# Evaluation in mapping constructs [closed]

I'm trying to use ReplacePart inside of Map, where the address of the element being replaced is the domain of the map, like this:

f[{i_,j_}]:=i+j;
out = ConstantArray[{}, {3,3}];
pairs = {{1,3}, {2,2}, {2,3}};
Map[(out = ReplacePart[out, #->f[#]])&, pairs]


I expect out to end up with {} in every position except out[[1,3]] == out[[2,2]] == 4 and out[[2,3]] == 5. However, out ends up with the value ReplacePart[out, 3]&. Clearly, f[] is getting evaluated the first time through, and ReplacePart is never getting evaluated. I suspect some judicious arrangement of Hold and Evaluate might solve this problem, but I can't figure it out. Can someone please help?

(For now, I've solved the problem by wrapping ReplacePart in a function and passing out and the current value of # as arguments to it, but it would be much more elegant just to do it in place.)

Thanks!

-
 here out ends up being {{{}, {}, 4}, {{}, 4, 5}, {{}, {}, {}}}. Also you can use Scan rather than Map here – acl Oct 26 '12 at 17:31 Try it in a clean kernel and confirm if you still get the same results – Rojo Oct 26 '12 at 21:18 Thanks, @Rojo, the clean kernel did it. And thanks, @acl, for pointing out Scan. – Van Oct 26 '12 at 21:51

## closed as too localized by Sjoerd C. de Vries, rm -rf♦Oct 27 '12 at 1:03

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