14 Ways to Reverse a String! (And solve the exercise on Exercism)

[@adityapalpattuwar]

30:20 Displayed code has compile error. You need to add 'char' keyword before '[input.Length]' - > Span chars = stackalloc char[input.Length]; Thanks for the detailed video.

[@exercism_org]

Ah - nice catch. Thanks for letting us know. Looks like it's been updated in the approach already, but we can't change the video now sadly.

[@khoguan]

That C++ solution is efficient both time- and space-wise. But to make C++ handle UTF-8 strings is still a hassle.

[@exercism_org]

Yeah - agreed!

[@daniel1564..]

Now I really understand the benefits of using StringBuilder, thank you for that detailed and exploratory overview of solutions!

[@exercism_org]

Awesome to read! Thanks for letting us know :)

[@rsalmei]

The key in the C++ one, for me, is not that it does it in less iterations, it is that it does it in-place, without allocating any extra memory.

[@ErikSchierboom]

True. Many languages don't support mutation of strings.

[@TroyLaurin]

I agree with this sentiment, and disagree with the performance analysis in the video. The algorithm may perform half as many iterations, but it does twice as much work per iteration so the performance is practically the same compared to a version that iterates and copies character-wise over the whole string. It doesn't even avoid allocating extra memory, because the parameter is defined using copy semantics. The input string is copied into the function's stack on invocation, THAT COPY of the string is modified in-place and returned as the result of the function. Depending on a whole host of context, that value may need to be copied AGAIN at the call site of the function, unless the copy can be optimised away by something like NVRO. If I was the interviewer on this question and the candidate offered this solution without showing any understanding of what value is being modified and what copies are being made, that is a clear red mark, but not before prompting about the difference between copies and references. (note: only for jobs where this matters, which is rare. Then again, if you're applying for C++ jobs...)

[@dolorsitametblue]

Guys, you're killing it with the quality of the videos lately. Also, that little interlude at 23:54 is very funny =D.

[@exercism_org]

Thanks so much! Yeah - at some point I'll have to put together an out-takes video of us talking over each other too. The 1sec lag while recording is too much for us to handle 😂

[@ronaaron6935]

As the developer of 8th, I'd have given a much different answer. First of all, just "s:rev" ... which does everything (much like the first, "cheating", answer). Or one could do 'null s:/ a:rev "" a:join' if one were inclined to write much code. Bruce's answer is esoteric, and perhaps meant to show off more techniques that go far beyond the needs of the exercise.

[@ErikSchierboom]

True. The aim of this video is not to showcase the language's idiomatic solution, but that there are different ways in which the exercise can be solved. We'll be sure to mention this explicitly going forwards.

[@glennjackman759]

18:40 -- 8th: -- s:len -- s is not the input string, s is the namespace for string "words" (aka functions) -- s:len is the string length of the string at the top of the stack.

[@ErikSchierboom]

I have to start reading up on 8th!

[@ronaaron6935]

I was going to say, but I forgot...

[@karo4131]

I really enjoyed the video. I was hoping to see the Python solution, it's pretty similar to Julia: def reverse(string): return string[::-1]

[@ErikSchierboom]

That's a nice one! Almost identical than the Julia one, but even more succinct.

[@robert36902]

While I've never had to reverse a string to solve an actual problem in my programming career so far, this exercise shows important points in string handling that are good to know in order to correctly process strings that may contain arbitrary Unicode. After all, you want your code to be able to handle names of any person or city, not just those whose name can be represented with ASCII characters...

[@NadCAtarun]

No recursive version? Base case: if length < 2 return the string unchanged. Recursive case: return last char + reverse(string[1::-1]) + first char

[@ErikSchierboom]

We probably should have added one, sorry!

[@NadCAtarun]

It's okay @@ErikSchierboom ! I'm just a huge recursion buff 😉

[@ErikSchierboom]

Well I am too, which is why I should not have forgotten! :)

[@NadCAtarun]

@user-tx4wj7qk4t The point of the #48in24 challenge is to experiment not only with different languages but with different ways of solving the problems... What's wrong with using recursion in that context, then? Why should I care about the single responsibility principle or data coupling in the context of this challenge?

[@NadCAtarun]

@user-tx4wj7qk4t "better code" is such a context-dependent and subjective thing. You can value runtime performance, flexibility, legibility, style cohesion, resilience, composability, security... Ever heard of code golf? That makes my skin crawl, but you won't see me bother code golfers with all the code qualities they're throwing down the drain to have fun. Please let me enjoy my recursions in peace.

[@GiriPrasath.D]

In python def rev_string(word): return word[::-1]

[@canopusinthenorth]

In Golang section: Small correction, rune is not just a codepoint representing a byte but it could be a multibyte codepoint, which is why indexing into a string is not a good idea.

[@ulissescruz5540]

I thought that the Go solution using `runes` would also work in reversing words with graphene. Doesn't the `rune` concept treat a graphene as a single unit as well?

[@exercism_org]

I think not. Graphemes are clusters of unicode codepoints. A rune is a single codepoint. So in Go, a grapheme is a slice of runes. This is a nice example: go.dev/play/p/5J32XwYJulh

[@ulissescruz5540]

@@exercism_org Thank you for the great example. Now I think I understand runes better.

