CODEWITHSUNDEEP
emacselisphashtableloops
Mirzhan Irkegulov
Mirzhan Irkegulov

Reputation: 18075

Iterate over Emacs Lisp hash-table

How to iterate over items (keys, values) in Elisp hash-tables?

I created a hash-table (map, dictionary) using (make-hash-table) and populated it with different items. In Python i could iterate over dicts by:

for k in d # iterate over keys
for k in d.keys() # same
for k in d.values() # iterate over values
for k in d.items() # iterate over tuples (key, value)

How can i do the same the most succinct and elegant way possible, preferably without loop-macro?

Upvotes: 9

Views: 3900

Answers (5)

Mirzhan Irkegulov
Mirzhan Irkegulov

Reputation: 18075

Starting from 2013 there is a third-party library ht, which provides many convenient functions to operate on Elisp hash-tables.

Suppose you have a hash-table, where keys are strings and values are integers. To iterate over a hash-table and return a list, use ht-map:

(ht-map (lambda (k v) (+ (length k) v)) table)
;; return list of all values added to length of their keys

ht-each is just an alias for maphash. There are also anaphoric versions of the above 2 functions, called ht-amap and ht-aeach. Instead of accepting an anonymous function, they expose variables key and value. Here's the equivalent expression to the one above:

(ht-amap (+ (length key) value) table)

Upvotes: 2

kdb
kdb

Reputation: 4426

I would have preferred to put this into a comment, but my reputation rating ironically prevents me from writing this in the appropriate format...

loop is considered deprecated and so is the cl library, because it didn't adhere to the convention of prefixing all symbols by a common library prefix and thus polluted the obarray with symbols without clear library association.

Instead use cl-lib which defines the same functions and macros but names them e.g. cl-loop and cl-defun instead of loop and defun*. If you need only the macros, you can import cl-macs instead.

Upvotes: 0

mathk
mathk

Reputation: 8143

maphash is the function you want. In addition I would suggest you to look at the manual (info "(elisp) Hash Tables")

Upvotes: 3

user797257
user797257

Reputation:

I'm going to advertise myself a bit, so take it with a grain of salt, but here are, basically, your options:

  • maphash - this is the built-in iteration primitive, fundamentally, no more ways to do it exist.

  • (loop for KEY being the hash-key of TABLE for VALUE being the hash-value of TABLE ...) is available in cl package. It will internally use maphash anyway, but it offers you some unification on top of different iterating primitives. You can use loop macro to iterate over multiple different things, and it reduces the clutter by removing the technical info from sight.

  • http://code.google.com/p/i-iterate/ Here's a library I'm working on to provide more versatile ways of iterating over different things and in different ways in Emacs Lisp. It is inspired by Common Lisp Iterate library, but it departed from it quite far (however, some basic principles still hold). If you were to try this library, the iteration over the hash-table would look like this: (++ (for (KEY VALUE) pairs TABLE) ...) or (++ (for KEY keys TABLE) ...) or (++ (for VALUE values TABLE) ...).

I will try to describe cons and pros of using either cl loop or i-iterate.

  • Unlike loop, iterate allows iterating over multiple hash-tables at once (but you must be aware of the additional cost it incurs: the keys of the second, third etc. hash-tables must be collected into a list before iterating, this is done behind the scenes).
  • Iterate provides arguably more Lisp-y syntax, which is easier to format in the editor.
  • With iterate you have more (and potentially even more in the future) options to combine iteration with other operations.
  • No one else so far is using it, beside myself :) It probably still has bugs and some things may be reworked, but it is near feature-freeze and is getting ready for proper use.
  • Significantly more people are familiar with either the built-in iteration primitives or the cl library.

Just as an aside, the full version of the iterate on hash-tables looks like this: (for VAR pairs|keys|values TABLE &optional limit LIMIT), where LIMIT stands for the number of element you want to look at (it will generate more efficient code, then if you were to break from the loop using more general-purpose tools).

Upvotes: 9

Anton Kovalenko
Anton Kovalenko

Reputation: 21517

(maphash (lambda (key value) ....your code here...) hash-table)

Upvotes: 16

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