clojure: howto build a string from two sequences?

Question

I struggling with a problem for hours now... I want to build a link with values from two sequences.

(doseq [item photoset-name] (prn item ))
(doseq [item-name photoset-id] (prn item-name ))

output:

"AlbumTitel2"
"test"
"AlbumTitel"
"album123"
"speciale"
"neues B5 Album"
"Album Nr 2"
"72157632764328569"
"72157632769231530"
"72157632769092584"
"72157632768156156"
"72157632762740183"
"72157632724688181"
"72157632760876608"

Now I want to build a link like this (for every id/name):

<a href="http://example.com?id=72157632764328569">AlbumTitel2</a>

And it should be a sequence or map... anything I can iterate though.

Does anyone have an idea how to archive this?

Thanks!

score 2 · Answer 1 · answered Aug 07 '13 at 20:59

2

(map #(str "<a href=\"http://example.com?id="
           %1
           "\">"
           %2
           "</a>") photoset-ids photoset-names)

answered Aug 07 '13 at 20:59

Leon Grapenthin

9,246
24
37

I hope these aren't user-provided albums - make sure you escape if they are! – Alister Lee Aug 07 '13 at 23:24
What do you mean? If he reads them in using edn they should be escaped, right? – Leon Grapenthin Aug 08 '13 at 00:40
I am referring to html escaping to avoid XSS. What if a user provided an album with the title `X`? – Alister Lee Aug 08 '13 at 07:45
How wold you do it? I don't deal with generating weblinks a lot. – Leon Grapenthin Aug 08 '13 at 09:19
This question goes into it: [html escaping](http://stackoverflow.com/questions/14233777/how-do-i-display-html-within-html) – Alister Lee Aug 08 '13 at 12:37

score 1 · Answer 2 · edited May 23 '17 at 11:56

1

You could try using a map in one of two ways (I don't have Clojure handy at the moment so I can't verify):

(map #(prn "<a href=\"" %1 "\">" %2 "</a>") item item-name)

or

(doseq [pair (map vector item item-name)]
  (prn "<a href=\"" (first pair) "\">" (second pair) "</a>"))

EDIT: be careful using map; it's lazy so if you don't use the result returned by it then it might not actually be run!

This might also work:

(doseq [[url title] (map vector item item-name)]
  (prn "<a href=\"" url "\">" title "</a>"))

Combining items from multiple lists into pairs in a single list is commonly referred to as "zipping". The (map vector ...) I found in this answer

edited May 23 '17 at 11:56

Community

1
1

answered Aug 07 '13 at 20:57

paul

1,655
11
23

1

Why would you want it to be run if you don't use the result? It's the whole point of laziness, isn't it? – Leon Grapenthin Aug 07 '13 at 22:05
@lgrapenthin you are correct; however, if you're unaware that `map` is lazy in Clojure and you try to do some sort of IO operation in it, using it as a general loop construct and not consuming the result, then you might be surprised, [as I was](http://stackoverflow.com/questions/12046515/listbox-jlist-wont-update-dynamically-from-custom-listmodel), when nothing happens. I just thought I'd mention this in case the OP didn't know. – paul Aug 07 '13 at 22:45
In the case that you linked to you were trying to invoke a function with side-effects using map. There, instead of using `doseq` as suggested in the answer you got, wrapping the call to map in a `dorun` usually performs better, as doseq should only be used when doing additional, `for`-like filtering/combining in the bindings. Btw., the map returned from the first line of your code will return a seq full of `nil`s. – Leon Grapenthin Aug 07 '13 at 23:14
Might be better to avoid doing the IO during the transformation (use str as per @lgrapenthin 's answer. – Alister Lee Aug 07 '13 at 23:20
1

@lgrapenthin: That's not true. `map` always makes all the intermediate allocations, while `doseq` does not. For that reason alone one would expect it to be faster. Then there's the fact that `dorun` is not chunked-seq-aware, whereas `doseq` is; `doseq` inlines the body in the expansion, while `map` uses the passed-in function in a higher order fashion; etc. In simple microbenchmarks using Criterium, with only one input collection and no processing of items beyond executing a body of expressions for side effects, `doseq` wins by fairly significant margins. – Michał Marczyk Aug 07 '13 at 23:26
1

Example microbenchmark: https://www.refheap.com/17369 (604 µs `doseq` vs. 987 µs `dorun map` when iterating over a range mutating a box at each step). I am of course only disagreeing with the assertion that `dorun map` should be preferred to `doseq` where both are applicable. Here `doseq` does not seem to be applicable in the first place, since the question asks for a seqable to be returned. – Michał Marczyk Aug 07 '13 at 23:33
1

Better microbenchmark (results: 624 µs vs. 977 µs): https://www.refheap.com/17370 – Michał Marczyk Aug 07 '13 at 23:43
I am surprised. I remember opposite results from my own benchmarks involving file IO. Well then I'll have to look into that again. – Leon Grapenthin Aug 08 '13 at 00:38
@lgrapenthin I was using `prn` in the iterated function since that's the function the OP was using in their example -- I seem to have missed the line where they wanted something iterable as a result. As you say, it should be using `str` instead so that the returned list isn't all `nil` values. – paul Aug 08 '13 at 01:13
@lgrapenthin Also, thanks for letting me know about `dorun`. I've been learning Clojure in a piecemeal fashion by reading docs for/finding functions when I need them, so I haven't had exposure to a lot of core's functions yet. – paul Aug 08 '13 at 01:15

clojure: howto build a string from two sequences?

2 Answers2