Clojure - map function

Question

As we know, (map f [a b c]) is equivalent to [(f a) (f b) (f c)].

My question is: The evaluation result of (map #(- (int %) (int \0)) "1234") is (1 2 3 4), why does it return the results of applying #(- (int %) (int \0)) to every digits of "1234", rather than the string "1234" as a whole? How should I understand this code example?

One qualification: The result of (map f [a b c]) isn't exactly the same as the result of [(f a) (f b) (f c)], since map returns a lazy sequence rather than a vector. I think it's correct to say that the result of (map f [a b c]) is the same as the result of (lazy-seq [(f a) (f b) (f c)]), and that result of (vec (map f [a b c])) is the same as the result of [(f a) (f b) (f c)]. (If I'm wrong about some details, more knowledgeable people will hopefully correct me.) — Mars
– Mars, Commented Aug 5, 2014 at 4:32
You don't need map if you just want to call a function using a single argument. If that's what you want, then don't use map: (#(- (int %) (int \0)) "1234") — DaoWen
– DaoWen, Commented Aug 5, 2014 at 15:01
@DaoWen Your code produces ClassCastException java.lang.String cannot be cast to java.lang.Character ... - surely not what's intended. — Thumbnail
– Thumbnail, Commented Aug 5, 2014 at 18:59
@Thumbnail - Ha, you're right, thanks. You would need to use Integer/parseInt to convert the string to an int instead of the int "cast" that works for chars. The point is that I don't really understand the OP's question—*if you just want to apply the function to the string (not the individual characters), why don't you just call it directly rather than using map?* Apparently I'm missing something... — DaoWen
– DaoWen, Commented Aug 5, 2014 at 20:23

noisesmith · Accepted Answer · 2014-08-04 16:58:54Z

4

map calls seq on all arguments after the first. seq turns a string into a sequence of characters.

answered Aug 4, 2014 at 16:58

noisesmith

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Thumbnail · Accepted Answer · 2014-08-06 19:03:49Z

1

Clojure can treat a string as a sequence - of characters. This is useful because you can:

map things over the string
partition the string
get locations by index
do everything else sequences do.

It's perhaps a bit annoying having to remember to put the resulting sequence back into a string by wrapping the sequence manipulating expression in a call to str.

edited Aug 6, 2014 at 19:03

Thumbnail

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answered Aug 4, 2014 at 16:58

Arthur Ulfeldt

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2 Comments

noisesmith Over a year ago

it may make sense to merge our answers into yours, since yours is more complete anyway - perhaps making it clear that it is specifically map which calls seq on args, and showing (apply str (map f string)) as the canonical way to get a string back out the other side.

Mars Over a year ago

I value the simplicity, elegance, and implicit generality of your answer, @noisesmith, as well as Arthur Ulfeldt's discussion of the ramifications and purpose of map's behavior.

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