reverse_iterator stores iterator with offset of 1, so that charMap.rbegin().base() == charMap.end(). Chart over here illustrates this.

You should instead use std::prev or std::next from <iterator>:

iter == std::prev(charMap.end())

or

std::next(iter) == charMap.end()

Whichever you find more meaningful.

Upvotes: 4

BartoszKP
BartoszKP

Reputation: 35891

base() doesn't return the same element:

The base iterator refers to the element that is next (from the std::reverse_iterator::iterator_type perspective) to the element the reverse_iterator is currently pointing to. That is &*(rit.base() - 1) == &*rit.

This works as intended:

#include <iostream>
#include <map>

int main()
{
    std::map <char, int> charMap = { {'a', 1}, {'b', 2}, {'c', 3}, {'d', 4} };
    auto last = charMap.rbegin();
    ++last;

    for (auto iter = charMap.begin(); iter != charMap.end(); iter++)
    {
        std::cout << iter->first << ":\t" << iter->second;
        if (iter == last.base())
            std::cout << "\t//This is the last element.\n";
        else
            std::cout << "\n";
    }
}

Upvotes: 2

AlexStepanov
AlexStepanov

Reputation: 481

base() method returns iterator to an element that follows the element to which a reverse iterator is pointing. This means that rbegin().base() is equivalent to end()

To accomplish your task, you can do this:

#include <iostream>
#include <map>

int main()
{
    std::map <char, int> charMap = { {'a', 1}, {'b', 2}, {'c', 3}, {'d', 4} };
    for (auto iter = charMap.begin(); iter != charMap.end(); )
    {
        std::cout << iter->first << ":\t" << iter->second;
        if (++iter == charMap.end())
            std::cout << "\t//This is the last element.\n";
        else
            std::cout << "\n";
    }
}

Upvotes: 5

vsoftco
vsoftco

Reputation: 56547

Use std::next from <iterator> as 

if (std::next(iter) == charMap.end())
    std::cout << "\t//This is the last element.\n";

instead.

Upvotes: 17

learnvst
learnvst

Reputation: 16195

Ugly . . . but . . . 

    auto iterCpy = iter;
    if (++iterCpy == charMap.end())
        std::cout << "\t//This is the last element.\n";

use vsoftco's answer :)

Upvotes: 0

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