Dev Notes

Various Cheat Sheets and Resources by David Egan/Carawebs.

First & Last Element in C++ Iterator Loop


C++
David Egan

Iterator-based iteration in C++ works for all containers, including lists and associative containers.

When looping with an iterator, the index of the current iteration isn’t immediately obvious, unless you resort to incrementing a counter.

First Element

It’s pretty straightforward to get the first element - you just compare the iterator at the current iteration through container with container.begin()

Last Element

To get the last element in an iterator loop you can use std::next() (from C++11). The loop is generally terminated by iterator != container.end(), where end() returns an iterator that points to the past-the-end element. If container.next(iterator) == container.end() returns true, you’re on the last element.

Example

#include <iostream>
#include <string>
#include <map>

int main()
{
    std::map<std::string, int> myMap{{"Rocky", 1}, {"Ronnie", 2}};
    std::map<std::string, int>::const_iterator it;

    for (it = myMap.begin(); it != myMap.end(); it++) {
        // Test for first element
        std::cout << (it == myMap.begin() ? "| " : "")
        << it->first << " = " << it->second

        // Test for last element
        << (std::next(it) != myMap.end() ? ", " : " |");
    }
    std::cout << std::endl;
    return 0;
}

// Output:
// | Rocky = 1, Ronnie = 2 |

References


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