Add a number to each negative element of a matrix

Question

I have the next matrix (Dimensions: {178, 178, 3}):

{{-1.22,2.44,3.14},{1.22,0.54,0.01},{1.21,-0.14,-2.31},{0.21,1.24,-0.42},...,{1.24,-0.25,1.34},{3.14,-3.14,-2.89}}

I want to add some quantity to each negative number of the matrix. Is it possible do that with control strucuture in Mathematica? How can I do it? thnk!

related: Replace values in a list based on conditions – WReach Oct 15 '16 at 22:02 — WReach, Oct 15 '16 at 22:02

bill s · Answer 1 · 2016-10-16T01:53:44.863

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Say mat is your matrix and val is the number you wish to add to all the negative values:

mat - val Clip[Sign[mat], {-1, 0}]

Another similar method:

mat + val (1 - UnitStep[mat])

edited Oct 16 '16 at 01:53

answered Oct 16 '16 at 01:11

bill s

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Furthermore, if mat is a packed array, it will stay packed with this method. – Michael E2 Oct 16 '16 at 01:47
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You might give mat + Clip[mat, {0, 0}, {val, 0}] consideration, I'd imagine faster (at least on packed arrays)... – ciao Oct 16 '16 at 04:32
Now I am curious. Has this solution any advatanges to a simple replace? E.g. far higher perfomance? – meneken17 Oct 16 '16 at 10:51

score 2 · Accepted Answer · answered Oct 15 '16 at 21:25

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This operation is pretty easy to do in Matheamtica. Let mat be your matrix and val your value. The you simply do

mat/. {x_ /; Negative[x] :> x + val}

If you want me to explain it just ask.

answered Oct 15 '16 at 21:25

meneken17

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The problem with this is that it won't work if the elements are symbolic expressions rather than numbers. How could that be modified to make it work in that case as well? – usumdelphini Jan 10 '19 at 15:13

Add a number to each negative element of a matrix

2 Answers2