Replacing a column of a matrix

Question

I want to replace a column of a matrix. The best I've been able to come up with is

a=RandomReal[9,{5,5}]
b=ConstantArray[0,5]
replacepos=4
Transpose[ReplacePart[Transpose[a], replacepos -> b]],

which does the trick, but looks awkward to me.

Did you search the site before posting? This has been asked a bunch of times... — Marius Ladegård Meyer, Dec 05 '18 at 12:13
@MariusLadegårdMeyer, yes I did, but previous questions seemed to look for more complex things. Thanks — Patricio, Dec 05 '18 at 18:54
Related (not dupe): https://mathematica.stackexchange.com/questions/3069/elegant-operations-on-matrix-rows-and-columns — Michael E2, Jun 28 '22 at 15:53

score 5 · Answer 1 · answered Dec 05 '18 at 13:15

5

What about

a[[All, replacepos]] = b

?

answered Dec 05 '18 at 13:15

Ulrich Neumann

Henrik Schumacher · Answer 2 · 2018-12-05T14:42:19.380

4

Indeed, Transpose has to perform superfluous reorderings which are memory bound. This does the trick and should be fairly efficient:

c = Module[{buffer = a},
  buffer[[All, replacepos]] = b;
  buffer
  ]

edited Dec 05 '18 at 14:42

answered Dec 05 '18 at 12:13

Henrik Schumacher

1

It wd be nice to compare with these suggestions to use ReplacePart or SubsetMap. – Alan May 02 '23 at 17:30

score 1 · Answer 3 · answered Dec 05 '18 at 12:21

1

a = RandomReal[9, {5, 5}]
(a[[#, 4]] = 0) & /@ Range@5
a

answered Dec 05 '18 at 12:21

ZaMoC

3 Answers3