fix usage of underscore in combinat loops

[fchapoton]

underscore should be used as a variable loop only iff its value is not used

otherwise, one should use a normal variable

CC: @tscrim @slel @kliem @orlitzky

Component: combinatorics

Author: Frédéric Chapoton

Branch/Commit: b34161e

Reviewer: Michael Orlitzky

Issue created by migration from https://trac.sagemath.org/ticket/32978

[fchapoton]

Commit: 34b411a

[fchapoton]

New commits:

ea5d3f5	using _ for some loop variables in combinat
34b411a	fix usage of _ in combinat loops (partial)

[fchapoton]

Branch: u/chapoton/32978

[fchapoton]

comment:2

green bot, but not sure if this is a good idea. Maybe only the first commit ?

[orlitzky]

comment:3

Code golf! These list-of-empty-list loops can all be sped up a little if you avoid creating the (unused) int objects:

sage: %timeit [[] for _ in range(1000)]
206 µs ± 11.8 µs per loop (mean ± std. dev. of 7 runs, 10000 loops each)
sage: %timeit [[] for _ in repeat(None,1000)]
163 µs ± 13.7 µs per loop (mean ± std. dev. of 7 runs, 10000 loops each)

The numbers vary, but the itertools.repeat version is always better.

[slel]

comment:4

Thanks @orlitzky for that tip! Modulo that, I like both commits.

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

49a0a8b

further tweaks as suggested

[sagetrac-git]

Changed commit from 34b411a to 49a0a8b

[orlitzky]

comment:6

Typo:

diff --git a/src/sage/combinat/subword_complex.py b/src/sage/combinat/subword_complex.py
index a5efdbf..fa1a364 100644
--- a/src/sage/combinat/subword_complex.py
+++ b/src/sage/combinat/subword_complex.py
@@ -111,7 +111,7 @@ REFERENCES:
 #
 #                  https://www.gnu.org/licenses/
 # ****************************************************************************
-
+from itertools import repeat
 from copy import copy
 from sage.misc.cachefunc import cached_method
 from sage.structure.element import Element
@@ -845,8 +845,8 @@ class SubwordComplexFacet(Simplex, Element):
 
         # list the pseudolines to be drawn
         pseudolines = [[(shift[0], shift[1] + i), .5] for i in range(last + 1)]
-        pseudolines_type_B = [[] for _ in range(last + 1)]
-        contact_points = []
+        pseudolines_type_B = [[] for _ in repeat(None, last + 1)]
+        contact_points = []x
         root_labels = []
         pseudoline_labels = []
         if labels is not False:

[sagetrac-git]

Changed commit from 49a0a8b to b34161e