(newbie) replace certain values of a map based on a predicate

[Colin Yates]

Hi all,

I think this is one of those questions which has quite a few answers, but given a map, how do I replace the values by applying a function to those values, but only if they meet a condition?

I understand the building blocks of (map..), (filter..), (assoc-in..) and (filter..) and I can see how something could work using those pieces but is would be pretty verbose.  I am sure there is probably a much more simple way using a more abstract function.

The actual use case is I have a list of maps as returned from the clojure.java.jdbc framework and they contain timestamps.  I want to replace all the timestamps with (for example) to a Joda LocalDate using the excellent clj-time library.

Any pointers?

Thanks!

Col

[John Walker]

I had to do something similar, and used clojure.walk.

[Jim - FooBar();]

You'll never really 'replace' any values so why not reduce/reduce-kv ?
Just build a new map out of the old one...

Jim

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[Jay Fields]

I use update-vals from https://github.com/jaycfields/jry fairly often. As long as you don't mind doing the pred check in the fn you pass to update-vals, it should do the trick.

Cheers, Jay

On Fri, Jul 5, 2013 at 5:14 PM, Jim - FooBar(); <jimpi...@gmail.com> wrote:

You'll never really 'replace' any values so why not reduce/reduce-kv ? Just build a new map out of the old one...

Jim

On 05/07/13 21:59, Colin Yates wrote:

Hi all,

I think this is one of those questions which has quite a few answers, but given a map, how do I replace the values by applying a function to those values, but only if they meet a condition?

I understand the building blocks of (map..), (filter..), (assoc-in..) and (filter..) and I can see how something could work using those pieces but is would be pretty verbose.  I am sure there is probably a much more simple way using a more abstract function.

The actual use case is I have a list of maps as returned from the clojure.java.jdbc framework and they contain timestamps.  I want to replace all the timestamps with (for example) to a Joda LocalDate using the excellent clj-time library.

Any pointers?

Thanks!

Col
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[Jim - FooBar();]

I 'd change the implentation of update-vals to use reduce-kv. you will avoid the cost of destructuring on every single step :)

Jim

On 05/07/13 22:15, Jay Fields wrote:

I use update-vals from https://github.com/jaycfields/jry fairly often. As long as you don't mind doing the pred check in the fn you pass to update-vals, it should do the trick.

Cheers, Jay

On Fri, Jul 5, 2013 at 5:14 PM, Jim - FooBar(); <jimpi...@gmail.com> wrote:

You'll never really 'replace' any values so why not reduce/reduce-kv ? Just build a new map out of the old one...

Jim

On 05/07/13 21:59, Colin Yates wrote:

Hi all,

I think this is one of those questions which has quite a few answers, but given a map, how do I replace the values by applying a function to those values, but only if they meet a condition?

I understand the building blocks of (map..), (filter..), (assoc-in..) and (filter..) and I can see how something could work using those pieces but is would be pretty verbose.  I am sure there is probably a much more simple way using a more abstract function.

The actual use case is I have a list of maps as returned from the clojure.java.jdbc framework and they contain timestamps.  I want to replace all the timestamps with (for example) to a Joda LocalDate using the excellent clj-time library.

Any pointers?

Thanks!

Col
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[Colin Yates]

Grrr.. google groups seems to be playing up so apologies if this is double posted.

Thanks both!

My attempt (inspired by Stuart Sierra's reply at https://groups.google.com/forum/#!topic/clojure/Hlfn4PdKg-k) was something like this (from memory so forgive me):

(defn val-or-date [result [k v]] (if (instance? java.sql.Date v) (assoc result k (local-time/to-local-date-time v)) (assoc result k v)))

(reduce val-or-date {} r)

I can see how val-or-date can be much more generic passing in a predicate and transformation function, sure.

Col

On Friday, 5 July 2013 22:15:43 UTC+1, Jay Fields wrote:

I use update-vals from https://github.com/jaycfields/jry fairly often. As long as you don't mind doing the pred check in the fn you pass to update-vals, it should do the trick.

Cheers, Jay

On Fri, Jul 5, 2013 at 5:14 PM, Jim - FooBar(); <jimpi...@gmail.com> wrote:

You'll never really 'replace' any values so why not reduce/reduce-kv ? Just build a new map out of the old one...

Jim

On 05/07/13 21:59, Colin Yates wrote:

Hi all,

I think this is one of those questions which has quite a few answers, but given a map, how do I replace the values by applying a function to those values, but only if they meet a condition?

I understand the building blocks of (map..), (filter..), (assoc-in..) and (filter..) and I can see how something could work using those pieces but is would be pretty verbose.  I am sure there is probably a much more simple way using a more abstract function.

The actual use case is I have a list of maps as returned from the clojure.java.jdbc framework and they contain timestamps.  I want to replace all the timestamps with (for example) to a Joda LocalDate using the excellent clj-time library.

Any pointers?

Thanks!

Col
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[Ben Wolfson]

You could use clojure.algo.generic.functor.fmap: (fmap #(if (pred? %) replacement-value %) your-map).

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[Cedric Greevey]

Even without reaching for libraries outside clojure.core, there's

(update-in m [:key-of-interest] #(if (condition? %) (transformation %) %))

and the ability to convert that into a function, e.g.

(defn conditional-update-in [m condition? transformation k]
  (update-in m [k] #(if (condition? %) (transformation %) %)))

and then, if you have many keys that should undergo the same transformation under the same condition,

(reduce (partial conditional-update-in m condition? transformation) [keys...]), and its encapsulation,

(defn conditional-update-multi [m condition? transformation ks]
  (reduce (partial conditional-update-in m condition? transformation) ks))

[Justin Kramer]

Besides post-processing results, you can also instruct java.jdbc to return Joda dates in the first place.

Using clojure.java.jdbc 0.3.0-alpha4:

(ns example

  (:require [clojure.java.jdbc :as j]

            [clj-time.local :as cl]))

...

(extend-protocol j/IResultSetReadColumn

  java.sql.Date

  (result-set-read-column [date]

    (cl/to-local-date-time date)))

Note that result-set-read-column's signature has changed in master; you'll need to update it to [date _ _] in the next release.

Justin

[Colin Yates]

Thanks Cedric,

I am constantly writing code and then finding something in the core library whivh already does what I need :). 

http://clojuredocs.org/clojure_core/1.2.0/clojure.walk/keywordize-keys was the latest gem.

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