Range-based for loop with special case for the first item

Question

I find myself often with code that looks like this:

bool isFirst = true;
for(const auto &item: items)
{
    if(!isFirst) 
    { 
       // Do something
    }
    // Normal processing
    isFirst = false;
}

It seems that there ought to be a better way to express this, since it's a common pattern in functions that act like a "join".

Answer 1

Maybe a for_first_then_each is what you're looking for? It takes your range in terms of iterators and applies the first function to the first element and the second function to the rest.

#include <iostream>
#include <vector>

template<typename BeginIt, typename EndIt, typename FirstFun, typename OthersFun>
void for_first_then_each(BeginIt begin, EndIt end, FirstFun firstFun, OthersFun othersFun) {
    if(begin == end) return;
    firstFun(*begin);
    for(auto it = std::next(begin); it != end; ++it) {
        othersFun(*it);
    };
} 

int main() {

    std::vector<int> v = {0, 1, 2, 3};

    for_first_then_each(v.begin(), v.end(),
        [](auto first) { std::cout << first + 42 << '\n'; },
        [](auto other) { std::cout << other - 42 << '\n'; }
    );

    // Outputs 42, -41, -40, -39

    return 0;
}

Answer 2

You can't know which element you are visiting in a range based for loop unless you are looping over a container like an array or vector where you can take the address of the object and compare it to the address of the first item to figure out where in the container you are. You can also do this if the container provides lookup by value, you can see if the iterator returned from the find operation is the same as the begin iterator.

If you need special handling for the first element then you can fall back to a traditional for loop like

for (auto it = std::begin(items), first = it, end = std::end(items); it != end; ++it)
{
    if (it == first)
    {
        // do something
    }
    // Normal processing
}

If what you need to do can be factored out of the loop then you could use a range based for loop and just put the processing before the loop like

// do something
for(const auto &item: items)
{
    // Normal processing
}

Answer 3

With Ranges coming in C++20, you can split this in two loops:

for (auto const& item : items | view::take(1)) {
    // first element only (or never executed if items is empty)
}

for (auto const& item : items | view::drop(1)) {
    // all after the first (or never executed if items has 1 item or fewer)
}

If you don't want to wait for C++20, check out range-v3 which supports both of these operations.

This won't work like this with an Input range (like if items is really a range that reads from cin) but will work just fine with any range that is Forward or better (I'm guessing items is a container here, so that should be fine).

---

A more straightforward version is actually to use enumerate (which only exists in range-v3, not in C++20):

for (auto const& [idx, item] : view::enumerate(items)) {
    if (idx == 0) {
         // first element only
    }
    // all elements
}