[@exercism_org]

Awesome. They're a really confusing topic! This was an interesting discussion the other day on the forum about them too: forum.exercism.org/t/potential-misleading-information-on-the-golang-runes-chapter/10082

[@edmundbertram8446]

Why do you say the string builder should, in the java solution, have been called with an input string length? There is no specified length of string in the instructions.

[@TroyLaurin]

When the function is called, you know the length of the string that was passed in, so you know what the length of the result will be.

[@ErikSchierboom]

Yes, this is precisely it! The reversed string's length will be equal to the input string's length, thus passing in the input string's length as the capacity for the StringBuilder will be exactly right.

[@ahans123]

C++ has also std::reverse, which essentially does the std::swap internally. It would be called as in `std::reverse(str.begin(), str.end());`. But maybe you'd call that cheating as well. ;-)

[@ErikSchierboom]

Yeah, most languages actually do support reversing using a built-in function or method. I showed the std:swap version as it was a great way to show the classic algorithm.

[@tamhuynhthanh874]

for Rust: pub fn reverse(input: &str) -> String { let mut reverse_str = String::new(); for c in input.chars().rev() { reverse_str.push(c); } reverse_str }

[@stefanwaldmann7636]

When using StringBuilder in Java, this would also work: (however I suppose that would count as cheating, too... ;-) public String reverse(String s) { return new StringBuilder(s).reverse().toString(); }

[@exercism_org]

Interesting! Is this better than s.reverse() in terms of speed and allocations?

[@stefanwaldmann7636]

The String class itself does not have a reverse() method. The implementation within the StringBuilder uses the same algorithm as the one from C++ shown in the video (swapping characters in place). However it's necessary to instantiate the StringBuilder and convert the result back to a String object again. Also, the version above instantiates a StringBuilder with a capacity of s.length() + 16. So an even better version (if you care about the additional 16 characters) with a capacity of only s.length() would be: new StringBuilder(s.length()).append(s).reverse().toString();

[@exercism_org]

Nice. Thank you for explaining!

[@ragectl]

Rust works well with graphemes

[@ErikSchierboom]

Absolutely. A lot of languages do actually, especially the more modern ones.

[@aedaed-f4x]

Bonsoir, s'il vous plaît j'ai besoin de votre aide, je veux faire quelques exercices en le langage C++ dans votre site puis savoir mon niveau par rapport aux solutions des autres, pouvez vous m'aider s'il vous plaît .. je sais pas les étapes pour faire les exercices; le lieu exact dans le site.. et merci infiniment pour vos efforts..

[@exercism_org]

Bonjour :) Vous pouvez résoudre des exercices C++ dans notre éditeur en ligne sur exercism.org/tracks/cpp. Si vous êtes bloqué, vous pouvez demander de l'aide en utilisant #get-help sur Discord (exercism.org/r/discord). Je recommande cependant de traduire d'abord vos messages en anglais (puis de traduire leurs réponses) pour augmenter les chances que les gens vous soutiennent car nous n'avons pas beaucoup de francophones. --- Hello :) You can solve C++ exercises in our online editor at exercism.org/tracks/cpp. If you get stuck, you can ask for help in #get-help on Discord (exercism.org/r/discord). I recommend translating your posts to English first though (then translating their responses back) to increase the chance people will support you as we don't have that many french speakers.

[@aedaed-f4x]

@@exercism_org Vraiment merciiiii énormément, d'accord je vais suivre vos recommandations.. juste je veux savoir est-ce que c'est nécessaire d'inscrire dans votre site pour éteindre ce que j'ai demandé dans mon 1 er msg ou non ? Parce que, hier, j'ai trouvé votre site exercism à pour but de résoudre les 82 exercices en C++ puis savoir mon niveau.. mais je sais pas tous les informations et les étapes pour faire ça.. je suis vraiment désolé d'avoir autant parlé 😢 et merciiii, je veux juste les étapes.. Autre fois, je vous réitère ma grande gratitude..

[@exercism_org]

Oui, vous devez vous inscrire et vous pourrez ensuite suivre le programme. Tout est gratuit. Si vous êtes totalement nouveau dans la programmation, vous aurez probablement un peu de mal et cela vaudrait la peine d'apprendre d'abord sur des sites plus basiques. J'envisagerais également un langage plus "convivial pour les débutants" comme JavaScript ou Python. Cependant, si vous savez déjà programmer un peu, alors notre cours de C++ sera parfait pour vous :) --- Yes, you need to sign up and then you can work through the syllabus. It's all free. If you're totally new to programming, you will probably struggle a little bit though and it would be worth learning from some more basic sites first. I'd also consider a more "beginnner-friendly" language like JavaScript or Python. However, if you can already program a little, then our C++ course will be great for you :)

[@aedaed-f4x]

@@exercism_org j'ai commencé la programmation depuis longtemps et d'abord j'ai terminé le C++ (ou plutôt, j'ai terminé les cours de C++ dans un site) et je veux faire qlqs exercices .. mais hier, comme j'ai dis avant, j'ai trouvé votre site, afin de faire qlqs exercices et connaître mon niveau par rapport aux solutions des autres, .. Bref, merci beaucoup à vous .. (j'attends votre réponse à propos des étapes que je dois suivre et mille mercis! Et désolé si je vous ai dérangé)

[@rsalmei]

You could have included Rust...

[@ErikSchierboom]

We could, but we only have a limited number of solutions (and tracks) to feature. We featured Rust last week, and we'll feature Rust in many other weeks to come.