Enhance _iter_comparable loop

[Vincent-Maladiere]

Checklist

• pytest passes
• code is well formatted

What does this implement/fix? Explain your changes
Hello @sebp!

The sksurv.metrics._iter_comparable function repeatedly creates a mask using np.zero, but this mask doesn't change for different elements j of the loop. Mallocs are computationally expensive, so this short PR proposes to reduce the number of calls to np.zero by creating the mask before the for-loop.

This will help make the c_index functions more efficient. WDYT?

cc @ogrisel

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.15%. Comparing base (03566f6) to head (2de9df5).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #465   +/-   ##
=======================================
  Coverage   98.15%   98.15%           
=======================================
  Files          37       37           
  Lines        3526     3526           
  Branches      539      539           
=======================================
  Hits         3461     3461           
  Misses         31       31           
  Partials       34       34           

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Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +0.00%	✅ ∅

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (03566f6)	3526	3495	99.12%
Head commit (03566f6)	3526 (+0)	3495 (+0)	99.12% (+0.00%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#465)	0	0	∅ (not applicable)

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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[sebp]

Thanks a lot for the PR.

Sorry for initially messing up your commit, but it's merged with commit ecd6dad now.

[Vincent-Maladiere]

Awesome, thanks!