for vs map

[letronje]

Context: http://www.4clojure.com/problem/99

One way to solve this is:

(fn [a b]
  (map #(- (int %) 48)
       (str (* a b))))

A slight variant would be to use a for comprehension,

(fn [a b]
  (for [d (str (* a b))]
    (- (int d) 48)))

From the above two, Which is preferable/recommended and why ?

The general question is whether to use 'for' or 'map' while generating new values from values in a single collection.

[Baishampayan Ghose]

Manoj,

`map` is a higher order function that maps a function over a sequence.
The length of the result sequence will always be the same as that of
the original sequence.

`for` on the other hand is a list-comprehension function and as such
allows you to "generate" elements given certain conditions.

In your case, there is really no difference between the two and I'd
prefer a `map` here as you don't really need `for`, but in certain
cases you must use `for` as `map` won't just cut it.

For example, what if I want to take in a sequence of numbers and emit
only the odd numbers from the sequence after adding one?

You can't use `map`, but `for` will do the trick -

(for [x xs :when (odd? x)]
(inc x))

;; a combination of `filter` & `map` will work as well

And this is how I solved #99 -

(fn [x y]
(map #(Integer/parseInt (str %)) (str (* x y))))

Regards,
BG

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--
Baishampayan Ghose
b.ghose at gmail.com