Some time ago I’ve seen an inspiring talk from CppCon 2013: “C++ Seasoning” by Sean Parent. One of the main points of this presentation was not to use raw loops. Instead, prefer to use existing algorithms or write functions that ‘wraps’ such loops. I was curious about this idea and searched for nice code examples. Here is my short list of usage of algorithms from the C++ std library that might help in writing better code.

Of course. I could not skip two prominent examples from the original “C++ Seasoning” talk: slide and gather.

The code

Source code can be found here: beautiful_std_alg.cpp @github

Solution (VS2013) is located here vc_solution @github

Insertion sort

In just two lines of code!

``````for (auto i = start; i != end; ++i)
std::rotate(std::upper_bound(start, i, *i), i, std::next(i));
``````

How it works?

`Rotate(first, middle, last)`- takes a range `[first, last)` and rotates it so that the `middle` element becomes the first in that range.

`upper_bound` - Returns an iterator pointing to the first element in the range `[first,last)` which compares greater than `val`. The range should be already sorted (or at least partitioned).

How do those two elements combine into Insertion sort?

`std::upper_bound(start, i, *i)` returns position of the first element greater than `*i`. Then, the range is shifted, so that `i-th` element becomes first.

Let’s look at one example:

Pretty nice!

Quick sort

Found on Stack Overflow:

``````template<class FwdIt, class Compare = std::less<>>
void quickSort(FwdIt first, FwdIt last, Compare cmp = Compare{})
{
auto const N = std::distance(first, last);
if (N <= 1) return;
auto const pivot = std::next(first, N / 2);
std::nth_element(first, pivot, last, cmp);
quickSort(first, pivot, cmp);
quickSort(pivot, last, cmp);
}
``````

How it works?

I will not describe quick sort algorithm… you should know how it works already! In this implementation `std::nth_element` is used to do most of the job. This function partially sorts the range so that given `n-th` elements is placed in proper position. All of the elements before `n-th` element are less than or equal to the elements after the `n-th` element.

Slide

Example from the Sean Parent’s talk:

``````template <typename It>
auto slide(It f, It l, randIter p) -> std::pair<It, It>
{
if (p < f) return { p, std::rotate(p, f, l) };
if (l < p) return { std::rotate(f, l, p), p };
return { f, l };
}
``````

How it works?

As an example, you can imagine a list of items on a UI dialog. User selects a continuous range and then algorithm takes this range and move it into some other place of the list.

• this function uses `std::rotate`: to move elements forward or backward.
• it returns two iterators - the start and the end of the new sequence. In C++11 `std::rotate` got new version and now can return iterator to the new position of `p` element.
• if you are not interested in returning of this iterator pair, you can simplify this code much more.

Implementation note:

• In GCC 4.9 (and previous versions) `std::rotate` does not return an iterator, but only void. So currently, this code will not work there.

Gather

Another example from Sean Parent’s talk:

``````template <typename BiIt, typename UnPred>
auto gather(BiIt f, BiIt l, BiIt p, UnPred s) -> std::pair <BiIt, BiIt>
{
return { stable_partition(f, p, not1(s)),
stable_partition(p, l, s) };
}
``````

How it works?

It’s use case can be similar to `slide`: select elements - using a predicate `s` (so this time continuous range is not needed), then gather those elements into a range and move this range to position around `p`. It returns the start and the end of the selected range.

`UnPred` is a predicate that returns if a given element is selected or not.

`std::stable_partition`: from cppreference

Reorders the elements in a given range in such a way that all elements for which the predicate returns `true` precede the elements for which predicate returns `false`. Relative order of the elements is preserved.

`std::stable_partition` is used twice:

Implementation note:

• `std::not1` does not work with the code correctly, so there is proposal to use simple lambda. Read more here in Sean’s comment.

String trim

Found on Stack Overflow

``````std::string trim(const std::string &s) {
return trimLeft(trimRight(s));
}

std::string trimLeft(const std::string &s) {
auto temp = s;
temp.erase(std::begin(temp),
std::find_if(std::begin(temp), std::end(temp),
[](char c){return !std::isspace(c, std::locale());
}));
return temp;
}

std::string trimRight(const std::string &s) {
auto temp = s;
temp.erase(std::find_if(std::rbegin(temp), std::rend(temp),
[](char c){return !std::isspace(c, std::locale()); }).base(),
std::end(temp));
return temp;
}
``````

How it works?

Another beautiful usage of Standard Library:

• in order to trim the string we trim from right and then from the left (what a discovery!)
• trim left: `std::find_if` returns iterator to the first non space character in the string. Then we erase those characters.
• trim right: also uses `std::find_if` but this time we use reverse iterators

Note: you can also use boost string algorithm to make life even easier.

Bonus :)

What does this code do?

``````while (std::next_permutation(start, end));
``````

Simple, one line of code… should be nice! But…

Answer: it’s another and ‘wonderful’ method of sorting containers - permutation sort! But please do not use it at home :)

Complexity: O((n+1)!)

This algorithm is a variation of Bogosort and other similar ‘sorting’ algorithms. Read more on wiki. As victor_zverovich noticed, in Bogosort the next permutation is chosen at random, but std::next_permutation gives the next lexicographically greater permutation.

Sumup

I’ve showed several, I think nice, code examples where algorithms from C++ Standard Library are heavily used. Maybe next time, when I’ll be writing some ugly piece of code I’ll stop, think for a minute, and maybe some existing algorithm/function could be called instead.

Side note: there’s a Pluralsight course from Kate Gregory with a similar name: Beautiful C++: STL Algorithms. So you might want to check it out later

Do you know some more interesting examples? My list, definitely, does not show all of them!

Thanks for a discussion at reddit: here @r/programming and here @r/cpp

CodeProject