various details in combinat

[fchapoton]

• about range(0, *)
• about isinstance() or isinstance()

CC: @tscrim @slel @trevorkarn

Component: combinatorics

Author: Frédéric Chapoton

Branch: 2967d9f

Reviewer: Trevor K. Karn

Issue created by migration from https://trac.sagemath.org/ticket/31918

[fchapoton]

Branch: u/chapoton/31918

[fchapoton]

New commits:

ee487be

various details about combinat (range and isinstance)

[fchapoton]

Commit: ee487be

[sagetrac-git]

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

2967d9f

one detail

[sagetrac-git]

Changed commit from ee487be to 2967d9f

[fchapoton]

comment:3

bot is morally green, please review

[trevorkarn]

Reviewer: Trevor K Karn

[trevorkarn]

comment:4

Looks good to me.

[trevorkarn]

Changed reviewer from Trevor K Karn to Trevor K. Karn

[vbraun]

Changed branch from u/chapoton/31918 to 2967d9f

[darijgr]

Changed commit from 2967d9f to none

[darijgr]

comment:7

-        result = [None] * len(grouping)
+        result = []
         j = 0
-        for i in range(len(grouping)):
-            result[i] = sum(self[j:j+grouping[i]])
-            j += grouping[i]
+        for gi in grouping:
+            result.append(sum(self[j:j + gi]))
+            j += gi

Sure this is an improvement?

sage: def xfun():
....:     a = []
....:     for i in range(10000):
....:         a.append(i)
....:     return a
....:
sage: def yfun():
....:     a = [None]*10000
....:     for i in range(10000):
....:         a[i] = i
....:     return a
....:
sage:
sage: timeit.eval("xfun()")
625 loops, best of 3: 1.6 ms per loop
sage: timeit.eval("yfun()")
625 loops, best of 3: 1.32 ms per loop

(I get even starker differences on occasion.)
Appending to a list over and over might be worse than initializing it to the right length.

[fchapoton]

comment:8

indeed. Undone among the other changes in #32310.